Microbial interaction – Definition, Types, Characteristics, Examples

Microbial interaction is a biological interaction in which the effect of microorganisms on other biotic components of an ecosystem can be investigated. Microbiology is the study of microorganisms, and microbial ecology is the study of microbial interactions within an ecosystem.

Positive and negative microbial interactions are possible, and microbes can affect (positively or negatively) other elements of an ecosystem, such as plants, animals, and humans. The beneficial microbial interaction may result in no harm to any of the populations or only benefit one population (without affecting the other).

In contrast, negative interaction may result in a positive outcome for one or both parties involved. All biotic components, or living organisms, are dependent on one another for sustenance or survival in an ecosystem. Here, you will learn the definition of positive and negative microbial interactions, as well as examples of each.

What is Microbial interaction?

• Microbial interaction, as understood in the realm of biology, refers to the influence that microorganisms exert on other biotic components within an ecosystem. This interaction is a subset of the broader field of microbiology, which is dedicated to the study of microorganisms. More specifically, when discussing the interactions of microorganisms within their environment, this falls under the domain of microbial ecology.
• Microbial interactions can manifest in various ways, either positively or negatively. These interactions can influence other elements of an ecosystem, including plants, animals, and humans.
• Positive microbial interactions typically result in no harm to any involved populations, and in some cases, only one population benefits without affecting the other. On the contrary, negative interactions might be advantageous to one or both involved parties.
• In any given ecosystem, all biotic components, which encompass living organisms, rely on each other for survival and sustenance. Therefore, understanding microbial interactions, both positive and negative, is crucial.
• For instance, a microbial interaction is characterized by one group of microorganisms engaging with another, establishing a relationship that can be either beneficial or detrimental. When the interaction occurs between the same organisms, it is termed an intraspecific interaction. Besides, when the interaction is between different organisms, it is labeled as an interspecific interaction.
• Biological interactions, in general, denote the effects organisms within a community impose on one another. Delving deeper into microbial interactions, one can identify various types.
• These include interactions with other microbes, interactions promoting plant growth (Plant-Germ interactions), interactions with animals, humans, and even water. It is imperative to note that microbial interactions are not only diverse and omnipresent but also play a pivotal role in the functioning of any biological community. Then, considering the broader perspective, these interactions are integral to global biogeochemistry.
• Among the myriad of microbial interactions, cooperative ones are frequently observed, often resulting in mutual benefits. The nature of these interactions can be categorized based on the benefits or detriments to the involved populations.
• For instance, there are several forms of symbiotic relationships, such as mutualism, parasitism, amensalism, commensalism, competition, predation, and protocooperation. Each of these relationships emphasizes the intricate balance and interdependence between organisms, underscoring the importance of understanding microbial interactions in the broader context of ecosystem dynamics.

Definition of Microbial interaction

Microbial interaction refers to the relationships and interactions between microorganisms within an ecosystem, which can be positive, negative, or neutral, influencing the function and balance of the community.

Types of Microbial Interaction

These interactions can be broadly categorized into positive and negative types, each with its distinct characteristics and subtypes. In this exposition, we will delve into the various types of microbial interactions, elucidating their functions and significance.

1. Positive Microbial Interaction

Positive microbial interaction refers to the relationship where both participating organisms derive benefits. In such interactions, organisms from two distinct populations establish a connection that can be persistent, transient, or obligatory, all aiming for mutual advantage. There are four primary types of positive interactions:

• Mutualism: In mutualism, both organisms benefit from the interaction. For instance, certain bacteria in the human gut aid in digesting food, while they receive nutrients in return.
• Protocooperation: This interaction is beneficial but not essential for the survival of the organisms involved. It’s a voluntary relationship where both parties benefit, but they can also survive independently.
• Syntrophism: Here, two or more organisms collaborate to degrade a compound, which individually they might not be able to. The end product of one organism serves as a substrate for another, ensuring mutual survival.
• Commensalism: In commensalism, one organism benefits while the other is neither harmed nor benefited. An example is certain bacteria living on the human skin, where they get a habitat and food without affecting the host.

2. Negative Microbial Interaction

Negative microbial interactions are characterized by a relationship where one organism benefits at the expense of another. In these interactions, one organism might assault or suppress others to secure survival and food sources. Four primary types of negative interactions are:

• Predation: This involves one organism (the predator) feeding on another organism (the prey). For example, certain protozoa feed on bacteria.
• Parasitism: In parasitism, one organism (the parasite) benefits while causing harm to another organism (the host). Many pathogenic bacteria that cause diseases in humans and animals exhibit this type of interaction.
• Amensalism: Here, one organism is harmed while the other remains unaffected. An example is when certain fungi produce antibiotics that kill bacteria, without the bacteria affecting the fungi.
• Competition: This occurs when two organisms vie for the same limited resource. In microbial communities, bacteria might compete for nutrients, space, or light.

A. Positive interaction

1. Mutualism

• Mutualism is a biological interaction characterized by a relationship where both participating organisms derive benefits. This type of interaction is foundational in many ecosystems and plays a crucial role in the survival and functioning of various species.
• Mutualism is defined as a relationship in which each organism involved benefits from the association. This is not a mere casual relationship; it is obligatory in nature. In mutualism, the mutualist and the host are metabolically interdependent, meaning they rely on each other for certain metabolic functions. This interdependence is so specific that one member of the association cannot be easily replaced by another species. Therefore, the specificity of mutualistic relationships is paramount.
• For mutualism to function effectively, close physical contact between the interacting organisms is often required. This proximity ensures that the benefits of the relationship are efficiently exchanged between the participants. Besides, mutualism enables organisms to thrive in habitats that might be inhospitable to them individually. When in a mutualistic relationship, the combined abilities of the organisms allow them to exist in environments that neither could occupy on their own.
• Furthermore, the relationship of mutualism is so intertwined that the participating organisms can sometimes function as if they were a single organism. This level of cooperation and synergy emphasizes the depth and importance of mutualistic relationships in nature.
• In essence, mutualism is a type of symbiotic relationship where both organisms live closely together, each benefiting from the other. The outcome of this interaction is positive for both parties, often denoted as +/+. Both individuals involved in mutualism are referred to as symbionts, highlighting their close and beneficial association.

Characterstics of Mutualism

1. Beneficial Association: Mutualism is defined by a relationship where each organism involved gains advantages from the association. Both parties benefit, making it a positive interaction.
2. Obligatory Nature: Often, mutualism is an obligatory relationship, meaning both the mutualist and the host are metabolically dependent on each other. Neither can thrive or even survive without the other.
3. Specificity: The mutualistic relationship is highly specific. One member of the association typically cannot be replaced by another species without disrupting the mutualistic benefits.
4. Close Physical Contact: Mutualism often requires close physical contact between the interacting organisms. This proximity ensures that both organisms can access the benefits of the relationship.
5. Expansion of Habitats: The relationship of mutualism allows organisms to exist in habitats they might not be able to occupy individually. By working together, they can overcome challenges that would be insurmountable alone.
6. Unified Function: In some mutualistic relationships, the organisms function almost as a single organism. Their actions and benefits are so intertwined that they operate in harmony, each supporting the other’s needs.
7. Interdependence: Both microbial populations in mutualism are interdependent for mutual benefit. Neither can fully benefit without the contribution of the other.
8. Symbiotic Nature: Both interacting individuals in mutualism are termed symbionts. Mutualism is a subset of symbiotic relationships where organisms live closely together. The effect of mutualism is often represented as +/+, indicating a positive outcome for both parties as a result of the interaction.

Examples of Mutualism

• Lichens: Lichens serve as a prime example of mutualism. They represent a unique association between specific fungi and certain genera of algae. In this relationship, the fungal component is termed the “mycobiont,” while the algal component, which belongs to cyanobacteria and green algae (like Trabauxua), is called the “phycobiont.” The phycobiont, being photoautotrophic, supplies organic carbon directly to the fungal partner. In return, the fungus offers protection to the phycobiont against extreme conditions and furnishes it with water and essential minerals. Lichens, though slow-growing, can colonize habitats that are inhospitable to other organisms. They exhibit remarkable resistance to high temperatures and desiccation.
• Protozoan-Termite Relationship: The relationship between flagellated protozoans and termites epitomizes mutualism. These protozoans reside in the termite’s gut. They feed on carbohydrates, specifically cellulose or lignin, ingested by their host termites and metabolize them into acetic acid, which the termites then utilize.
• Paramecium and Chlorella: Paramecium, a type of protozoa, can host the algae Chlorella within its cytoplasm. This mutualistic relationship allows Chlorella to provide the Paramecium with organic carbon and oxygen. In exchange, the Paramecium offers protection, mobility, carbon dioxide, and other growth factors. The presence of Chlorella enables the Paramecium to survive in anaerobic conditions, provided there is adequate light.
• Gut Flora and Humans: The human digestive tract is home to a diverse community of microbes, collectively termed gut microflora or gut microbiota. This community, comprising bacteria, archaea, and fungi, lives symbiotically within the human gut. While the microflora benefits from the energy stored in the human body, they reciprocate by offering resistance against external microbial colonization, aiding in vitamin synthesis, assisting in digestion, and fostering a robust immune system.
• Trichonympha and Termites: Trichonympha, a protozoan, plays a crucial role in breaking down the complex carbohydrate cellulose found in wood into simpler sugars, which termites then utilize. Living symbiotically in the termite’s gut, Trichonympha benefits from shelter and a consistent food supply, thanks to the termite’s chewing action.
• Chlorella and Paramecium: Paramecium bursaria, a ciliated protist, harbors algal cells from the Chlorella species. This mutualistic relationship sees the Chlorella residing within the cytoplasm of P. bursaria. While P. bursaria provides carbon dioxide and protection to the algal cells, Chlorella, in turn, enables P. bursaria to survive in anaerobic conditions and offers maltose as an energy source.

2. Protocooperation

• Protocooperation is defined as a relationship where organisms in an association mutually benefit from their interaction. However, a defining feature of protocooperation is that, unlike mutualism, the relationship between the organisms is not obligatory. This means that while both parties benefit from the association, they are not metabolically or functionally dependent on each other for survival.
• Protocooperation bears similarities to mutualism in that both interactions result in mutual benefits for the involved organisms. However, a crucial distinction lies in the nature of their relationship. In mutualism, the relationship is obligatory, meaning that the organisms are closely interdependent and often cannot thrive without each other. In contrast, protocooperation does not involve such a stringent dependency. The organisms in protocooperation interact primarily for the advantages they derive from the association, but they can exist independently without significant detriment.
• The effect of protocooperation on the involved organisms is positive for both parties, often represented as +/+. This indicates that both organisms gain advantages from the interaction, enhancing their chances of survival or reproduction. However, it’s essential to note that the absence of one organism does not severely impact the other’s survival, underscoring the non-obligatory nature of this interaction.

Characterstics of Protocooperation

• Mutual Benefit: In protocooperation, both organisms involved gain advantages from the association, similar to mutualism. This mutual benefit is the driving force behind their interaction.
• Lack of Obligation: Unlike mutualism, where the relationship is obligatory and both organisms are closely interdependent, protocooperation does not bind the organisms in such a manner. They interact because of the benefits, but they can survive independently of each other.
• Positive Interaction: The outcome of protocooperation is positive for both parties involved, often represented as +/+. This means that both organisms benefit from the interaction without any detrimental effects on either side.

Examples of Protocooperation

1. Association of Desulfovibrio and Chromatium:
   • Function: Desulfovibrio and Chromatium engage in a mutually beneficial relationship that plays a vital role in both the carbon and sulfur cycles within ecosystems.
   • Desulfovibrio is a type of sulfate-reducing bacteria that utilizes sulfate (SO4²⁻) as an electron acceptor during anaerobic respiration. This process leads to the reduction of sulfate to hydrogen sulfide (H2S).
   • Chromatium is a photosynthetic sulfur bacterium that thrives in anoxic conditions. It utilizes light energy to fix carbon dioxide (CO2) and reduce sulfur compounds, producing elemental sulfur (S) as a byproduct.
   • Interaction: Desulfovibrio releases hydrogen sulfide (H2S) as a metabolic product, which serves as an essential electron donor for Chromatium during its photosynthesis. In return, Chromatium provides Desulfovibrio with organic carbon compounds produced through photosynthesis.
   • Contribution: This protocooperation results in a balanced carbon-sulfur cycle. Desulfovibrio benefits from the organic carbon compounds provided by Chromatium, while Chromatium gains a constant supply of electron donors in the form of H2S. Therefore, this interaction ensures the efficient cycling of carbon and sulfur in the ecosystem.
2. Interaction between N2-Fixing Bacteria and Cellulolytic Bacteria such as Cellulomonas:
   • Function: Nitrogen-fixing bacteria and cellulolytic bacteria, like Cellulomonas, engage in a mutualistic relationship that supports the nitrogen cycle.
   • Nitrogen-fixing bacteria are capable of converting atmospheric nitrogen (N2) into ammonia (NH3) through the process of nitrogen fixation.
   • Cellulomonas is a cellulolytic bacterium that specializes in breaking down cellulose, a complex organic compound found in plant cell walls.
   • Interaction: Nitrogen-fixing bacteria provide Cellulomonas with ammonia (NH3) as a source of nitrogen, which is essential for the synthesis of proteins and nucleic acids. In return, Cellulomonas assists nitrogen-fixing bacteria by breaking down cellulose-rich plant material, releasing organic carbon compounds.
   • Contribution: This protocooperation ensures the availability of nitrogen, a crucial nutrient, in ecosystems with abundant plant material. Nitrogen-fixing bacteria contribute to the nitrogen pool, while Cellulomonas aids in the decomposition of plant matter, facilitating nutrient recycling. Consequently, this interaction plays a vital role in maintaining ecosystem productivity.

3. Commensalism

• Commensalism is a biological relationship characterized by one organism, known as the commensal, benefiting from its association with another organism, referred to as the host, without causing any harm to the host. This type of interaction falls under the category of positive interactions, where one party benefits, while the other remains unaffected.
• In commensalism, the commensal organism derives advantages from the association, but the host organism neither gains nor suffers any consequences as a result of this interaction. This relationship is unidirectional, meaning that the commensal benefits, but the host is essentially neutral in the association.
• If the commensal organism is separated from the host, it can continue to survive independently. This independence demonstrates that the commensal does not rely on the host for its essential needs, and the association is primarily one-sided in terms of benefits.
• The impact of commensalism is denoted as +/0, indicating that it has a positive effect on the commensal organism (denoted by “+”) while having no significant impact on the host organism (denoted by “0”). This classification is essential in ecological studies as it helps describe the various types of interactions that occur in ecosystems.
• In summary, commensalism is a form of symbiotic relationship in which one organism benefits without causing any harm to another. This interaction is unidirectional, and if the commensal is separated from the host, it can continue to thrive on its own. This type of positive interaction is characterized by a +/0 impact, highlighting the asymmetry in the benefits derived from the association.

Characteristics of Commensalism

1. Unidirectional Benefit: In commensalism, only one of the two organisms involved benefits from the association. The organism benefiting is referred to as the commensal, while the other, the host, remains unaffected. This one-sided benefit is a defining feature of commensal relationships.
2. Neutral Effect on the Host: The host organism in a commensal relationship neither gains nor suffers any significant consequences as a result of the association. It continues its normal life processes without being influenced by the commensal.
3. Independence of Commensal: Commensals are typically capable of surviving and functioning independently if they are separated from the host. This independence distinguishes commensalism from other forms of symbiosis where the organisms may have a more interdependent relationship.
4. Lack of Harm to the Host: Unlike parasites, which harm their hosts, commensals do not negatively impact the host organism in any way. They do not feed on the host’s resources or cause damage to the host’s tissues.
5. Examples in Nature: Commensal relationships can be found in various ecosystems and among a wide range of organisms. For instance, certain species of birds may build nests in the trees without harming the tree, and epiphytic plants may grow on the branches of larger trees, utilizing them for support but not causing harm.
6. Benefit to the Commensal: The commensal organism derives some form of benefit from the association, which can include access to resources, protection, or support. This benefit allows the commensal to thrive or gain advantages it might not have on its own.
7. Variability in Intensity: Commensal relationships can vary in the degree of benefit that the commensal receives. In some cases, the benefit may be significant, while in others, it may be relatively minor.
8. Interactions in Ecosystems: Commensalism is one of the ways in which organisms interact in ecosystems. It contributes to the overall diversity and complexity of ecological communities by providing opportunities for different species to coexist and utilize available resources.
9. Positive Sign in Ecological Notation: In ecological notation, commensalism is often represented with a “+” sign for the commensal and a “0” (zero) for the host to indicate the positive benefit to the commensal and the lack of impact on the host.

Examples of Commensalism

1. Non-pathogenic E. coli in the Intestinal Tract of Humans:
   • Components: This commensal relationship involves the non-pathogenic strain of Escherichia coli (E. coli) as the commensal and the human intestinal tract as the host.
   • Function: E. coli, being a facultative anaerobe, utilizes oxygen during its metabolic processes. As a result, it lowers the oxygen concentration in the gut environment, creating conditions suitable for obligate anaerobes like Bacteroides. In this association, E. coli serves as the host, while Bacteroides benefits from the anaerobic environment.
   • Benefit: Bacteroides benefits from the reduced oxygen levels, allowing it to thrive in the intestinal tract. E. coli, on the other hand, remains unaffected by Bacteroides and continues to inhabit the gut without harm.
2. Flavobacterium (Host) and Legionella pneumophila (Commensal):
   • Components: Flavobacterium, a bacterium, serves as the host, while Legionella pneumophila, another bacterium, is the commensal in this interaction.
   • Function: Flavobacterium excretes cystine, a sulfur-containing amino acid, into its aquatic habitat.
   • Benefit: Legionella pneumophila benefits from the cystine excreted by Flavobacterium. Cystine serves as a source of nutrients and energy for Legionella pneumophila, allowing it to survive and thrive in the aquatic environment. Flavobacterium, as the host, remains unaffected by this interaction.
3. Association of Nitrosomonas (Host) and Nitrobacter (Commensal) in Nitrification:
   • Components: Nitrosomonas and Nitrobacter are both bacteria involved in the nitrification process.
   • Function: Nitrosomonas oxidizes ammonia (NH3) into nitrite (NO2⁻), while Nitrobacter utilizes nitrite as a source of energy and further oxidizes it into nitrate (NO3⁻).
   • Benefit: Nitrosomonas initiates the nitrification process by converting ammonia to nitrite. Nitrobacter subsequently uses nitrite as an energy source and converts it to nitrate. This sequential interaction is essential for the conversion of ammonia to nitrate in the nitrogen cycle, which plays a crucial role in nutrient cycling in ecosystems.

4. Syntrophism

Syntrophism represents a form of biological association in which the growth and metabolic activities of one organism are either entirely dependent on or significantly improved by the substrates provided by another organism. In this mutualistic interaction, both organisms involved benefit from their association, and their collaboration is essential for carrying out specific metabolic processes.

Components of Syntrophism:

1. Population 1: This refers to the first group of microorganisms in the syntrophic relationship.
2. Population 2: This is the second group of microorganisms involved in the interaction.
3. Compound A: Compound A is a substrate or molecule that can be utilized by Population 1.
4. Compound B: Compound B is a product formed by the metabolic activity of Population 1 when it utilizes Compound A.
5. Compound C: Compound C is a product formed by the metabolic activity of Population 2 when it utilizes Compound B.
6. Products: These are the end products resulting from the collaborative metabolic reactions between Population 1 and Population 2.

Syntrophism in Action:

In a theoretical example of syntrophism, Population 1 possesses the capability to utilize and metabolize Compound A, leading to the formation of Compound B. However, Population 1 cannot metabolize beyond Compound B without the cooperation of Population 2. On the other hand, Population 2 is unable to utilize Compound A, but it can metabolize Compound B, forming Compound C.

Therefore, the sequence of events in syntrophism is as follows:

1. Population 1 utilizes Compound A, forming Compound B.
2. Population 2 metabolizes Compound B, generating Compound C.
3. The combined actions of both Population 1 and Population 2 enable a metabolic reaction that leads to the production of end products that neither population could produce independently.

In essence, syntrophism highlights the interdependence of microorganisms in carrying out complex metabolic processes. Each population contributes a crucial step in the overall reaction sequence, and together, they achieve the production of end products that benefit both parties. This cooperative relationship is a fundamental aspect of microbial ecology and plays a vital role in various biogeochemical cycles, including those in anaerobic environments such as sediment and the human gut, where syntrophic interactions are prevalent.

Characteristics of Syntrophism

1. Interdependence of Organisms: Syntrophism involves an interdependent relationship between two or more microorganisms. The growth and metabolic activities of one organism depend on the products or substrates produced by another organism in the association.
2. Mutual Benefit: Both organisms involved in a syntrophic relationship benefit from their association. Each organism plays a crucial role in the overall metabolic pathway, and their collaboration is necessary for the successful utilization of specific substrates and the production of end products.
3. Sequential Metabolic Reactions: Syntrophism typically involves a sequence of metabolic reactions that occur in a stepwise manner. Each organism contributes to a specific stage of the process, and the reaction products of one organism serve as substrates for the next organism in the sequence.
4. Utilization of Different Compounds: Organisms in a syntrophic relationship often utilize different compounds or substrates. One organism may metabolize a particular compound to produce a product that the other organism can use as a substrate for further metabolic reactions.
5. Anaerobic Environments: Syntrophic interactions are commonly found in anaerobic environments where oxygen is limited or absent. Anaerobic syntrophs play a crucial role in various biogeochemical cycles, such as carbon and sulfur cycling in sediments and the decomposition of organic matter in anaerobic ecosystems.
6. End Product Formation: The ultimate goal of syntrophic interactions is the formation of end products that are beneficial to both organisms. These end products may include various compounds such as methane, hydrogen, acetate, or other metabolites depending on the specific metabolic pathway.
7. Metabolic Cooperation: Syntrophic microorganisms cooperate metabolically to overcome thermodynamic barriers associated with certain reactions. This cooperation allows them to carry out processes that would be energetically unfavorable for a single organism.
8. Energy Conservation: In syntrophism, the efficient conservation of energy is crucial. By working together, microorganisms can extract energy from substrates that would otherwise be wasted or unavailable for individual organisms.
9. Ubiquitous in Nature: Syntrophic interactions are prevalent in natural ecosystems, including environments like anaerobic digesters, wetlands, and the intestinal tracts of animals, where they contribute to the breakdown of complex organic matter.
10. Biotechnological Applications: Understanding syntrophic interactions has practical applications in biotechnology and wastewater treatment processes, where syntrophic consortia of microorganisms are used to efficiently degrade organic compounds.

Examples of Syntrophism

a. Methanogenic Ecosystem in a Sludge Digester:

In a methanogenic ecosystem within a sludge digester, syntrophic interactions play a crucial role in methane production, a valuable biogas. Here’s how it works:

1. Methanogenic Bacteria (Methanobacter): Methanogenic bacteria are responsible for producing methane (CH4). However, they cannot directly utilize complex organic compounds like carbohydrates.
2. Anaerobic Fermentative Bacteria: These fermentative bacteria are capable of breaking down complex organic compounds such as carbohydrates, producing carbon dioxide (CO2) and hydrogen gas (H2) as metabolic byproducts.
3. Syntrophic Interaction: The fermentative bacteria generate CO2 and H2 during the breakdown of carbohydrates. These compounds are then utilized by methanogenic bacteria (Methanobacter) in a syntrophic relationship. Methanobacter uses CO2 and H2 to produce methane (CH4) through methanogenesis.

Function: In this syntrophic relationship, fermentative bacteria break down complex organic compounds, releasing CO2 and H2, which serve as substrates for methanogenic bacteria. Methanogenic bacteria, in turn, convert these substrates into valuable methane, which is the end product of the metabolic process.

b. Lactobacillus arobinosus and Enterococcus faecalis:

In this example, Lactobacillus arobinosus and Enterococcus faecalis exhibit a synergistic syntrophic relationship, where both organisms rely on each other for specific nutrients in a minimal media environment.

Components:

• Lactobacillus arobinosus: This organism produces folic acid.
• Enterococcus faecalis: This organism produces phenylalanine.

Syntrophic Interaction:

1. Folic Acid Production by Lactobacillus arobinosus: Lactobacillus arobinosus produces folic acid as a metabolic byproduct.
2. Phenylalanine Requirement of Enterococcus faecalis: Enterococcus faecalis requires phenylalanine for its growth and metabolic processes.

Function: In this syntrophic relationship, Enterococcus faecalis depends on Lactobacillus arobinosus for the production of folic acid, while Lactobacillus arobinosus relies on Enterococcus faecalis for phenylalanine. Therefore, their coexistence is essential for each organism’s growth and metabolic functions in a minimal media environment.

In both examples, syntrophism highlights the functional interdependence of microorganisms to efficiently utilize substrates, perform metabolic reactions, and produce valuable end products. These interactions are vital in various ecological and biotechnological contexts, contributing to the cycling of nutrients and the generation of useful compounds.

B. Negative interaction

1. Predation

• Predation, a fundamental biological interaction, is characterized by one organism, referred to as the predator, attacking or engulfing another organism, termed the prey. This interaction typically culminates in the death of the prey. Therefore, it plays a pivotal role in shaping the dynamics of ecological communities.
• In the vast tapestry of nature, the size of the prey in relation to the predator can vary. Contrary to common perception, prey is not always smaller than the predator. In some instances, prey organisms can be larger, yet they still fall victim to the predatory tactics of their adversaries. Besides size, various other factors, such as speed, camouflage, and defensive mechanisms, play a role in the outcome of these interactions.
• Predator-prey interactions are generally of short duration. However, the implications of these brief encounters reverberate through the ecosystem. For instance, they influence population dynamics, species distribution, and even evolutionary trajectories. The balance between predators and their prey is crucial for the stability of ecosystems. An overabundance of predators can lead to a drastic reduction in prey populations, while a scarcity of predators can result in overpopulation of certain prey species, which may then overexploit their food resources.
• Then, it’s essential to note the technical vocabulary used in understanding predation. Terms such as “camouflage,” which refers to an organism’s ability to blend into its surroundings, and “defensive mechanisms,” which are strategies or adaptations used by prey to deter or escape from predators, are crucial in grasping the intricacies of these interactions.
• Furthermore, the function of predation in maintaining ecological balance cannot be overstated. It acts as a natural control mechanism, ensuring that no single species dominates an ecosystem to the detriment of others. Therefore, predation, though often viewed in a negative light due to the death of the prey, is a vital component of nature’s intricate web.
• In conclusion, predation is a complex and multifaceted interaction that has profound implications for the functioning and stability of ecosystems. Through a detailed and sequential exploration of its components, one can appreciate its significance in the broader context of biology and ecology.

Characteristics of Predation

1. Predator-Prey Relationship: At its core, predation involves two main participants: the predator, which captures and consumes, and the prey, which is consumed.
2. Result in Death: Unlike other interactions like parasitism, predation typically results in the immediate death of the prey.
3. Size Dynamics: Contrary to popular belief, predators are not always larger than their prey. For instance, a small spider might prey on insects larger than itself.
4. Short Duration: Predator-prey interactions are generally brief, especially when compared to other relationships like parasitism, where the interaction can last for extended periods.
5. Influence on Population Dynamics: Predation can regulate prey populations. An increase in predator numbers can lead to a decrease in prey populations and vice versa.
6. Evolutionary Implications: Predation drives evolutionary changes in both predators and prey. Prey may develop defensive mechanisms like camouflage or toxins, while predators might evolve better hunting strategies or tools.
7. Functional Response: Predators might change their rate of consumption of prey depending on prey density. For instance, as prey becomes more abundant, a predator might consume more of them, but this rate might plateau at very high prey densities.
8. Diverse Strategies: Predators employ a variety of hunting strategies. Some are ambush predators, lying in wait for their prey, while others are active hunters.
9. Trophic Levels: Predation establishes trophic levels in ecosystems. Predators occupy higher trophic levels, while their prey occupies lower levels.
10. Ecological Balance: Predation plays a crucial role in maintaining ecological balance. It ensures that no single species dominates an ecosystem unchecked.
11. Co-evolution: The continuous “arms race” between predators and prey leads to co-evolution, where changes in one species drive evolutionary changes in the other.
12. Specialized Adaptations: Both predators and prey may have specialized physiological, behavioral, or morphological adaptations. Predators might have sharp claws, keen senses, or stealthy behaviors, while prey might have swift movements, protective shells, or mimicry abilities.

Examples of Predation

1. Protozoan-Bacteria Interaction in Soil: Protozoans, single-celled eukaryotic organisms, actively engage in predation by feeding on bacteria present in the soil. This interaction serves a crucial function in the soil ecosystem. By consuming bacteria, protozoans help regulate bacterial populations, ensuring that they remain at optimum levels. Therefore, this predation not only provides sustenance to the protozoans but also contributes to the stability and health of the soil microbial community.
2. Predatory Bacteria: Besides protozoans, certain bacteria themselves act as predators, targeting other bacterial species. Examples of such predatory bacteria include Bdellovibrio, Vampirococcus, and Daptobacter. These bacteria have evolved specialized mechanisms to attack and consume a wide range of bacterial populations. For instance, Bdellovibrio invades the periplasmic space of its prey and feeds on its contents, leading to the eventual death of the prey bacterium. The function of these predatory bacteria is paramount. They play a role in controlling bacterial populations, thereby maintaining a balance in microbial communities. Furthermore, their predatory nature can be harnessed for potential applications, such as biocontrol agents against pathogenic bacteria.

2. Parasitism

Parasitism stands as one of the primary symbiotic relationships observed in nature. It is defined by a unique set of characteristics that distinguish it from other forms of biological interactions.

1. Nature of the Relationship: Parasitism is a relationship where one organism, the parasite, benefits at the expense of another, the host. In this association, the parasite derives its nutrition and other essential resources from the host, which is invariably harmed in the process. Therefore, while the parasite thrives and reproduces, the host may suffer various degrees of damage, ranging from minor inconveniences to severe health implications or even death.
2. Duration of Interaction: A defining feature of the host-parasite relationship is the duration of their interaction. Unlike predation, where the interaction is typically brief, parasitism is characterized by a relatively long period of contact. This contact can be either physical, where the parasite attaches itself to the host, or metabolic, where the parasite taps into the host’s biological processes for sustenance.
3. Location of the Parasite: Based on their location relative to the host, parasites can be categorized into two main types:
   • Ectoparasites: These are parasites that live on the external surface of the host. Examples include lice, ticks, and fleas. They often have specialized structures to attach to the host and feed on its blood or other external resources.
   • Endoparasites: These parasites reside inside the host’s body. They can be found in various organs, tissues, or even within cells. Examples of endoparasites include tapeworms, which live in the intestines, and Plasmodium species, which invade red blood cells and cause malaria.

Characteristics of Parasitism

1. One-sided Benefit: In parasitism, the parasite benefits by deriving nutrition or other advantages from the host. The host, on the other hand, is harmed in the process.
2. Long-term Interaction: Unlike predation, which is typically a short-term interaction, parasitism involves a prolonged association between the parasite and its host. This extended relationship can last from days to the entire lifespan of the host or parasite.
3. Physical or Metabolic Contact: The relationship between the host and the parasite is characterized by either physical contact, where the parasite attaches to or resides in the host, or metabolic contact, where the parasite derives nutrients or other benefits from the host’s biological processes.
4. Variability in Location:
   • Ectoparasites: These parasites live on the external surface of the host. They might attach to the skin, fur, or feathers and often feed on the host’s blood or other external secretions.
   • Endoparasites: These reside within the host’s body, occupying spaces in organs, tissues, or even cells.
5. Specificity: Many parasites exhibit host specificity, meaning they are adapted to infect a particular species or group of species. This specificity can be due to evolutionary adaptations that allow the parasite to exploit the biology of its preferred host.
6. Reproductive Strategy: Parasites often have complex life cycles and reproductive strategies to ensure their survival and propagation. Some might require multiple hosts at different stages of their life cycle.
7. Adaptations: Parasites possess a range of adaptations to facilitate their parasitic lifestyle. These can include specialized structures for attachment, mechanisms to evade the host’s immune response, or enzymes to digest host tissues.
8. Impact on Host: The presence of a parasite can lead to a range of effects on the host, from mild discomfort to severe disease or even death. The severity of these effects can depend on factors like the number of parasites, the host’s health, and the presence of other stressors.
9. Co-evolution: The continuous interaction between parasites and their hosts often leads to co-evolution. As hosts develop defenses against parasitic invasion, parasites evolve countermeasures to bypass these defenses, leading to an ongoing evolutionary “arms race.”

Examples of Parasitism

1. Viruses: Viruses represent a unique category of parasitic entities. They are obligate intracellular parasites, meaning they cannot reproduce or carry out most of their life processes outside a host cell. Their parasitic nature is underscored by their reliance on the host’s cellular machinery for replication. Furthermore, viruses exhibit a high degree of host specificity. For instance, bacteriophages are viruses that specifically target bacteria. There are also viruses that parasitize fungi, algae, and protozoa. The function of these viruses is to hijack the host’s cellular processes to produce more viral particles. Therefore, while the virus benefits by producing progeny, the host cell is often damaged or destroyed in the process.
2. Bdellovibrio: Bdellovibrio is a fascinating example of bacterial parasitism. Unlike many parasites that target multicellular organisms, Bdellovibrio preys on other bacteria, specifically gram-negative bacteria. Functioning as an ectoparasite, Bdellovibrio attaches itself to the outer surface of its bacterial prey. Once attached, it penetrates the prey’s outer membrane and resides in the periplasmic space. Here, it feeds on the host’s resources, eventually leading to the host’s death. The presence of Bdellovibrio in microbial communities serves as a natural control mechanism, ensuring that no single bacterial species dominates the environment unchecked.

3. Ammensalism (antagonism)

Ammensalism, also known as antagonism, is a specific type of interaction observed among microbial populations. This relationship is characterized by the production of inhibitory substances by one population that adversely affects another, while the producing population remains largely unaffected. By delving into the details of this interaction, one can better appreciate its role and implications in microbial ecosystems.

1. Nature of the Relationship: Ammensalism or antagonism is defined by an interaction where one microbial population produces substances that are inhibitory to another microbial population. This relationship stands out as it results in a negative impact on one population without necessarily benefiting the other.
2. Outcome of the Interaction: The relationship is inherently negative for the population that is inhibited. The microbial population producing the inhibitory substances remains unaffected by its own secretions. In some cases, this population may even gain a competitive advantage by suppressing potential competitors, thereby enhancing its chances of survival in the habitat.
3. Chemical Inhibition – Antibiosis: The specific phenomenon where one microbial population produces chemical substances that inhibit another is termed antibiosis. This is a form of chemical warfare in the microbial world, where organisms produce compounds, often secondary metabolites, to suppress the growth or activity of potential competitors. Examples of such compounds include antibiotics produced by certain bacteria and fungi.
4. Implications in Microbial Communities: The presence of antagonistic interactions in microbial communities can shape the composition and dynamics of these communities. By suppressing certain populations, the producing organisms can alter the balance of species, potentially leading to changes in nutrient cycling, community resilience, and overall ecosystem function.

Characteristics of Ammensalism (antagonism)

• Unidirectional Harm: In ammensalism, one organism negatively affects another without receiving any benefit or harm in return. This unidirectional impact distinguishes ammensalism from other interactions like parasitism, where one organism benefits at the expense of another.
• Chemical Inhibition: Often, the inhibitory effect in ammensalism arises from the production of specific chemical substances by one organism. This phenomenon, termed antibiosis, results in the suppression or inhibition of the growth or activity of another organism.
• No Benefit to the Producer: The organism producing the inhibitory substance does not necessarily derive a direct benefit from the harm it causes to another organism. However, in some contexts, especially in microbial communities, this action might indirectly provide a competitive advantage by reducing competition for resources.
• Variability in Impact: The degree of harm caused by the inhibitory substance can vary. Some organisms might be severely affected, while others might exhibit only mild sensitivity or even resistance.
• Environmental Influence: The effectiveness of the inhibitory substance can be influenced by environmental factors such as pH, temperature, and nutrient availability. These factors can modulate the potency or availability of the inhibitory compounds.
• Specificity: While some inhibitory substances might have a broad spectrum of activity, affecting multiple species, others might be highly specific, targeting only a particular species or group of organisms.
• Temporal Dynamics: The production of inhibitory substances might not be constant. It can be influenced by the life stage of the organism, environmental cues, or the presence of specific competitors.
• Evolutionary Implications: Over time, organisms subjected to ammensalistic interactions might evolve resistance or tolerance to the inhibitory substances. This can lead to an evolutionary “arms race” where the producer organism might further refine its inhibitory compounds.

Examples of Ammensalism (antagonism)

1. Lactic Acid in the Vaginal Tract: The vaginal tract is home to a diverse community of microorganisms, with lactic acid bacteria being one of the predominant members. These bacteria produce lactic acid as a metabolic byproduct. This acidification of the environment serves a crucial function. The lowered pH created by the lactic acid is inhibitory to many pathogenic organisms, such as the fungus Candida albicans. Therefore, while the lactic acid bacteria do not directly benefit from inhibiting Candida albicans, their metabolic activity creates an environment that suppresses potential pathogens, contributing to the overall health of the vaginal ecosystem.
2. Fatty Acids from Skin Flora: The skin, our body’s largest organ, is colonized by a myriad of microorganisms, collectively referred to as the skin flora. Some members of this microbial community produce fatty acids as metabolic byproducts. These fatty acids have an inhibitory function. They create an environment on the skin’s surface that is hostile to many pathogenic bacteria. Thus, the skin flora, through the production of fatty acids, plays a protective role, preventing potential pathogenic invasions.
3. Thiobacillus thiooxidans and Sulfuric Acid Production: Thiobacillus thiooxidans is a bacterium known for its unique metabolic capability to oxidize sulfur. This oxidation process results in the production of sulfuric acid. The function of this acid production goes beyond mere metabolism. The sulfuric acid significantly lowers the pH of the surrounding culture media. This acidic environment inhibits the growth of most other bacteria that might compete with Thiobacillus thiooxidans for resources. While the primary aim of Thiobacillus thiooxidans is not to inhibit other bacteria, its metabolic activity creates conditions that give it a competitive edge in its habitat.

4. Competition

Competition, a fundamental biological interaction, is prevalent among microbial populations. This interaction is characterized by the simultaneous demand of two or more microbial populations for the same limited resources, leading to potential adverse effects on their growth and survival. By delving into the specifics of this interaction, one can gain insights into the dynamics of microbial communities.

1. Nature of the Relationship: Competition is defined by a negative relationship between microbial populations. In this interaction, both populations are adversely affected in terms of their growth and survival. This mutual detriment sets competition apart from other interactions like parasitism or mutualism.
2. Resource Limitation: The crux of competition lies in the shared utilization of limited resources. When two microbial populations vie for the same resources, whether it’s space, nutrients, or other essential factors, they inevitably impact each other’s ability to thrive. As a result, these competing populations often achieve a lower maximum density or growth rate than they would in the absence of competition.
3. Types of Resources: Microbial populations can compete for a variety of growth-limiting resources. These can include essential elements like carbon, nitrogen, and phosphorus, or more specific requirements like vitamins and growth factors. The availability of these resources can significantly influence the outcome of the competitive interaction.
4. Ecological Implications: Competition has profound implications for the structure and function of microbial communities. It prevents two populations from occupying the exact same ecological niche. Over time, one population might outcompete the other, leading to its dominance in that particular niche. This dynamic ensures diversity within microbial ecosystems, as no single population can monopolize all available resources. Therefore, while competition might seem detrimental on an individual level, it plays a pivotal role in maintaining the balance and diversity of microbial communities.

Characteristics of competition

1. Mutual Detriment: One of the defining features of competition is that it results in negative effects for all parties involved. Each competitor experiences reduced access to resources, leading to potential decreases in growth, reproduction, or survival.
2. Shared Resources: Competition arises due to the simultaneous demand for the same limited resources. These resources can be tangible, like food, water, or space, or they can be intangible, like sunlight in plant communities.
3. Intensity Variability: The intensity of competition can vary based on resource availability. When resources are abundant, competition may be minimal, but as resources become scarcer, competition intensifies.
4. Direct and Indirect Competition:
   • Direct (Interference) Competition: Organisms directly hinder the access of others to resources, often through aggressive behaviors or physical obstruction.
   • Indirect (Exploitative) Competition: Organisms consume available resources, making them unavailable to others without direct interaction.
5. Intraspecific and Interspecific Competition:
   • Intraspecific Competition: Occurs between members of the same species.
   • Interspecific Competition: Occurs between members of different species.
6. Competitive Exclusion Principle: This principle posits that two species competing for the same limiting resource cannot coexist indefinitely. Over time, one species will outcompete the other, leading to the latter’s local extinction or a shift in its ecological niche.
7. Resource Partitioning: To reduce competition, species often evolve to utilize different parts of a resource or use resources at different times. This division allows multiple species to coexist without directly competing for the exact same resources.
8. Evolutionary Implications: Competition can drive evolutionary changes, leading to adaptations that enhance an organism’s competitive ability. Over time, this can result in niche differentiation, where species evolve to occupy different ecological niches, reducing direct competition.
9. Temporal and Spatial Dynamics: Competition can vary in intensity and outcome based on temporal (time) and spatial (location) factors. For instance, two plants might compete more intensely during a dry season compared to a wet season.
10. Density-Dependence: The effects of competition are often density-dependent. As the population density of a competing species increases, the effects of competition can become more pronounced.

Examples of competition

1. Competition Among Protozoa: A classic example of competition in the microscopic world involves two species of protozoa: Paramecium caudatum and Paramecium aurelia.
   • Shared Resource: Both these species of Paramecium feed on the same bacterial population. When they coexist in the same environment, they vie for this common food source, leading to a competitive interaction.
   • Outcome of the Interaction: Observations have shown that when both species are placed together, P. aurelia tends to grow at a superior rate compared to P. caudatum. This indicates that P. aurelia has a competitive advantage in this scenario, potentially due to more efficient feeding mechanisms, faster reproduction rates, or other advantageous traits.
   • Implications: The competitive advantage of P. aurelia over P. caudatum in shared environments can lead to reduced populations of P. caudatum. Over time, if resources remain limited, P. caudatum might face significant population declines or even local extinction due to this competitive pressure.

Types of Microbial Interaction	Description	Examples
Positive Interaction
Mutualism	A relationship where each organism benefits. It’s obligatory, and both organisms are metabolically dependent on each other.	Lichens, Protozoan-termite, Paramecium-Chlorella
Syntrophism	An association where the growth of one organism depends on or is improved by substrates provided by another organism.	Methanogenic ecosystem in sludge digester, Lactobacillus arobinosus and Enterococcus faecalis
Protocooperation	A mutually beneficial relationship, but not obligatory.	Association of Desulfovibrio and Chromatium, Interaction between N2-fixing bacteria and cellulolytic bacteria
Commensalism	One organism benefits, while the other is neither harmed nor benefited.	Non-pathogenic E. coli in the human intestinal tract, Flavobacterium and Legionella pneumophila
Negative Interaction
Ammensalism (Antagonism)	One organism produces substances inhibitory to another.	Lactic acid in vaginal tract, Fatty acid on skin, Thiobacillus thiooxidant
Competition	Both organisms are adversely affected due to vying for the same resources.	Competition between Paramecium caudatum and Paramecium aurelia
Parasitism	One organism benefits at the expense of another.	Viruses, Bdellovibrio
Predation	One organism engulfs or attacks another, leading to the prey’s death.	Protozoan-bacteria in soil, Bdellovibrio, Vamparococcus, Daptobacter

Mutualism vs commensalism vs parasitism

Criteria	Mutualism	Commensalism	Parasitism
Definition	A type of interaction where both organisms benefit.	An interaction where one organism benefits, and the other is neither harmed nor benefited.	An interaction where one organism (the parasite) benefits at the expense of the other (the host).
Benefit/Harm	Both organisms benefit.	One organism benefits; the other is neutral (neither harmed nor benefited).	The parasite benefits, while the host is harmed.
Dependency	Often obligatory; both organisms rely on each other for survival or growth.	Not obligatory; the benefiting organism can typically survive without the host.	Often obligatory for the parasite; the host is typically harmed but not always killed.
Examples	Lichens (fungi and algae), nitrogen-fixing bacteria and leguminous plants.	Barnacles on whales, epiphytic plants on trees.	Tapeworms in mammals, fleas on dogs, mistletoe on trees.
Function	Both organisms provide some service or resource to the other, such as nutrients, protection, or transportation.	The benefiting organism might use the host for habitat, transportation, or as a source of food without affecting the host.	The parasite derives nutrients or other benefits from the host, often leading to disease or harm to the host.
Duration	Typically long-term and persistent.	Can be temporary or long-term.	Can be temporary (e.g., mosquito feeding on blood) or long-term (e.g., tapeworm in intestines).

Mutualism vs Symbiosis

Criteria	Mutualism	Symbiosis
Definition	A type of interaction where both organisms benefit from the relationship.	A close and long-term biological interaction between two different biological organisms.
Benefit/Harm	Both organisms benefit.	Can be mutualistic (both benefit), commensalistic (one benefits, the other is unaffected), or parasitic (one benefits, the other is harmed).
Dependency	Often obligatory; both organisms rely on each other for survival or growth.	Can be obligatory (one or both organisms depend on the relationship) or facultative (the relationship is beneficial but not essential).
Examples	Lichens (fungi and algae), nitrogen-fixing bacteria and leguminous plants.	Lichens, human gut flora, clownfish and sea anemones, tapeworms in mammals.
Function	Both organisms provide some service or resource to the other, such as nutrients, protection, or transportation.	The nature of the interaction varies. It can be beneficial, neutral, or harmful depending on the specific type of symbiosis.
Duration	Typically long-term and persistent.	Typically long-term, but the duration can vary based on the specific type of symbiotic relationship.

Mutualism vs Cooperation

Criteria	Mutualism	Cooperation
Definition	A type of interaction, typically between different species, where both organisms benefit from the relationship.	A behavior where individuals, either of the same species or different species, work together in a way that benefits them all.
Context	Often used in the context of ecological interactions between different species.	Often used in the context of behavioral interactions, especially within the same species, but can also apply to interspecies interactions.
Benefit/Harm	Both organisms benefit.	All individuals involved benefit, but the degree of benefit might vary.
Dependency	Can be obligatory (both organisms rely on each other for survival) or facultative (beneficial but not essential for survival).	Not necessarily obligatory; individuals can often survive without cooperation, but cooperating might enhance survival or reproductive success.
Examples	Lichens (fungi and algae), nitrogen-fixing bacteria and leguminous plants.	Pack hunting in wolves, collaborative nesting in birds, or humans working together in teams.
Scale	Typically describes interactions at the species level.	Can describe interactions at the individual level or group level.
Duration	Can be short-term or long-term, but often persistent.	Can be temporary (e.g., during a specific task) or long-term (e.g., lifelong partnerships).

Commensalism vs amensalism

Criteria	Commensalism	Amensalism
Definition	An interaction between two species where one species benefits and the other is neither harmed nor benefited.	An interaction where one species is harmed, and the other is unaffected.
Benefit/Harm	One organism benefits; the other is neutral (neither harmed nor benefited).	One organism is harmed; the other is unaffected.
Examples	Barnacles growing on the shell of a turtle (the barnacles benefit by being carried to different feeding areas, while the turtle is unaffected).	A large tree providing shade, which inhibits the growth of plants beneath it (the plants are harmed by the lack of sunlight, while the tree is unaffected).
Mechanism	The benefiting organism might use the other for habitat, transportation, or as a source of food without affecting the other organism.	Often involves the production of chemical substances by one organism that inhibit the growth or reproduction of another organism. This chemical inhibition is sometimes referred to as “antibiosis.”
Dependency	Not obligatory; the benefiting organism can typically survive without the other.	Not obligatory; the unaffected organism doesn’t rely on the harmed organism for survival.

FAQ

References

• Braga RM, Dourado MN, Araújo WL. Microbial interactions: ecology in a molecular perspective. Braz J Microbiol. 2016 Dec;47 Suppl 1(Suppl 1):86-98. doi: 10.1016/j.bjm.2016.10.005. Epub 2016 Oct 26. PMID: 27825606; PMCID: PMC5156507.
• https://biologyreader.com/microbial-interaction.html
• https://microbialinteractions.expertconferences.org/
• https://microbialinteraction.conferenceseries.com/2017/call-for-abstracts.php
• https://www.onlinebiologynotes.com/microbial-interaction-and-types-mutualism-syntropism-proto-cooperation-commensalism-antagonism-parasitism-predation-competition/
• https://www.omicsonline.org/conferences-list/microbial-association-microbial-interactions
• https://faculty.ksu.edu.sa/sites/default/files/lecture_2microbial_interactionsppt.pdf
• https://www.jsps.go.jp/file/storage/general/english/e-plaza/e-sdialogue/data/Slide_C.pdf