Alternaria alternata – Habitat, Life Cycle, Morphology, Pathogenesis

Alternaria alternata is a fungus that causes leaf spots, rots, blights, and other plant maladies. It is an opportunistic pathogen that affects over 380 plant species.

Additionally, it can cause upper respiratory tract infections and asthma in immunocompromised humans.

• Alternaria alternata, the most prevalent mould in arid, warm climates, has air spores that typically peak in the afternoon and dissipate in warm, dry air. Therefore, in temperate climates, the peak season for Alternaria alternata spore counts is typically summer.
• After imbibing this fungus’ spores, those allergic to it may experience symptoms.Alternaria alternata is primarily an outdoor fungus that develops on plant matter.
• Nonetheless, the species can also be found indoors, where it prefers humid environments such as bathrooms and frequently generates large brown spores that are a well-known allergen and asthma trigger.
• Nevertheless, indoor Alternaria alternata concentrations are typically influenced by outdoor concentrations. In other words, if outdoor pathogen levels are high, indoor levels may also be elevated.
• Alternaria thrives at temperatures between 20 and 25 degrees Celsius (68 and 77 degrees Fahrenheit), but can tolerate temperatures between 1 and 35 degrees Celsius (approximately 34 and 95 degrees Fahrenheit).
• Various studies report varying rates of mould sensitization, but Alternaria alternata allergy is strongly linked to allergic rhinitis (also known as hay fever) and asthma and affects approximately 5 percent of the population.
• In children, the reported prevalence of sensitization ranges from less than 1 percent in Austria to 50 percent in Arizona, United States.
• And according to a large study of children with asthma residing in inner cities in the United States, 38 percent of study participants had a positive skin test for Alternaria.
• Alternaria allergy in conjunction with exposure to Alternaria is a risk factor for the development and worsening of allergic diseases such as allergic rhinitis and asthma, and it can lead to severe asthma exacerbations.

Classification of Alternaria alternata

Kingdom	Fungi
Phylum	Ascomycota
Class	Euascomycetes
Order	Pleosporales
Family	Pleosporaceae
Genus	Alternaria

Habitat of Alternaria alternata

• Alternaria alternata is a fungus that has a wide range of habitats. It can be found in soil, water, and air, and can grow on a variety of substrates, including plants, foodstuffs, and building materials.
• In particular, it is commonly found on decaying plant material, including leaves, fruits, and vegetables. It is also a common contaminant of foodstuffs, such as cereals, nuts, and spices.
• Outdoors, it can be found in soil, on plant debris, and in water sources. Alternaria alternata is able to survive in a range of temperatures and can even grow in low-moisture environments.

Morphological of Alternaria alternata

Alternaria alternata is a filamentous fungus belonging to the family Pleosporaceae. It has a complex morphology and its structure is composed of different parts, including:

• Hyphae: Alternaria alternata hyphae are thin, cylindrical, and septate, meaning that they are divided by cross-walls or septa. The hyphae can be branched or unbranched and are responsible for the growth of the fungus.
• Conidiophores: These are specialized hyphae that produce asexual spores called conidia. The conidiophores are typically unbranched and arise from the mycelium. They are often dark in color and can be up to several millimeters long.
• Conidia: Alternaria alternata conidia are single-celled spores that are typically dark in color, ranging from brown to black. They are oval or oblong in shape and can be smooth or have ridges or warts on their surface. The conidia are produced in chains at the tips of the conidiophores and are easily dispersed by wind or water.
• Mycelium: The mycelium is a mass of interconnecting hyphae that forms the vegetative body of the fungus. It grows through the substrate, absorbing nutrients and water to support the growth of the fungus.

Cultural Features of Alternaria alternata

Alternaria alternata has distinctive cultural features that can help to identify it in the laboratory. These features include:

1. Colony morphology: Alternaria alternata colonies are typically flat, velvety, and olive green to black in color. The colonies can range in size from a few millimeters to several centimeters in diameter. As the colony matures, it may develop a dark, concentric ring around the edge.
2. Texture: The texture of Alternaria alternata colonies is typically velvety or woolly. The surface of the colony may also be slightly wrinkled or raised in places.
3. Pigmentation: The pigmentation of Alternaria alternata colonies can vary depending on the growth conditions. The colonies are typically dark in color, ranging from olive green to brown or black.
4. Sporulation: Alternaria alternata produces conidia on the surface of the colony, which can be observed as dark specks. The conidia are typically single-celled and oval to oblong in shape.
5. Growth rate: Alternaria alternata is a fast-growing fungus, and colonies can typically be seen within a few days of inoculation. The growth rate may be affected by environmental factors such as temperature and humidity.

Life Cycle of Alternaria alternata

Alternaria alternata has a complex life cycle that includes both sexual and asexual reproduction. Here are the main stages in the life cycle of Alternaria alternata:

1. Asexual reproduction: Alternaria alternata primarily reproduces asexually through the production of conidia, which are single-celled spores. The conidia are formed on specialized hyphae called conidiophores and are released into the air or water when mature. Conidia can then germinate and form new hyphae, which grow into new colonies.
2. Sexual reproduction: Alternaria alternata can also reproduce sexually through the formation of ascospores. This occurs when two compatible strains of the fungus come together and form a sexual structure called an ascocarp. Inside the ascocarp, specialized cells called asci produce ascospores, which can be released and dispersed in the same way as conidia.
3. Survival: Alternaria alternata can survive for long periods of time as spores in the environment. The spores can be dispersed by wind or water, and can remain viable for months or even years.
4. Infection: Alternaria alternata can infect a wide range of hosts, including plants, animals, and humans. The fungus can enter the host through wounds, natural openings, or by penetrating the host tissue directly.
5. Disease development: Once inside the host, Alternaria alternata can cause a range of diseases, depending on the host and the environmental conditions. In plants, it can cause leaf spots, fruit rot, and other symptoms. In humans, it can cause respiratory allergies, skin infections, and other health problems.

Pathogenesis of Alternaria alternata

Alternaria alternata is a pathogenic fungus that can cause disease in plants, animals, and humans. Here are some of the key mechanisms involved in the pathogenesis of Alternaria alternata:

1. Production of enzymes and toxins: Alternaria alternata produces a range of enzymes and toxins that can damage host tissues and suppress the immune system. For example, it produces cell-wall-degrading enzymes that can break down plant cell walls and facilitate infection, as well as mycotoxins that can cause allergic reactions in humans and animals.
2. Invasion of host tissues: Alternaria alternata can invade host tissues by penetrating through natural openings, such as stomata in plants, or by breaking through the host cell wall directly. Once inside the host tissue, the fungus can grow and spread, causing damage to the host cells.
3. Induction of host defense responses: When Alternaria alternata infects a host, it triggers a range of defense responses from the host immune system. For example, in plants, it can induce the production of pathogenesis-related (PR) proteins and phytohormones that help to limit the spread of the fungus.
4. Host susceptibility factors: The ability of Alternaria alternata to cause disease can also be influenced by host factors such as genetics, age, and immune status. Hosts with compromised immune systems are more susceptible to infection, and some genetic variants may be more resistant or susceptible to the fungus.

Virulance factors of Alternaria alternata

The virulence factors of Alternaria alternata are the factors that allow the fungus to cause disease in its host. Here are some of the known virulence factors of Alternaria alternata:

1. Cell wall-degrading enzymes: A. alternata produces a number of enzymes that help it to break down the cell walls of the host plant. These enzymes include chitinases, glucanases, and proteases. Chitinases break down chitin, which is a major component of the cell walls of plants. Glucanases break down glucans, which are also a major component of plant cell walls. Proteases break down proteins, which are essential for the structure and function of plant cells.
2. Mycotoxins: Alternaria alternata produces several mycotoxins, such as alternariol, tenuazonic acid, and alternariol monomethyl ether, which can cause tissue damage and suppress the immune system of the host.
   • Tentoxin: Tentoxin is a toxin that inhibits the production of chlorophyll in plants. This leads to the death of the plant cells.
   • Alternariol: Alternariol is a toxin that damages the cell membranes of plants. This leads to the death of the plant cells.
   • Alternariol monomethyl ether: Alternariol monomethyl ether is a toxin that damages the mitochondria of plants. This leads to the death of the plant cells.
3. Melanin: Alternaria alternata produces melanin, a pigment that can protect the fungus from host defense mechanisms such as oxidative stress and phagocytosis.
4. Extracellular polysaccharides/Adhesive proteins: Alternaria alternata produces extracellular polysaccharides that can help the fungus to adhere to host tissues and evade the host immune system. These proteins help the fungus to attach to the surface of the host plant.
5. Phytohormones: Alternaria alternata produces phytohormones, such as auxin and abscisic acid, which can manipulate the host plant’s growth and development to facilitate infection.

Alternaria alternata Pathogenesis in Human Host

In humans, Alternaria alternata can cause a range of health problems, including respiratory allergies, skin infections, and other conditions. Here are some of the key steps involved in the pathogenesis of Alternaria alternata in humans:

1. Exposure to spores: Alternaria alternata spores are commonly found in the environment, particularly in damp and warm conditions. Humans can be exposed to the spores through inhalation or skin contact.
2. Immune response: When Alternaria alternata spores enter the human body, they can trigger an immune response, particularly in individuals with allergies or asthma. This immune response can cause inflammation, which can lead to symptoms such as coughing, wheezing, and skin rashes.
3. Production of toxins: Alternaria alternata produces several mycotoxins, such as alternariol and tenuazonic acid, that can cause tissue damage and further exacerbate the immune response.
4. Colonization of tissues: In some cases, Alternaria alternata can colonize human tissues and cause infection. This can occur in individuals with weakened immune systems or in those with pre-existing skin conditions.
5. Disease development: Alternaria alternata can cause a range of health problems in humans, depending on the site of infection and the individual’s immune status. For example, it can cause respiratory allergies, such as allergic rhinitis and asthma, as well as skin infections, such as dermatitis and onychomycosis.

Alternaria alternata is a ubiquitous fungus that can cause a variety of diseases in humans, including:

• Allergic bronchopulmonary aspergillosis: This is a chronic lung infection that occurs in people with asthma or other lung diseases.
• Allergic Aspergillus sinusitis: This is a chronic sinus infection that occurs in people with allergies.
• Invasive aspergillosis: This is a serious infection that can occur in people with weakened immune systems.
• Food poisoning: This is a rare but serious illness that can occur after eating food that is contaminated with A. alternata.
• Allergic hypersensitivity pneumonitis (AHP): AHP is an inflammatory lung disease that occurs when people are exposed to spores of A. alternata. It is characterized by fever, cough, shortness of breath, and fatigue.
• Sepsis: Sepsis is a life-threatening condition that occurs when the body’s response to an infection damages its own tissues and organs. A. alternata can cause sepsis in people with weakened immune systems.
• Invasive aspergillosis: Invasive aspergillosis is a serious infection that occurs when A. alternata enters the bloodstream and spreads to other organs. It is a life-threatening condition, especially in people with weakened immune systems.
• Allergic rhinitis: This is an allergic reaction to the spores of A. alternata. Symptoms include sneezing, runny nose, and itchy eyes.
• Allergic asthma: This is an allergic reaction to the spores of A. alternata that can cause wheezing, chest tightness, and shortness of breath.
• Aspergillosis: This is a serious infection that can affect the lungs, sinuses, and other organs. Symptoms include fever, cough, and shortness of breath.

Alternaria alternata Pathogenesis in plants

Alternaria alternata is a pathogen that can cause disease in many different plant species, including crops such as fruits and vegetables. Here are some of the key steps involved in the pathogenesis of Alternaria alternata in plants:

1. Spore germination and penetration: Alternaria alternata spores can germinate on plant surfaces and produce germ tubes that penetrate through natural openings, such as stomata, or directly through the plant cuticle.
2. Production of enzymes and toxins: Once inside the plant, Alternaria alternata produces a range of enzymes and toxins that can damage plant tissues and suppress the plant’s immune system. For example, it produces cell-wall-degrading enzymes that can break down plant cell walls, as well as mycotoxins that can cause necrosis and chlorosis.
3. Induction of host defense responses: When Alternaria alternata infects a plant, it triggers a range of defense responses from the plant immune system. For example, the plant can produce phytohormones, such as jasmonic acid, and pathogenesis-related (PR) proteins that help to limit the spread of the fungus.
4. Spread and colonization: Alternaria alternata can spread within the plant through the vascular system or by colonizing neighboring tissues. The fungus can cause tissue damage and necrosis, leading to plant death in severe cases.
5. Production of spores: Alternaria alternata can produce spores on infected plant tissues, which can be dispersed by wind or water and infect other plants.

Alternaria alternata can cause a variety of diseases in plants, including:

• Leaf spot: This is a common disease that causes small, dark spots on leaves.
• Fruit rot: This disease causes fruit to rot and decay.
• Stem rot: This disease causes stems to rot and die.
• Wilt: This disease causes plants to wilt and die.

There are a number of factors that can contribute to the development of A. alternata infection in plants, including:

• Moisture: The fungus needs moisture to germinate and grow.
• Temperature: The fungus grows best at temperatures between 20 and 25 degrees Celsius.
• Nutrients: The fungus needs nutrients to grow.
• Host susceptibility: Some plants are more susceptible to infection than others.

The pathogenesis of A. alternata can be summarized as follows:

1. The fungus produces virulence factors that help it to attach to and infect the host plant.
2. The fungus produces enzymes that break down the cell walls of the host plant.
3. The fungus produces toxins that damage the cells and tissues of the host plant.
4. The fungus colonizes the host plant and causes disease.

Laboratory Diagnosis of Alternaria alternata infections

Laboratory diagnosis of Alternaria alternata infections can be done through a combination of different methods. Here are some of the commonly used diagnostic methods:

1. Microscopic examination: Alternaria alternata can be identified microscopically by its characteristic morphology. The fungus produces conidia, which are asexual spores that are produced on conidiophores. Conidiophores are specialized hyphae that are responsible for producing conidia. Conidia of A. alternata are light brown to dark brown in color and are typically 10-15 µm in length. They have 1-3 longitudinal septa.
2. Culture isolation: Alternaria alternata can be cultured on different types of media, such as Potato Dextrose Agar (PDA) or Sabouraud Dextrose Agar (SDA). The colonies of Alternaria alternata are usually dark, greenish to black, with a downy or velvety texture.
3. DNA-based methods/molecular methods: Alternaria alternata can also be identified molecularly by using DNA sequencing. DNA sequencing is a technique that is used to determine the order of the nucleotides in a DNA molecule. The DNA sequence of A. alternata is unique and can be used to identify the fungus. Polymerase Chain Reaction (PCR) and DNA sequencing can be used to identify Alternaria alternata from environmental or clinical samples. Specific PCR primers can be designed to target specific genes of Alternaria alternata for rapid and sensitive identification.
4. Serological tests: ELISA and immunofluorescence tests can be used to detect antibodies against Alternaria alternata in human serum or other biological samples. These tests can be used to diagnose respiratory allergies or other infections caused by Alternaria alternata.
5. Histology: Histology is a technique that is used to examine tissues under a microscope. Tissues from infected areas can be stained and examined to look for fungal hyphae.

Symptoms of Alternaria alternata infections

Alternaria alternata is a fungus that can cause a variety of diseases in humans and plants. The symptoms of Alternaria alternata infections vary depending on the site of infection.

In humans, Alternaria alternata can cause:

• Skin infections: Skin infections caused by Alternaria alternata can present as a variety of lesions, including:
  • Erythema: This is a redness of the skin.
  • Pustules: These are small, raised bumps filled with pus.
  • Vesicles: These are small, fluid-filled blisters.
  • Ulcers: These are open sores on the skin.
• Respiratory infections: Respiratory infections caused by Alternaria alternata can present as:
  • Cough: This is a sudden, forceful expulsion of air from the lungs.
  • Wheezing: This is a whistling sound that occurs when you breathe.
  • Shortness of breath: This is a difficulty breathing.
  • Chest pain: This is a pain in the chest.
• Eye infections: Eye infections caused by Alternaria alternata can present as:
  • Conjunctivitis: This is an inflammation of the conjunctiva, which is the clear membrane that lines the eyelids and covers the white of the eye.
  • Keratitis: This is an inflammation of the cornea, which is the clear front part of the eye.
• Systemic infections: Systemic infections caused by Alternaria alternata can present as:
  • Fever: This is a high body temperature.
  • Chills: These are sudden feelings of coldness.
  • Fatigue: This is a feeling of tiredness.
  • Muscle aches: These are pains in the muscles.
  • Joint pain: These are pains in the joints.

In plants, Alternaria alternata can cause:

• Leaf spots: Leaf spots caused by Alternaria alternata are small, brown or black spots that can appear on leaves.
• Fruit rots: Fruit rots caused by Alternaria alternata are soft, brown or black spots that can appear on fruits.
• Stem cankers: Stem cankers caused by Alternaria alternata are sunken, black or brown areas that can appear on stems.
• Wilts: Wilts caused by Alternaria alternata are a sudden wilting of plants.
• Seedling blights: Seedling blights caused by Alternaria alternata are a death of seedlings.

Treatment of Alternaria alternata infections

Treatment of Alternaria alternata infections depends on the location and severity of the infection. In general, treatment options for Alternaria alternata infections include antifungal drugs, immunotherapy, and environmental control measures. Here are some of the commonly used treatment options:

1. Antifungal drugs: Antifungal drugs such as amphotericin B, itraconazole, fluconazole, and voriconazole are commonly used to treat Alternaria alternata infections. The choice of antifungal drug and duration of treatment depend on the location and severity of the infection. The most common antifungal medications used to treat Alternaria alternata infection are:
   • Itraconazole: Itraconazole is an oral antifungal medication that is effective against a wide range of fungi, including A. alternata.
   • Voriconazole: Voriconazole is an intravenous antifungal medication that is effective against a wide range of fungi, including A. alternata.
   • Posaconazole: Posaconazole is an oral antifungal medication that is effective against a wide range of fungi, including A. alternata.
   • Amphotericin B: Amphotericin B is an intravenous antifungal medication that is used to treat serious fungal infections, including Alternaria alternata infections. Amphotericin B is available as a powder that is mixed with water before administration.
2. Immunotherapy: Immunotherapy, such as allergy shots, can be used to treat respiratory allergies caused by Alternaria alternata. Immunotherapy involves administering small doses of the allergen to gradually build up immunity to the allergen.
3. Environmental control measures: Environmental control measures such as reducing humidity and moisture levels in indoor environments can help to reduce exposure to Alternaria alternata spores. Proper ventilation, air filtration, and regular cleaning can also help to reduce the risk of exposure.
4. Surgery: In severe cases, surgery may be required to remove infected tissues, especially in cases of invasive Alternaria alternata infections.

Prevention and control of Alternaria alternata infections

There are a number of things that can be done to prevent and control Alternaria alternata infections. These include:

1. Environmental control: Alternaria alternata spores thrive in warm, humid environments. Reducing humidity levels and keeping indoor environments clean and dry can help to reduce the risk of exposure to Alternaria alternata spores.
2. Air filtration: Air filtration systems, such as HEPA filters, can help to remove Alternaria alternata spores from indoor air. Regular cleaning and maintenance of air filters are important to ensure their effectiveness.
3. Personal protective equipment: When working in environments where exposure to Alternaria alternata spores is likely, wearing appropriate personal protective equipment, such as gloves and masks, can help to reduce the risk of exposure.
4. Allergy immunotherapy: Immunotherapy, such as allergy shots, can be used to desensitize individuals with respiratory allergies to Alternaria alternata spores.
5. Treatment of underlying health conditions: Treating underlying health conditions, such as asthma, can help to reduce the severity of respiratory allergies caused by Alternaria alternata spores.
6. Avoidance of outdoor activities: During peak Alternaria alternata spore season, individuals with respiratory allergies may consider avoiding outdoor activities, especially on windy days when spore levels are high.
7. Good hygiene: Good hygiene practices, such as washing your hands frequently, can help to prevent the spread of infection.
8. Avoiding contact with contaminated surfaces: If you come into contact with contaminated surfaces, it is important to wash your hands thoroughly with soap and water.
9. Using protective gear: If you are working with materials that may be contaminated with Alternaria alternata, it is important to wear protective gear, such as gloves and a mask.
10. Proper storage of food: Food should be stored properly to prevent the growth of fungi. This includes storing food in airtight containers and keeping food at a cool temperature.
11. Crop rotation: Crop rotation can help to reduce the incidence of Alternaria alternata infections in plants. Crop rotation helps to break the life cycle of the fungus and prevents it from building up in the soil.
12. Sanitation: Sanitation practices, such as removing debris and dead plants, can help to reduce the incidence of Alternaria alternata infections in plants.
13. Chemical control: Chemical control can be used to control Alternaria alternata. However, the use of chemicals should be limited to prevent the development of resistance.
14. Biological control: Biological control can be used to control Alternaria alternata. Biological control agents include bacteria, fungi, and nematodes.

FAQ

References

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