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Classification of Bacteria

Table of Contents

Bacteria Definition

Bacteria are prokaryotic, unicellular organisms without a real nucleus and a few organelles.

  • Bacteria; singular bacterium, common noun bacteria) are pervasive, largely free-living creatures typically composed of a single cell.
  • Bacteria inhabit every potential habitat on the globe, including soil, the ocean, the Earth’s crust, and severe situations such as acidic hot springs and radioactive waste.
  • They make up the majority of prokaryotic microbes. Bacteria, which are typically a few micrometres long, were among the first life forms to appear on Earth and are prevalent in the majority of its habitats.
  • Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the Earth’s crust’s deep biosphere.
  • Bacteria play a crucial role in numerous stages of the nutrient cycle by recycling nutrients, such as the fixation of atmospheric nitrogen.
  • The decomposition of dead corpses is part of the nutrient cycle; microorganisms are responsible for the putrefaction stage of this process.
  • Extremophile bacteria convert dissolved chemicals, such as hydrogen sulphide and methane, into energy in the biological communities surrounding hydrothermal vents and cold seeps.
  • Bacteria also coexist with plants and animals in symbiotic and parasitic interactions. The majority of bacteria have not been characterised, and numerous species cannot be cultured in a laboratory.
  • The study of bacteria is referred to as bacteriology, which is a subfield of microbiology.
  • The majority of animals, including humans, carry millions of microorganisms. There are numerous on the skin and in the intestines.
  • The majority of bacteria within and on the body are harmless or rendered harmless by the immune system, and many are useful, especially those in the gut.
  • Several types of bacteria, including cholera, syphilis, anthrax, leprosy, TB, tetanus, and bubonic plague, are harmful and cause infectious diseases. Respiratory infections are the most common causes of bacterial death.
  • Antibiotics are used to treat bacterial illnesses and in farming, making antibiotic resistance an increasing concern.
  • Bacteria are essential in sewage treatment and the breakdown of oil spills, the fermentation-based production of cheese and yoghurt, the mining industry’s recovery of gold, palladium, copper, and other metals, biotechnology, and the synthesis of antibiotics and other chemicals.
  • Bacteria were once considered plants of the class Schizomycetes (“fission fungi”), but are now classed as prokaryotes.
  • Unlike animal and other eukaryotic cells, bacterial cells lack a nucleus and have membrane-bound organelles seldom.
  • Due to the finding in the 1990s that prokaryotes are comprised of two distinct groups of organisms that originated from a common ancestor, the scientific classification of bacteria has changed. The names of these evolutionary realms are Bacteria and Archaea.

Phenotypic Characteristics of Bacteria

Morphologic Characteristics

  • Bacterial cell suspensions that are both wet-mounted and correctly stained can give a considerable deal of information.
  • These straightforward tests can reveal the organism’s Gram response, acid-fastness, motility, flagellar organisation, presence of spores, capsules, and inclusion bodies, and, of course, its form.
  • This information frequently permits identification of an organism to the genus level or reduces the likelihood that it belongs to a particular group.
  • Colony features and pigmentation are also incredibly beneficial. Several Porphyromonas species colonies autofluoresce under long-wavelength UV light, and Proteus species colonies swarm on suitable substrates.

Growth Characteristics

  • Whether an organism grows aerobically, anaerobically, facultatively (i.e., either in the presence or absence of oxygen), or microaerobically is a defining trait (i.e., in the presence of a less than atmospheric partial pressure of oxygen).
  • Essential for isolating and identifying microorganisms are optimal meteorological conditions. Other essential growth measures include the incubation temperature, pH, needed nutrients, and antibiotic resistance.
  • Campylobacter jejuni, for instance, grows well at 42° C in the presence of multiple antibiotics, although Y. enterocolitica grows better than most other bacteria at 4° C.
  • Some infections, like Legionella, Haemophilus, and others, require particular growth factors, while E. coli and the majority of other Enterobacteriaceae can grow on minimal media.

Antigens and Phage Susceptibility

  • Cell wall (O), flagellar (H), and capsular (K) antigens are utilised to aid in the classification of some organisms at the species level, to serotype strains of medically significant species for epidemiological purposes, and to identify serotypes of public health significance.
  • Serotyping is also occasionally employed to differentiate strains of extreme virulence or public health significance, such as with V. cholerae (O1 is the pandemic strain) and E. coli (enterotoxigenic, enteroinvasive, enterohemorrhagic, and enteropathogenic serotypes).
  • Phage typing (determining the sensitivity pattern of an isolate to a collection of distinct bacteriophages) has mostly been utilised as an assistance in the epidemiologic surveillance of infections caused by Staphylococcus aureus, mycobacteria, P, aeruginosa, V. cholerae, and S. typhi.
  • Additionally, susceptibility to bacteriocins has been utilised as an epidemiologic marker of strain. Molecular approaches have replaced phage and bacteriocin typing in the majority of instances in recent years.

Biochemical Characteristics

  • The majority of bacteria are recognised and categorised based on their responses to a set of biochemical tests.
  • Others are confined to a specific family, genus, or species (oxidase, nitrate reduction, amino acid degrading enzymes, fermentation, or carbohydrate utilisation) (coagulase test for staphylococci, pyrrolidonyl arylamidase test for Gram-positive cocci).
  • Depending on the group of organisms being identified, the required number of tests and the tests themselves will vary. Consequently, each laboratory must determine the extent to which it should go in detecting and identifying organisms based on its function, the sort of population it serves, and its resources.
  • Today, clinical laboratories determine the extent of their work based on the clinical significance of an isolate to a specific patient, the public health significance of thorough identification, and the overall cost-benefit analysis of their processes.
  • For instance, the reference laboratory at the Centers for Disease Control and Prevention (CDC) uses at least 46 tests to identify members of the Enterobacteriaceae family, whereas the vast majority of clinical laboratories use commercial identification kits or simple rapid tests to identify isolates based on significantly fewer criteria.

Reproduction in Bacteria

  • Bacteria reproduce asexually through a process known as binary fission. The division of a single bacterium into two daughter cells. These are identical to the mother cell and to one another.
  • Beginning with the replication of DNA within the parent bacteria, fission occurs. Eventually, the cell divides into two daughter cells.
  • The rate and timing of reproduction rely on environmental factors such as temperature and nutrient availability. When conditions are favourable, E.coli or Escherichia coli produces approximately 2 million bacteria every seven hours.
  • Bacterial reproduction is strictly asexual, however in extremely rare instances it can be sexual.
  • In bacteria, genetic recombination may occur by conjugation, transformation, or transduction. As there is variety in the genetic material, antibiotic-resistant bacteria may develop in such instances (as opposed to asexual reproduction where the same genetic material is present in generations)

Useful Bacteria

Not all germs pose a threat to human health. There are bacteria that are advantageous in many ways. Below are a few advantages of bacteria:

  • Change milk to curd — Lactobacillus or lactic acid bacteria
  • Streptococcus and Bacillus are used to ferment foods.
  • Beneficial to digestion and the immune system – Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria
  • Antibiotics are produced for the treatment and prevention of bacterial illnesses. – Soil microbes

Harmful Bacteria

  • There are bacteria that are capable of causing a variety of ailments. They cause numerous infectious diseases, including pneumonia, tuberculosis, diphtheria, syphilis, and tooth rot. By taking antibiotics and recommended medication, their effects can be reversed.
  • However, vigilance is far more efficient. The majority of these pathogens can be eradicated by sterilising or cleaning exposed surfaces, equipment, and other implements. These techniques include the use of heat, disinfectants, ultraviolet radiation, pasteurisation, and boiling, among others.

Classification Of Bacteria On The Basis Of The Cell Wall And Staining Reaction

On the basis of the cell wall and staining reaction, bacteria are categorised into three groups;

Cell Wall Structure
Cell Wall Structure

1. Gram-Positive Bacteria

  • Gram-positive bacteria, classified under the phylum Firmicutes, are a significant group within the bacterial domain. These microorganisms are distinctly recognized by their ability to retain the crystal violet stain during the Gram staining procedure, resulting in a characteristic purple hue when observed microscopically. This staining property is attributed to their unique cellular architecture.
  • The cell wall of gram-positive bacteria is characterized by a thick layer of peptidoglycan, which provides structural integrity. Contrary to some bacterial types, gram-positive bacteria do not possess an outer cell envelope. Their cell wall composition is notably different, with a reduced lipid content and a higher presence of teichoic acids. Notably, species such as Staphylococcus aureus and Streptococcus faecalis have cell walls enriched with teichoic acid.
  • Locomotion in these bacteria is facilitated by specialized organelles, including cilia and flagella. This mobility aids in their survival and adaptability in various environments.
  • Several gram-positive bacteria play crucial roles in industrial and biological processes. For instance, Corynebacterium species are employed in the synthesis of enzymes, amino acids, and nucleotides. Similarly, various Bacillus species are harnessed for large-scale enzyme production. In the realm of food production, certain gram-positive bacteria are indispensable. They contribute to the characteristic holes in Emmentaler or Swiss cheese and are involved in the maturation of various cheese types. Additionally, the bacterium Bacillus amyloliquefaciens is a prominent source of the natural antibiotic protein, Barnase.
  • Furthermore, gram-positive bacteria can be categorized based on their morphology and characteristics:
    • Cocci: Examples include Streptococcus and Staphylococcus.
    • Spore-forming rods: These can be aerobic, such as Bacillus, or anaerobic like Clostridium.
    • Non-spore forming rods: These can be non-filamentous, like Corynebacterium and Listeria, or filamentous, represented by Actinomyces and Nocardia.
  • In the human body, many of these bacteria are commensals, residing harmlessly in regions like the mouth, skin, intestines, and upper respiratory tract. However, it’s essential to note that while many are non-pathogenic, some can cause diseases under specific conditions.
  • In conclusion, gram-positive bacteria, with their diverse roles and characteristics, are pivotal both in nature and various industries. Their study and understanding are crucial for advancements in biotechnology, medicine, and food production.

Gram-Positive Bacteria Examples

  • Gram-positive cocci are Staphylococcus and Streptococcus.
  • Gram-positive rods that produce spores.
    • Aerobic: Bacillus.
    • Anaerobic: Clostridium.
  • Non-spore forming rods
    • Non-filamentous bacteria include Corynebacterium and Listeria.
    • Filamentous: Actinomyces, Nocardia.

2. Gram-negative Bacteria

  • Gram-negative bacteria represent a distinct class within the bacterial domain, specifically under the phylum Firmicutes. These microorganisms are characterized by their inability to retain the crystal violet stain during the Gram staining procedure. As a result, they exhibit a pink hue when visualized under a microscope.
  • The cellular architecture of gram-negative bacteria is notably different from their gram-positive counterparts. Their cell wall is relatively thin and primarily composed of peptidoglycan. Surrounding this cell wall is a unique outer membrane, which is absent in gram-positive bacteria. This outer membrane is a bilayer structure, enriched with phospholipids, lipopolysaccharides (LPS), lipoproteins, and various surface proteins. Notably, the presence of LPS gives these bacteria their endotoxic properties, which can inhibit immune responses when released.
  • Embedded within the outer membrane are porin proteins, which play a pivotal role in regulating the transport of molecules into and out of the bacterial cell. The absence of teichoic acid and magnesium ribonucleate, coupled with a higher lipid content and the presence of every type of amino acid, further distinguishes gram-negative bacteria from gram-positive ones.
  • Medically, gram-negative bacteria hold significant importance. Many members of the Enterobacteriaceae family, for instance, are crucial in clinical settings. Genera such as Vibrio, Campylobacter, and Pseudomonas are typically associated with the gastrointestinal tract and can be pathogenic under certain conditions.
  • Classifying gram-negative bacteria based on their morphology yields the following categories:
    • Cocci: An example is Neisseria.
    • Coccobacilli: This includes genera like Haemophilus, Bordetella, Legionella, and others.
    • Rods (Enterics): This group comprises bacteria like Escherichia, Enterobacter, and Salmonella, among others.
    • Curved Rods: This category includes bacteria such as Campylobacter, Helicobacter, and Vibrio.
  • In summary, gram-negative bacteria, with their unique cellular structure and significant medical relevance, are integral to the microbial world. Their study is vital for understanding bacterial pathogenesis, antibiotic resistance, and for the development of therapeutic interventions.

Examples of Gram-Negative Bacteria

  • Gram-negative cocci: Neisseria
  • Gram negative coccobacilli: Haemophilus, Bordetella, Legionella, Brucella, Francisella, Pasteurella, Yersinia
  • Gram-negative rods (Enterics): Escherichia, Enterobacter, Serratia, Klebsiella, Salmonella, Shigella, Proteus
  • Gram-negative curved rods: Campylobacter, Helicobacter, Vibrio

3. Acid Fast Bacteria

  • Acid-fast bacteria, commonly known as acid-fast bacilli or AFB, are a group of bacteria that share the property of acid resistance.
  • Acid fastness is the capacity of a bacteria to resist decolorization by acids during staining techniques.
  • This indicates that after the bacteria has been stained, it cannot be decolored using the acids typically employed in the process.
  • Using very straightforward laboratory techniques, such as microscopy, it is possible to categorise and discover particular bacteria based on this essential and distinctive characteristic.

Examples of Acid Fast Bacteria

Bacteria displaying acid fastness include:

  • Genus Mycobacterium – M. leprae, M. tuberculosis, M. smegmatis, M. Avium complex, M. kansasii.
  • Genus Nocardia – N. brasiliensis, N. cyriacigeorgica, N. farcinica, and N. nova.

Classification Of Bacteria On The Basis Of Oxygen Requirements

Bacteria are divided into three categories based on their oxygen requirements: aerobes, anaerobes, and microaerophiles.

1. Aerobic bacteria (aerobes)

  • Aerobic bacteria are bacteria that develop in the presence of oxygen.
  • With the aid of enzymes, they are capable of oxygen detoxification.
  • The ultimate electron acceptor is oxygen molecules.
  • From the last electron acceptor, water is generated. When in a liquid medium, they are observed on the liquid’s surface.
  • Example: Nocardia, Bacillus.

Types of Aerobic Bacteria

Aerobic Bacteria can be classified as:

  • Obligate Aerobes
  • Facultative Aerobes
  • Microaerophiles
  • Aerotolerant Aerobes

a. Obligate aerobes

  • Organisms that cannot thrive in the absence of oxygen are obligate aerobes. Therefore, the presence of oxygen in their environment is required for the survival and growth of obligatory aerobes.
  • Aerobic bacteria utilise molecular oxygen as a terminal electron acceptor, oxidise sugars and lipids, and produce ATP/energy via glycolysis, the electron transport chain, and the Krebs TCA cycle.
  • In obligate aerobes, the enzymes engaged in the respiratory chain are catalase, peroxidase, and superoxide dismutase.
  • These three enzymes are crucial to the aerobic biology of aerobes, as they counteract the harmful effects of reactive oxygen species produced by the presence of molecular oxygen.
  • Example: The obligatory aerobes Bacillus, Mycobacterium, and Pseudomonas are examples.

b. Facultative aerobes

  • In their environment, facultative aerobic bacteria do not rely solely on the presence of oxygen.
  • Instead, these bacteria create ATP/energy molecules via anaerobic processes. Therefore, they can survive in the lack of oxygen.
  • Example: Enterobacteriaceae is an instance of a facultative aerobe.

c. Microaerophiles

  • As their name implies, microaerophiles require only a small amount of oxygen for energy production.
  • A high concentration of oxygen can be fatal to microaerophiles. Microaerophiles lack an electron transport system and derive their energy from the fermentation reaction.
  • Example: Microaerophiles include Helicobacter and Campylobacter.

d. Aerotolerant aerobes

  • Aerotolerant organisms do not require oxygen for metabolic processes or energy production.
  • Additionally, they are unaffected by the presence of oxygen. Aerotolerant bacteria lack the enzymes essential for aerobic respiration, specifically catalase, peroxidase, and superoxide dismutase.
  • Example: Aerotolerant microorganisms include Lactobacilli and Streptococci.

Aerobic Bacteria Examples

  • Pseudomonas aeruginosa is a Gram-negative rod-shaped bacterium species that can infect plants, animals, and humans (infections in lungs or blood)
  • Nocardia is a genus of Gram-positive rod-shaped bacteria. Nocardiosis is a lung condition caused by inhaling dust particles harbouring infective Nocardia species, which can be caused by certain species.
  • Mycobacterium tuberculosis is the causal agent of tuberculosis, another lung disease.
  • E. Coli Proteus is a saprophyte commonly found in manure soil, animal excrement, and sewage.
  • Salmonella is a genus of rod-shaped, Gram-negative bacteria; specific species of Salmonella are frequently related with food-borne diseases.
  • Achromobacter is a genus of rod-shaped Gram-negative bacteria whose peritrichous flagella are a distinguishing feature. They are purely aerobic and thrive in soils and water.
  • Klebsiella is a genus of rod-shaped Gram-negative bacteria that are common in nature and a natural component of the flora of the human digestive tract.
  • Citrobacter is a genus of coliform Gram-negative bacteria. The species are facultative pathogens. Certain species can cause urinary tract infections, pneumonia, central nervous system infections, and newborn infections in humans.

2. Anaerobic bacteria (anaerobes)

  • The bacteria that grow without oxygen are known as anaerobic bacteria. It is incapable of decontaminating oxygen.
  • Carbon dioxide, sulphur, fumarate, or ferric are the final electron acceptors. These bacteria create compounds resembling acetate, methane, nitrate, and sulphide.
  • When in the liquid medium, they are observed near the bottom.
  • Example: Bacteroids, E.Coli.

Types of Anaerobic bacteria (anaerobes)

According to their tolerance for oxygen, anaerobic bacteria can be categorised into three distinct types:

  1. Facultative anaerobes: In biology, facultative anaerobes are classified as bacteria that can thrive in the presence or absence of oxygen. In the presence of oxygen, they generate most of their energy through aerobic respiration. Without oxygen, they produce energy through fermentation. This category is exemplified by E. coli facultative anaerobes. Salmonella is another example; it is a gram-negative anaerobic rod.
  2. Obligate anaerobes: Bacteria that cannot grow in the presence of oxygen are obligate anaerobes. Oxygen is hazardous to these microorganisms. Therefore, the fermentation process is their primary energy production method. When organisms are exposed to high levels of oxygen, they produce endospores to survive in this harsh environment. Obligate bacteria are present as natural flora in the mucous membranes of the human body. Peptostreptococcus, Clostridium, and actinomyces are examples of anaerobes that are oxygen-dependent.
  3. Aerotolerant anaerobes: Aerotolerant anaerobes are anaerobes that can tolerate oxygen for a limited amount of time, between 8 and 72 hours. However, they cannot grow in the presence of oxygen. As an obligatory type, the only avenue to make energy is fermentation. Clostridium and Streptococcus are the most prominent examples of aerotolerant anaerobic microorganisms.

Examples of Anaerobic bacteria (anaerobes)

  • Bacteroides: Bacteroides are present as typical flora in the human colon and female genital. However, they have the potential to be pathogenic and cause gastrointestinal illnesses, abscesses, aspiration pneumonia, and brain abscesses.
  • Campylobacter: The leading cause of acute gastroenteritis is Campylobacter.
  • Fusobacterium: Not all groupings are anaerobic. Only Fusobacterium necrophorum is anaerobe-producing. It primarily causes tonsillar abscesses, leading to thrombosis of the jugular vein.
  • Prevotella melaninogenica: Prevotella is a non-spore-forming gram-negative coccobacilli bacteria. They are primarily encountered as opportunistic pathogens in the oral cavity.
  • Bilophila wadsworthia: Gram-negative bacteria constitute Bilophila wadsworthia. It causes penicillin-resistant B lactamase enzymes to be released. It can be extracted from gangrenous tissues, pleural fluids, and scrotal abscess pus.
  • Clostridium: Clostridium is an anaerobic rod that produces spores. This group contains three members, including Clostridium difficile, Clostridium perfringens, and Clostridium septicum, which cause gas gangrene and other dangerous diseases.
  • Propionibacterium: Propionibacterium are natural inhabitants of the skin and mucosa. Acne vulgaris is the most prevalent skin disorder, which is typically caused by Propionibacterium acne.
  • Lactobacillus: Lactobacillus inhabit the digestive tract as part of the natural flora. They are present in numerous foods. Therefore, they can be restored by consuming such meals. They are the least dangerous gram-positive organisms. In newborns, however, it may produce an abdominal abscess.
  • Peptostreptococcus anaerobius bacteria and finegoldia: Peptostreptococcus anaerobius and finegoldia are further examples of anaerobic bacteria.
  • Parvimonas micra: There are cocci-shaped Parvimonas micra. It is primarily present on mucous membranes and the skin’s surface.

3. Microaerophilic bacteria

  • They cannot survive atmospheric air and require a low oxygen concentration of 2-10% for growth.
  • Microaerophiles are microorganisms that are not destroyed by oxygen per se, but can survive subatmospheric quantities of oxygen in their environment.
  • This places them between obligate anaerobes, which are really destroyed by oxygen, and obligate aerobes, which require the same overall level of oxygen as you and most other organisms.
  • Oxygen makes up roughly 21 percent of the Earth’s atmosphere; microaerophiles thrive in environments with significantly less oxygen, but they require at least a small amount to survive.
  • Aerotolerant anaerobes include microaerophilic microorganisms. Oxygen is comparable to the mineral iron for microaerophiles. All humans require a little amount of iron to survive, but an excess of iron is toxic.
  • These bacteria provide the enzymes necessary for degrading the hazardous byproducts of aerobic metabolism, albeit in smaller quantities than aerobic species.
  • In the realm of infectious (pathogenic) disease, examples of aerotolerant microorganisms abound. This is largely due to the fact that oxygen can reach some inside body components, but not in the same quantities that the exterior and lungs are exposed to.

Examples of Microaerophilic bacteria

Classification of Bacteria on The Basis of pH Requirements

Bacteria exhibit diverse growth preferences, adapting to various environmental conditions. One of the critical factors influencing bacterial growth is the pH of their surroundings. Based on their pH growth requirements, bacteria can be broadly classified into three primary categories: Acidophiles, Neutrophiles, and Alkaliphiles.

  1. Acidophiles:
    • Growth Preference: Acidophiles thrive in acidic environments, with an optimal pH range of 0 to 5.5.
    • Cytoplasmic Nature: The internal environment or cytoplasm of these bacteria is typically acidic.
    • Special Characteristics: Some acidophiles also possess thermophilic properties, allowing them to survive in high-temperature conditions. Such bacteria are termed Thermoacidophiles.
    • Representative Species: Examples of acidophiles include Thiobacillus thioxidans, Thiobacillus ferroxidans, Thermoplasma, and Sulfolobus.
  2. Neutrophiles:
    • Growth Preference: Neutrophiles flourish in environments with a neutral pH, typically within the range of 5.5 to 8.0.
    • Predominance: A significant portion of known bacteria fall under this category, indicating the prevalence of neutral pH conditions in many habitats.
    • Representative Species: Common neutrophiles include Escherichia coli and Salmonella.
  3. Alkaliphiles:
    • Growth Preference: Alkaliphiles are adapted to alkaline conditions and exhibit optimal growth in a pH range of 8.0 to 11.5.
    • Representative Species: Examples of alkaliphiles are Bacillus alcalophilus and Natronobacterium. Notably, Vibrio cholerae has an optimal growth pH of 8.2, showcasing its preference for slightly alkaline conditions.

In conclusion, the pH growth preferences of bacteria are a testament to their adaptability and resilience. Understanding these preferences is crucial for various applications, from industrial processes to medical research, as it provides insights into bacterial behavior, survival mechanisms, and potential vulnerabilities.

Classification of Bacteria on The Basis of Temperature Requirements

Bacteria are divided into five categories based on their required growth temperature: psychrophiles, psychrotrophs, mesophiles, thermophiles, and hyperthermophiles.

1. Psychrophiles

  • Psychrophiles and cryophiles are extremophilic organisms capable of growth and reproduction in temperatures ranging from 20 °C (4 °F)[2] to 20 °C (68 °F).
  • The optimal growing temperature is 15 degrees Celsius (59 degrees Fahrenheit).
  • They inhabit persistent frigid environments, such as the arctic regions and the depths of the ocean.
  • They can be contrasted with thermophiles, organisms that flourish at abnormally high temperatures, and mesophiles, species that thrive at moderate temperatures.
  • Numerous psychrophilic creatures are bacteria or archaea, but lichens, snow algae, phytoplankton, fungus, and wingless midges are also psychrophiles.
  • Example: Pseudomonas, Vibrio, Alcaligenes, Bacillus, Arthrobacter, Moritella, Photobacterium, and Shewanella are among examples.

2. Psychrotrophs

  • Minimum growth temperature for psychrotrophs is between 0°C and 7°C, optimal is between 20°C and 30°C, and maximum is 35°C.
  • Also known as facultative psychrophiles.
  • Psychrotrophic bacteria contaminate refrigerated foods.
  • Example: Listeria monocytogenes, Pseudomonas fluorescens.

3. Mesophiles

  • A mesophile is an organism that thrives optimally in temperatures between 20 and 45 °C (68 and 113 °F), which is neither too hot nor too cold.
  • The optimal temperature for growth of these organisms is 37°C.
  • Primarily, the term refers to microbes. Extremophiles are organisms that prefer harsh environmental conditions.
  • Mesophiles have several classifications, as they belong to two domains: Bacteria and Archaea, as well as the kingdom Fungi of the domain Eucarya.
  • Mesophiles that belong to the Bacteria domain can be either gram-positive or gram-negative.
  • Mesophile oxygen requirements might be aerobic or anaerobic.
  • There are three fundamental mesophile shapes: cocci, bacilli, and spiral.
  • The majority of human pathogens are mesophiles.
  • Example: Escherichia coli, Neisseria gonorrhoeae etc. 

4. Thermophiles

  • A thermophile is a type of extremophile that thrives at temperatures between 41 and 122 degrees Celsius (106 and 252 degrees Fahrenheit).
  • Although many thermophiles are archaea, they might also be bacteria or fungi. It is believed that thermophilic eubacteria were among the earliest bacteria.
  • Thermophiles are found in many geothermally heated locations of the Earth, such as Yellowstone National Park’s hot springs (see image) and deep sea hydrothermal vents, as well as decaying plant waste like peat bogs and compost.
  • Thermophiles are able to survive at high temperatures, when other bacteria or archaea would be harmed or even killed by the same temperatures.
  • The thermophilic enzymes are active at high temperatures. Several of these enzymes are utilised in molecular biology, such as the Taq polymerase employed in PCR.
  • Composts, self-heating hay stacks, hot water pipes, and hot springs all contain thermophiles.
  • Example: Bacillus stearothermophilus, Thermus aquaticus, etc.

5. Hyperthermophiles

  • A hyperthermophile is an organism that thrives in settings above 60 °C (140 °F) in temperature. Typically, the ideal temperature for the existence of hyperthermophiles is above 80 degrees Celsius (176 degrees Fahrenheit).
  • Although most hyperthermophiles belong to the class Archaea, some bacteria can also endure high temperatures.
  • Some of these bacteria can survive at temperatures beyond 100 °C, when high pressures increase the boiling point of water in the deep ocean.
  • Many hyperthermophiles can endure additional environmental extremes, such as high acidity or radiation levels.
  • Extremophiles comprise a subset of hyperthermophiles. Their existence may provide credence to the idea of extraterrestrial life, demonstrating that life can flourish in severe environments.
  • Example: Sulfolobus, Pyrococcus, etc. 

Classification of Bacteria on The Basis of salt and tolerance

Bacteria are divided into halophilic bacteria (halophiles) and non-halophilic bacteria (non-halophiles) based on their salt requirements.

  1. Non-halophiles: Bacteria incapable of surviving in this salty environment. It can grow in less than 0.2 m, or less than one percent of NaCl.
  2. Halophilic bacteria (halophiles): Bacteria that can thrive in salty environments (NaCl).
  1. Based on salt tolerance, halophiles are categorised as follows:
  2. Halotolerant bacteria: Bacteria that can thrive at moderate salt concentrations and develop optimally in the absence of NaCl. For instance, Staphylococcus aureus (can tolerate 5-10% NaCl).
  3. Slight halophiles: The optimal salt concentration for slight halophiles is 0.2-0.85M (1-5%). NaCl. E.g., Halomonas
  4. Moderate halophiles: optimal salt concentrations range from 0.85 to 3.40M (5 to 20%) NaCl. E.g., Bacillus, Marinococcus
  5. Extreme halophiles: 3.4-5.1M (20-30%) optimal salt concentration NaCl. E.g., Halobacterium, Haloarcula, Halococcus

Classification of Bacteria on The Basis of Nutritional Requirements

Based on the supply of carbon, energy, and electrons, bacteria are divided into the following categories.

A. Based on Source of Carbon

1. Autotrophic bacteria (autotrophs)

  • Bacteria can produce their own organic substances. CO2 is its primary source of carbon consumption.
  • The two types of autotrophic bacteria are as follows:
    1. Photoautotrophs – or photosynthetic. They obtain their energy from the sun.
    2. Chemoautotrophs – or chemosynthetic. They utilise chemical energy for food preparation.
  • In addition to energy requirements, both species of bacteria require a carbon source, such as carbon dioxide and other chemicals, to produce their food.
  • Example: Purple and green sulphur bacteria, for instance

2. Heterotrophic bacteria (heterotrophs)

  • Heterotrophic bacteria obtain their nutrition from organic molecules.
  • They are the most prolific and extensively dispersed species.
  • These organisms may be aerobic or anaerobic.
  • They are ubiquitous and can be found in food, soil, and water.
  • They contribute to the recycling of natural materials.
  • They are primarily responsible for organic matter breakdown.
  • They also exist as parasites and are responsible for a variety of diseases in plants, animals, and people. They are also present in organisms as symbionts, such as Rhizobium in the root nodules of legumes.
  • Bacteria are utilised in the production of curd, antibiotics, nitrogen-fixing, etc. Heterotrophic bacteria may breakdown cellulose, keratin, lignin, chitin, etc.
  • On the basis of their habitat, food source, and relationship with other organisms, heterotrophic bacteria can be categorised into three major groups.
    1. Parasitic – They obtain their nutrition from other organisms.
    2. Saprophytic – Saprophytic – They consume decomposing organic stuff.
    3. Symbiotic – They live in a symbiotic relationship with other creatures.

B. Based on Source of Energy

1. Phototrophs

  • Phototrophs are organisms that catch photons to generate complex chemical substances (such as carbohydrates) and energy.
  • They utilise the light’s energy to carry out cellular metabolic activities. It is a frequent misunderstanding that phototrophs must be photosynthetic.
  • Many, but not all, phototrophs photosynthesize: they transform carbon dioxide into organic material to be employed structurally, functionally, or as a source for further catabolic processes (e.g. in the form of starches, sugars and fats).
  • All phototrophs use either electron transport chains or direct proton pumping to build an electrochemical gradient, which is utilised by ATP synthase to generate the cellular energy currency, ATP.
  • Phototrophs may be heterotrophs or autotrophs. If their electron and hydrogen suppliers are inorganic compounds (such as Na2S2O3, as in some purple sulphur bacteria, or H2S, as in some green sulphur bacteria), they can also be referred to as lithotrophs; hence, some photoautotrophs are also referred to as photolithoautotrophs.
  • Example: These organisms are examples of phototrophs: Rhodobacter capsulatus, Chromatium, and Chlorobium.

2. Chemotrophs 

  • Chemotrophs are organisms that derive their energy from the oxidation of electron donors in their surroundings.
  • These molecules may be inorganic or organic (chemoorganotrophs) (chemolithotrophs).
  • The term chemotroph contrasts with phototrophs, which utilise photons. Both autotrophic and heterotrophic chemotrophs exist.
  • Chemotrophs can be found in places with a high concentration of electron donors, such as around hydrothermal vents.
  • Two forms of Chemotrophs exist;
    1. Chemoautotroph: In addition to deriving energy from chemical reactions, chemoautotrophs produce all essential organic molecules from carbon dioxide. Chemoautotrophs can create energy from either inorganic or organic sources, including hydrogen sulphide, elemental sulphur, ferrous iron, molecular hydrogen, and ammonia.
    2. Chemoheterotroph: Chemoheterotrophs (or chemotrophic heterotrophs) are incapable of fixing carbon to produce their own organic molecules. Chemoheterotrophs can be chemolithoheterotrophs, which use inorganic electron sources such as sulphur, or chemoorganoheterotrophs, which use organic electron sources such as carbohydrates, lipids, and proteins. The majority of animals and fungi are chemoheterotrophs, as are halophiles.

C. Source of Electrons

1. Lithotroph

  • Lithotrophs are a varied category of organisms that utilise an inorganic substrate (often of mineral origin) to produce reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation (e.g., ATP production) by means of aerobic or anaerobic respiration.
  • While lithotrophs include photolithotrophs such as plants, chemolithotrophs are microorganisms only; no known macrofauna can use inorganic substances as electron sources.
  • In symbiotic connections between macrofauna and lithotrophs, the lithotrophs are referred to as “prokaryotic symbionts.”
  • Chemolithotrophic bacteria in giant tube worms or plastids, organelles within plant cells that may have developed from photolithotrophic cyanobacteria-like organisms, are examples of this.
  • Chemolithotrophs are members of the Bacteria and Archaea domains.
  • The term “lithotroph” was derived from the Greek words “lithos” (rock) and “troph” (consumer), which together indicate “eating of rock.” Many lithoautotrophs, but not all, are extremophiles.
  • There are various forms of Lithotroph, including;
    1. Chemolithotrophs: Chemolithotrophs are able to utilise inorganic reduced compounds in their energy-producing processes. This process is carried out through oxidation and the synthesis of ATP.
    2. Photolithotrophs: To power biosynthetic reactions, photolithotrophic organisms, such as plants, employ exclusively inorganic electron donors, such as water (e. g., carbon dioxide fixation in lithoautotrophs).
    3. Lithoheterotrophs: Lithoheterotrophs are incapable of fixing carbon dioxide and must ingest extra organic substances in order to decompose them and utilise their carbon. Few microorganisms are exclusively lithoheterotrophic.
    4. Lithoautotrophs: Similar to plants, lithoautotrophs can use carbon dioxide from the air as a carbon source.
    5. Mixotrophy: Mixotrophy is the uptake and utilisation of organic matter to supplement their carbon dioxide fixation source (mix between autotrophy and heterotrophy). Numerous lithotrophs are known to be mixotrophic in terms of their C-metabolism.
    6. Chemolithotrophs: Chemolithotrophs utilise the inorganic chemicals listed above for aerobic or anaerobic respiration. The energy provided by oxidation of these chemicals is sufficient for ATP synthesis. Some of the electrons derived from inorganic donors are also required for biosynthesis. Reverse electron transfer processes are required to convert these reducing equivalents (mainly NADH or NADPH) into the forms and redox potentials required (primarily NADH or NADPH).
    7. Photolithotrophs: Photolithotrophs obtain their energy from the sun. These organisms are photosynthetic; purple bacteria (e.g., Chromatiaceae), green bacteria (Chlorobiaceae and Chloroflexota), and “Cyanobacteria” are examples of photolithotrophic bacteria. Purple and green bacteria oxidise sulphide, sulphur, sulfite, iron or hydrogen. Plants and cyanobacteria extract reducing equivalents from water, i.e., they oxidise water to oxygen. As long as there is light, the electrons received from electron donors are not employed for ATP synthesis; rather, they are utilised in biosynthetic activities. In the dark, some photolithotrophs switch to chemolithotrophic metabolism.

2. Organotrophs

  • An organism that obtains hydrogen or electrons from organic substrates is an organotroph.
  • In microbiology, this word is used to define and describe organisms according on how they receive electrons for their respiration activities.
  • Some heterotrophs, including mammals and numerous microorganisms, are also organotrophs. There are both anaerobic and aerobic organotrophs.
  • The word was proposed by Lwoff and colleagues in 1946.
  • E.g., Pseudomonas pseudoflava

Classification of Bacteria on The Basis of Shape And Arrangement

Bacteria are categorised into five classes according to their morphologies. They are cocci (spherical), bacilli (rod-shaped), spiral, vibrios (comma), and spirochetes.

Morphology of Bacteria - Sizes, Shapes, Arrangements, Examples With Images
Morphology of Bacteria – Sizes, Shapes, Arrangements, Examples With Images

1. Cocci

These bacteria have a spherical or elliptical form and are unicellular. Either they can remain as a single cell or they can aggregate to form a variety of structures. They are listed below:

  1. Monococcus: They are also known as micrococcus and are indicated by a single, distinct round shape. Example: Micrococcus flavus.
  2. Diplococcus: the cell of Diplococcus divides in a particular plane, and the cells remain linked to one another after division. Example: Diplococcus pneumonia.
  3. Streptococcus: Here, the cells divide repeatedly in a single plane to produce a chain of cells. Example: – Streptococcus pyogenes.
  4. Tetracoccus: Tetracoccus consists of four spherical cells arranged on two planes at right angles to one another. Example: – Gaffkya tetragena. Staphylococcus: Here, the cells are separated into three planes, like clusters of grapes and resulting in an irregular pattern. Example: – Staphylococcus aureus.
  5. Sarcina: In the instance of Sarcina, the cells divide in three planes yet form a cube-like arrangement of eight or sixteen cells with a regular shape. Example: –Sarcina lutea.

2. Bacillus (Rod-shaped)

  • These are rod- or cylinder-shaped bacteria that exist individually or in pairs.
  • Example: Bacillus cereus.

3. Vibrio (Comma-shaped)

  • The Vibra are bacteria that have the shape of a comma and are represented by an one genus.
  • Example: Vibro cholerae.

4. Spirilla or spirochete (Spiral)

  • These bacteria are helical or spring-like, with many curves and flagella at their ends.
  • Example: Spirillum volutans.

Classification of Bacteria on The Basis of Flagella

Flagella Staining Principle, Procedure, Result
Flagella Staining Principle, Procedure, Result

Bacteria exhibit a wide range of morphological features, and one of the distinguishing characteristics is the presence and arrangement of flagella. Flagella are whip-like appendages that serve as locomotory organs, enabling bacteria to move in response to environmental stimuli. Based on the presence and distribution of flagella, bacteria can be systematically classified into the following categories:

  1. Atrichous:
    • Description: Bacteria that lack flagella entirely fall under this category.
    • Example: Staphylococcus aureus and Corynebacterium diphtheriae are classic examples of atrichous bacteria.
  2. Monotrichous:
    • Description: These bacteria possess a single flagellum, which is typically located at one end, making it polar in nature.
    • Example: Pseudomonas aeruginosa and Vibrio cholerae are representative species of monotrichous bacteria.
  3. Lophotrichous:
    • Description: In lophotrichous bacteria, a tuft or cluster of flagella emerges from one end.
    • Example: Pseudomonas fluorescens is a classic lophotrichous bacterium.
  4. Amphitrichous:
    • Description: Amphitrichous bacteria are characterized by the presence of flagella at both poles. These can be singular or in clusters.
    • Example: Aquaspirillum serpens and Rhodospirillum rubrum are examples of amphitrichous bacteria.
  5. Peritrichous:
    • Description: Bacteria with a peritrichous flagellar arrangement have flagella distributed uniformly over their entire surface.
    • Example: Salmonella Typhi and Bacillus species exemplify peritrichous bacteria.

In summary, the flagellar arrangement in bacteria is not just a morphological feature but also provides insights into their behavior, habitat preferences, and interactions with their environment. Understanding these classifications is crucial for microbiologists and researchers, as it aids in bacterial identification and offers insights into their ecological roles and pathogenic potential.

Classification of Bacteria on The Basis of Capsule

Bacterial cells often exhibit external structures that play pivotal roles in their interaction with the environment and host organisms. One such structure is the capsule, a gelatinous layer enveloping the bacterial cell. The presence or absence of this capsule significantly influences the bacterial properties, especially in terms of virulence and immune evasion. Based on the presence of capsules, bacteria can be systematically categorized as follows:

  1. Non-Capsulated Bacteria:
    • Description: These bacteria lack an external capsule. The absence of a capsule often means these bacteria might be more susceptible to phagocytosis by immune cells.
    • Examples: Notable non-capsulated bacteria include Mycobacterium tuberculosis and Shigella spp.
  2. Capsulated Bacteria:
    • Description: These bacteria are characterized by the presence of a distinct capsule. The capsule offers several advantages, including protection against desiccation, resistance to phagocytosis, and enhanced virulence. It also plays a role in biofilm formation, aiding in bacterial adherence to surfaces.
    • Examples: Bacteria such as Streptococcus mutans, Klebsiella pneumoniae, and Bacillus anthracis are classic examples of capsulated bacteria.

In summary, the presence or absence of a capsule in bacteria is not just a morphological feature but also provides insights into their pathogenic potential and interactions with host organisms. Recognizing this classification is crucial for microbiologists and clinicians, as it aids in understanding bacterial pathogenesis, designing therapeutic interventions, and predicting disease outcomes.

Classification of Bacteria on The Basis of Ability to form spores

Location of Endospore
Location of Endospore

Bacteria exhibit a remarkable ability to adapt to varying environmental conditions. One such adaptation is the formation of spores, which are specialized, dormant structures that allow bacteria to survive in adverse conditions. Based on their ability to produce spores, bacteria can be systematically categorized into the following groups:

  1. Non-Spore Formers:
    • Description: These bacteria lack the capability to produce spores. They do not form any dormant structures and are typically sensitive to unfavorable conditions.
    • Examples: Escherichia coli (E. coli) and Staphylococcus aureus are representative non-spore-forming bacteria.
  2. Spore Formers: Bacteria that can produce spores are further classified based on the location and type of spore formation:a. Endospore Formers:
    • Description: Endospores are formed within the bacterial cell. These are highly resistant structures that can endure extreme conditions such as high temperatures, desiccation, and radiation.
    • Examples: Genera like Bacillus, Clostridium, and Sporosarcina are known to produce endospores.
    b. Exospore Formers:
    • Description: Exospores are produced outside the bacterial cell. These spores are typically less resistant than endospores but still offer a survival advantage in certain conditions.
    • Example: Methylosinus is a classic example of an exospore-forming bacterium.

In essence, the ability of bacteria to form spores is a testament to their evolutionary adaptability. Spore formation ensures the survival of bacterial species in hostile environments, allowing them to persist and thrive once conditions become favorable again. Understanding this classification is pivotal for microbiologists, especially in contexts like food preservation, where spore-forming bacteria can pose challenges, and in medical settings, where certain spore-formers can be pathogenic.

Classification of Bacteria on The Basis shape of Spores

  1. Oval and central spore E.g., Bacillus spp.
  2. Oval and sub-terminal spore: E.g., Clostridium spp. except C. tetani (round and terminal) and C. bifermentans (oval and central)


What is the basis for classifying bacteria?

Bacteria are classified based on various factors such as shape, cell wall composition, nutritional requirements, oxygen requirements, presence of flagella, and genetic makeup.

What are the main shapes of bacteria?

The primary shapes of bacteria are cocci (spherical), bacilli (rod-shaped), and spirilla (spiral-shaped).

How are bacteria classified based on oxygen requirements?

Bacteria can be classified as obligate aerobes (require oxygen), obligate anaerobes (cannot survive in the presence of oxygen), facultative anaerobes (can live with or without oxygen), and microaerophiles (require low levels of oxygen).

What are Gram-positive and Gram-negative bacteria?

Based on the Gram staining technique, bacteria are classified as Gram-positive (retain the violet stain) and Gram-negative (do not retain the violet stain). This classification is based on the differences in their cell wall composition.

What is the significance of bacterial spore formation?

Some bacteria can form spores, which are dormant and resistant structures, to survive in unfavorable environmental conditions. When conditions become favorable, the spore can germinate and revert to its vegetative form.

How are bacteria classified based on their nutritional requirements?

Bacteria can be classified as autotrophs (synthesize their own food) and heterotrophs (rely on external sources for food). Autotrophs can further be classified as photoautotrophs (derive energy from light) and chemoautotrophs (derive energy from chemical reactions).

What are capsulated bacteria?

Capsulated bacteria possess an outer covering called a capsule, which provides protection against the host’s immune system and helps in adherence to surfaces.

How do flagella help in classifying bacteria?

Based on the presence and arrangement of flagella, bacteria can be classified as monotrichous (single flagellum), lophotrichous (cluster of flagella at one end), amphitrichous (flagella at both ends), and peritrichous (flagella all over the surface).

What is the role of genetic studies in bacterial classification?

Genetic studies, especially DNA sequencing, have revolutionized bacterial classification by providing insights into the evolutionary relationships between different bacterial species.

Why is it important to classify bacteria?

Classification helps in understanding the relationships between different bacterial species, predicting their behavior, and devising strategies for their control, especially in the case of pathogenic bacteria.


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What is Karyotyping? What are the scope of Microbiology? What is DNA Library? What is Simple Staining? What is Negative Staining? What is Western Blot? What are Transgenic Plants? Breakthrough Discovery: Crystal Cells in Fruit Flies Key to Oxygen Transport What is Northern Blotting? What is Southern Blotting?
What is Karyotyping? What are the scope of Microbiology? What is DNA Library? What is Simple Staining? What is Negative Staining? What is Western Blot? What are Transgenic Plants? Breakthrough Discovery: Crystal Cells in Fruit Flies Key to Oxygen Transport What is Northern Blotting? What is Southern Blotting?
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