Alternaria alternata- An Overview
Alternaria spp are a group of Ascomycete fungi that are known for their saprophytic nature in decomposing soil and plants. They are ubiquitous, found in soil, air, and plants. They have a clinical impact as well as, it is a plant pathogen. There are over 299 species of Alternaria spp with the most commonly known species including:
- A. alternata
- A. botrytis
- A. leptinellae
- A. oudemansii
- A. scirpinfestans
Some species are endophytic, living on plant parts such as seeds and fruits and can cause plant and fruit damage such as the mango rot. The clinical significance of some species in causing animal infections to include allergic (hypersensitivity) pneumonitis. Alternaria alternata is a causative agent of subcutaneous phaeohyphomycosis and mycotic keratitis. It is the most common species in this group of fungi.
Alternaria spp belong to the phylum Ascomycota, which is characterized by the production of sexual spores called ascospores inside a sac-like structure called an ascus. However, Alternaria spp rarely produce ascospores and instead rely on asexual spores called conidia for reproduction and dispersal. The conidia are large, dark, and multicellular, with transverse and longitudinal septa. They have a characteristic beak-like projection at the tip. The conidia are produced on branched chains of conidiophores that emerge from the hyphae.
Alternaria spp are able to grow in a wide range of environmental conditions, such as temperature, pH, moisture, and nutrient availability. They can also tolerate high levels of salt, sugar, and organic acids. They can colonize various substrates, such as plant tissues, food products, textiles, wood, and leather. They can also cause spoilage and deterioration of these materials by producing enzymes and secondary metabolites that degrade them.
Alternaria spp can also produce various toxins that have harmful effects on plants, animals, and humans. Some of these toxins include alternariol, alternariol monomethyl ether, altenuene, tenuazonic acid, and altertoxins. These toxins can cause necrosis, chlorosis, wilting, blight, rotting, and mycotoxicosis in plants. They can also cause allergic reactions, respiratory infections, skin infections, eye infections, and systemic infections in animals and humans.
Alternaria spp are important pathogens of many crops and ornamental plants worldwide. Some of the most economically important diseases caused by Alternaria spp include early blight of potato and tomato, black spot of citrus and apple, brown spot of pear and peach, leaf spot of brassica and lettuce, black rot of carrot and olive, and black mold of tomato and onion. Alternaria spp can also infect seeds and reduce their germination and viability.
Alternaria spp are also opportunistic pathogens of animals and humans, especially those with compromised immune systems or underlying diseases. They can cause infections of the skin (cutaneous), subcutaneous tissues (subcutaneous), nails (onychomycosis), sinuses (rhinosinusitis), eyes (keratitis), lungs (pneumonia), blood (septicemia), brain (meningitis), and bones (osteomyelitis). They can also cause allergic reactions such as asthma, rhinitis, conjunctivitis, dermatitis, and hypersensitivity pneumonitis.
Alternaria spp are therefore important fungi that have both beneficial and detrimental roles in nature and human society. They are widely distributed in the environment and can adapt to various conditions. They can decompose organic matter and recycle nutrients in the soil. They can also cause diseases in plants and animals that affect their health and productivity. They can also produce toxins that have various biological activities and health implications.
Alternaria alternata is a saprophytic fungus that thrives in decomposing organic matter and soil. It is also commonly found in air, water, and plants. It can survive in a wide range of environmental conditions, such as temperature, humidity, pH, and light. It prefers warm and moist climates, but can also tolerate cold and dry conditions.
Alternaria alternata is also an endophytic fungus, meaning that it can live inside plant tissues without causing any apparent harm. It can colonize various plant parts, such as seeds, fruits, leaves, stems, and roots. It can also infect plants through wounds or natural openings, such as stomata or lenticels. Some of the plants that are susceptible to Alternaria alternata infection include:
Alternaria alternata can also be transmitted to animals and humans through contact with infected plants or contaminated materials. It can cause allergic reactions, respiratory infections, skin infections, eye infections, and nail infections in humans and animals. It can also produce mycotoxins that can cause toxicity and disease in plants and animals.
Morphological and Cultural Features of Alternaria alternata
Alternaria alternata is a filamentous fungus that produces distinctive spores and structures that can be observed under a microscope or in culture. Some of the morphological and cultural features of Alternaria alternata are:
- In culture, Alternaria alternata grows as long chains with dark brown conidiophores. Conidiophores are specialized hyphae that bear conidia, which are asexual spores.
- The conidia of Alternaria alternata are large, dark, and multicellular. They have short beaks and fine longitudinal septa. Septa are cross-walls that divide the cells of the conidia.
- The conidia of Alternaria alternata are arranged in chains or clusters on the conidiophores. They are easily detached and dispersed by wind or water.
- The colonies of Alternaria alternata are fast growing, black to olivaceous-black or greyish, and suede-like to floccose. Floccose means having a woolly or cottony appearance.
- The colonies of Alternaria alternata can vary in size from a few millimeters to several centimeters in diameter. They thrive in environments with moisture and good nutrition.
Here is an image of the colony and microscopic morphology of Alternaria alternata:
The life cycle of Alternaria alternata begins with the dispersion of the conidiospores, which are the asexual spores produced by the fungus. The conidiospores are carried by wind or water and land on a suitable substrate, such as plant leaves, fruits, or seeds. The spores germinate when there is enough moisture and temperature, and develop into hyphae, which are the filamentous structures of the fungus. The hyphae grow and branch out, forming a network called mycelium. The mycelium produces conidiophores, which are specialized hyphae that bear the conidiospores at their tips. The conidiophores are pale or dark brown, and have a straight or flexuous shape. The conidiospores are large and dark brown, with a short beak at one end. The conidiospores detach from the conidiophores and are dispersed again by wind or water, completing the cycle. The life cycle of Alternaria alternata is shown in the figure below.
Alternaria alternata is an opportunistic pathogen that can cause various types of infections in humans, especially in immunocompromised individuals. The most common clinical manifestations are cutaneous and subcutaneous infections, which account for about 74% of the reported cases. These infections usually occur after traumatic inoculation of the fungus into the skin or mucous membranes, resulting in localized lesions that may ulcerate, crust, or discharge pus. The lesions may be single or multiple, and may affect any part of the body, but are more frequent on the extremities. Cutaneous and subcutaneous infections may also spread to deeper tissues, such as bones, joints, or muscles, causing osteomyelitis, arthritis, or myositis.
Other less common forms of human infection caused by Alternaria alternata include rhinosinusitis, oculomycosis, onychomycosis, and pulmonary disease. Rhinosinusitis is an inflammation of the nasal and paranasal sinuses that may be invasive or non-invasive. It is often associated with bone marrow transplantation and chronic use of corticosteroids. Oculomycosis is an infection of the eye that may affect the cornea (keratitis), the conjunctiva (conjunctivitis), or the orbit (orbital cellulitis). It is usually related to exposure to soil or garbage and trauma to the eye. Onychomycosis is an infection of the nails that may cause discoloration, thickening, or brittleness of the nail plate. It is often preceded by contact with soil or injury to the nails. Pulmonary disease caused by Alternaria alternata may present as asthma, allergic bronchopulmonary mycosis, pneumonia, or granulomatous lung disease. It is more common in patients with underlying lung disorders or immunodeficiency.
Alternaria alternata can also produce mycotoxins that may cause toxicity in humans. However, this is a rare occurrence and the clinical effects are variable depending on the host immunity and the type and amount of toxin involved. Some of the mycotoxins produced by Alternaria alternata include alternariol, alternariol monomethyl ether, altenuene, tenuazonic acid, and altertoxins. These toxins may have mutagenic, carcinogenic, teratogenic, or immunosuppressive effects. Some of the symptoms of mycotoxicosis caused by Alternaria alternata include nausea, vomiting, diarrhea, abdominal pain, headache, fever, rash, and liver damage.
Alternaria alternata is a fungus that causes various diseases in plants, such as leaf spots, rots, blights, and mycotoxicosis. It is an opportunistic pathogen that can infect over 380 host species of plants. It can also produce host-selective toxins (HSTs) that are essential for its pathogenicity and host specificity.
The infection process of A. alternata in plants involves the following steps:
- Dispersion of conidia (asexual spores) by wind or water to the plant surface
- Germination of conidia under favorable conditions of moisture and temperature
- Penetration of the plant cuticle and epidermis by appressoria (specialized infection structures) or wounds
- Colonization of the plant tissue by hyphae (fungal filaments) and production of HSTs
- Induction of necrosis, chlorosis, lesions, and blights on the plant tissue by HSTs
- Formation of new conidia on conidiophores (spore-bearing structures) and release of conidia for secondary infection
Some of the common diseases caused by A. alternata in plants are:
- Brown and black spot: This disease affects the leaves and fruits of many crops, such as tomato, citrus, apple, pear, mango, olive, and carrot. It is characterized by brown or black necrotic spots that may coalesce and cause defoliation and fruit rot.
- Mycotoxicosis: This disease is caused by the production of secondary metabolites, such as phytotoxins and mycotoxins, by A. alternata. These toxins can cause toxicity and spoilage in plants and their products. Some examples of mycotoxicosis are tomato black mold, olive black rot, citrus black/grey rot, apple black rot, and carrot black rot.
The management of A. alternata diseases in plants involves cultural, biological, and chemical methods. Some of these methods are:
- Planting resistant or tolerant cultivars
- Avoiding overhead irrigation and excessive nitrogen fertilization
- Removing infected plant debris and pruning infected branches
- Applying fungicides or biocontrol agents at appropriate times and doses
One of the common plant diseases caused by Alternaria alternata is brown and blackspot, which affects the leaves and fruits of various crops such as citrus, tomato, pear, olive, apple, and carrot. The disease is characterized by the appearance of brown to black spots with yellow halos on both sides of the leaves. The spots may enlarge and coalesce, causing necrosis and chlorosis of the leaf tissue. The infected leaves may become brittle and fall off, resulting in defoliation and reduced photosynthesis. The disease can also affect the fruits, causing dark spots with yellow halos that later become covered by corky tissue. The fruit quality and yield are reduced by the fungal infection.
The disease is favored by warm and moist conditions, which promote the production and dispersal of spores by wind or water splash. The spores can germinate on the leaf or fruit surface within 36 to 48 hours after infection. The fungus can penetrate the plant tissue through natural openings or wounds, and colonize the intercellular spaces. The fungus can also produce toxins that cause damage to the plant cells.
The disease can be managed by using resistant cultivars, applying fungicides, practicing good sanitation, avoiding overhead irrigation, pruning infected twigs, and removing fallen leaves and fruits. The disease can also be prevented by using healthy plant material from certified sources and providing adequate drainage and aeration to the fields.
Alternaria alternata is capable of producing secondary metabolites such as phytotoxins and mycotoxins which cause spoilage in plants. These elements cause toxicity that eventually leads to disease. Mycotoxins are fungal metabolites that can contaminate food and feed and pose a threat to human and animal health. Some of the mycotoxins produced by Alternaria alternata include:
- Alternariol (AOH): A polyketide-derived compound with mutagenic, genotoxic, cytotoxic, and estrogenic effects.
- Alternariol monomethyl ether (AME): A methylated derivative of AOH with similar biological activities.
- Altenuene (ALT): A polyketide-derived compound with antimicrobial and phytotoxic properties.
- Tenuazonic acid (TeA): A tetramic acid-derived compound with antibiotic, antiviral, antitumor, and immunosuppressive effects.
- Altertoxins I, II, and III (ATX-I, ATX-II, and ATX-III): Epipolythiodioxopiperazine-derived compounds with mutagenic, genotoxic, and cytotoxic effects.
These mycotoxins can cause various diseases in plants depending on the host species, environmental conditions, and toxin concentration. Some of the infections that arise due to mycotoxicosis include:
- Tomato black mold: A disease characterized by dark brown to black lesions on tomato fruits caused by AOH and AME.
- Olive black rot: A disease characterized by blackening and shriveling of olive fruits caused by TeA.
- Citrus black/grey rot: A disease characterized by dark brown to black spots on citrus fruits caused by AOH, AME, ALT, and TeA.
- Black rot of apples: A disease characterized by dark brown to black lesions on apple fruits caused by AOH, AME, ALT, and TeA.
- Black rot of carrots: A disease characterized by dark brown to black lesions on carrot roots caused by AOH, AME, ALT, and TeA.
Mycotoxicosis in plants can result in reduced yield and quality of crops, as well as potential health risks for consumers and animals. Therefore, it is important to prevent and control the growth and spread of Alternaria alternata and its mycotoxins in agricultural settings. Some of the possible strategies include:
- Cultural practices: Such as crop rotation, sanitation, irrigation management, pruning, harvesting at optimal maturity, and post-harvest handling.
- Chemical control: Such as fungicides, biocontrol agents, antioxidants, and detoxifying agents.
- Genetic resistance: Such as breeding or genetic engineering of crops with enhanced tolerance or immunity to Alternaria alternata and its mycotoxins.
The diagnosis of Alternaria alternata infections can be challenging, as this fungus can be isolated from normal human skin or as a laboratory contaminant. Therefore, a good clinical correlation and histological confirmation are essential to establish the causative role of this fungus.
The specimens that can be collected for diagnosis include plant exudates, lesion biopsy, pus cells, nail clippings, corneal scrapings, and sinus aspirates, depending on the site and type of infection.
The specimens can be examined by direct microscopy using 10-20% KOH wet mount or calcofluor white stain. The characteristic features of Alternaria alternata are branched chains of large conidia with short beaks and fine longitudinal septa. The conidia are ovoid or ellipsoidal, pale brown, and smooth-walled.
The specimens can also be cultured on Sabouraud dextrose agar or potato dextrose agar at 28-32°C. The colonies grow rapidly and produce black-olivaceous-black/greyish and suede-like to floccose appearance. The microscopic morphology of the colonies is similar to that of the direct examination.
The identification of Alternaria alternata to the species level can be difficult by conventional methods, as there are many morphologically similar species in this genus. Molecular methods, such as polymerase chain reaction (PCR) and DNA sequencing, can be used to confirm the species identification and to differentiate between pathogenic and non-pathogenic strains.
Histological examination of tissue biopsies or pus exudates can also aid in the diagnosis of Alternaria alternata infections. The hematoxylin-eosin stain can reveal dark-colored filamentous hyphae in the infected tissues. The Fontana-Masson silver stain can be used to detect melanin in the fungal cell walls.
Human infections caused by Alternaria alternata can be treated with various antifungal drugs, depending on the type and severity of the infection. The choice of treatment may also depend on the patient`s immune status, underlying conditions, and drug interactions. Some of the common antifungal drugs used to treat Alternaria alternata infections are:
- Voriconazole: This is a broad-spectrum triazole antifungal that inhibits the synthesis of ergosterol, a key component of fungal cell membranes. Voriconazole has been shown to be effective against cutaneous, subcutaneous, ocular, and sinus infections caused by Alternaria alternata. Voriconazole can be administered orally or intravenously, depending on the site and severity of the infection. The usual dose is 200 mg twice daily for adults and 4 mg/kg twice daily for children. Voriconazole has some side effects such as visual disturbances, hepatotoxicity, rash, and QT prolongation.
- Itraconazole: This is another triazole antifungal that works by inhibiting ergosterol synthesis. Itraconazole has been used successfully to treat onychomycosis (nail infection), cutaneous, and subcutaneous infections caused by Alternaria alternata. Itraconazole can be taken orally as capsules or solution, with a dose of 200 mg once or twice daily for adults and 5 mg/kg once daily for children. Itraconazole can cause side effects such as nausea, headache, liver dysfunction, and drug interactions.
- Amphotericin B: This is a polyene antifungal that binds to ergosterol and disrupts the fungal cell membrane, causing leakage of cellular contents and death of the fungus. Amphotericin B is a potent antifungal that can treat systemic and invasive infections caused by Alternaria alternata. Amphotericin B is given intravenously as a lipid formulation or a conventional deoxycholate formulation, with a dose of 3-5 mg/kg per day for adults and 1-1.5 mg/kg per day for children. Amphotericin B can cause serious side effects such as nephrotoxicity, infusion reactions, electrolyte disturbances, and anemia.
- Flucytosine: This is a pyrimidine analog that interferes with fungal DNA and RNA synthesis. Flucytosine is often used in combination with amphotericin B to enhance its antifungal activity and prevent resistance development. Flucytosine can treat systemic and invasive infections caused by Alternaria alternata. Flucytosine is taken orally as capsules or suspension, with a dose of 100-150 mg/kg per day for adults and 50-100 mg/kg per day for children. Flucytosine can cause side effects such as bone marrow suppression, hepatotoxicity, gastrointestinal disturbances, and CNS toxicity.
The duration of treatment depends on the type and extent of the infection, the response to therapy, and the resolution of symptoms. In general, cutaneous and subcutaneous infections require 2-4 months of treatment, while ocular, sinus, and systemic infections require 6-12 months of treatment. The treatment may be longer for immunocompromised patients or those with relapsing or refractory infections.
In addition to antifungal therapy, some supportive measures may be helpful in managing Alternaria alternata infections. These include:
- Surgical debridement or excision of infected tissue or foreign bodies
- Drainage of abscesses or sinus tracts
- Correction of underlying immunodeficiency or comorbidities
- Avoidance of exposure to moldy environments or contaminated materials
- Use of protective eyewear or gloves when handling plants or soil
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