Aspergillus flavus- An Overview
Aspergillus flavus is a fungus that belongs to the genus Aspergillus, which comprises about 250 species of molds that are widely distributed in nature. Aspergillus flavus is one of the most important species of Aspergillus, as it can cause diseases in plants, animals, and humans, and produce a potent carcinogen called aflatoxin.
Aspergillus flavus is a saprotrophic fungus, which means that it feeds on dead organic matter, especially plant debris. It can be found in soil, air, water, and various substrates such as grains, legumes, nuts, spices, and dried fruits. It can also infect living plants and animals, including humans, as an opportunistic pathogen.
Aspergillus flavus has a complex life cycle that involves both sexual and asexual reproduction. The asexual spores, called conidia, are produced on specialized structures called conidiophores, which form clusters of spore-bearing cells called phialides. The conidia are usually yellow-green in color and have a rough surface. They can be dispersed by air currents or insects and germinate under favorable conditions. The sexual spores, called ascospores, are produced within a hard mass of fungal tissue called sclerotia, which can survive in harsh environments for long periods. The sclerotia can also produce conidia under certain stimuli. The sexual reproduction of Aspergillus flavus was discovered only recently and is still poorly understood.
Aspergillus flavus is a major threat to human health and food security, as it can cause various diseases and contaminate food products with aflatoxin. Aflatoxin is a group of toxic compounds that can cause acute poisoning, liver damage, immune suppression, and cancer in humans and animals. Aflatoxin is mainly produced by two strains of Aspergillus flavus: the L strain, which produces large sclerotia and low levels of aflatoxin; and the S strain, which produces small sclerotia and high levels of aflatoxin. Aflatoxin production is influenced by several factors such as temperature, moisture, substrate composition, and stress.
The diseases caused by Aspergillus flavus in humans are collectively called aspergillosis, which can range from mild allergic reactions to invasive infections that affect various organs such as the lungs, sinuses, brain, eyes, skin, bones, and blood vessels. Aspergillosis is more common and severe in people who have weakened immune systems due to diseases such as HIV/AIDS, cancer, diabetes, or organ transplantation. The diagnosis of aspergillosis is based on clinical signs and symptoms, radiological findings, laboratory tests such as microscopy, culture, serology, and molecular methods; and histopathology. The treatment of aspergillosis depends on the type and severity of the infection and may include antifungal drugs such as amphotericin B, itraconazole, voriconazole, posaconazole, or caspofungin; surgery to remove infected tissue; or immunotherapy to boost the host defense.
The diseases caused by Aspergillus flavus in plants are mainly ear rot in corn and yellow mold in peanuts; but it can also affect other crops such as cotton, rice, wheat, barley, sorghum, soybean, sunflower, pistachio, almond, walnut; and fruits such as figs and dates. The infection of plants by Aspergillus flavus can occur before or after harvest; but it is more common during storage or transit when the moisture and temperature are favorable for fungal growth. The infection of plants by Aspergillus flavus can reduce the yield and quality of the crops; and also increase the risk of aflatoxin contamination of food products derived from them. The prevention and control of plant diseases caused by Aspergillus flavus involve good agricultural practices such as crop rotation; pest management; irrigation; harvesting at optimal maturity; drying; cleaning; sorting; storing at low temperature and humidity; using resistant varieties; applying fungicides or biocontrol agents; or using physical or chemical methods to reduce aflatoxin levels.
Aspergillus flavus is a versatile and adaptable fungus that poses significant challenges to human health and food security. Therefore, it is important to understand its biology; ecology; epidemiology; pathogenicity; diagnosis; treatment; prevention; and control.
Aspergillus flavus is a type of mold that is widely distributed in nature and can be found in various habitats. Some of the common habitats of Aspergillus flavus are:
- Soil: Aspergillus flavus is a saprophytic fungus that decomposes organic matter in the soil. It can survive as conidia or sclerotia in the soil and colonize plant tissues as mycelia.
- Plants: Aspergillus flavus infects many important crops, such as corn, peanuts, cotton, and tree nuts. It can enter the plants through wounds, insect damage, or natural openings and cause diseases such as ear rot, yellow mold, and aflatoxin contamination.
- Indoor environments: Aspergillus flavus can also grow in indoor environments, especially in damp or moist areas such as bathrooms, kitchens, and basements. It can be found on walls, ceilings, carpets, furniture, and food items. Exposure to Aspergillus flavus spores can cause allergic reactions or respiratory infections in humans and animals.
Aspergillus flavus thrives in warm and humid conditions and is more common in tropical and subtropical regions. It can grow in a wide range of temperatures, from 12 °C to 48 °C, with an optimum of 37 °C. It also prefers high moisture levels, which vary depending on the substrate. For example, it can grow on starchy cereals at moisture levels of 13-13.2%, while on soybeans at 11.5-11.9%.
Aspergillus flavus is a fungus that has a complex morphology and can be classified into two groups based on the size of sclerotia produced. Sclerotia are hard masses of fungal mycelia that can survive in harsh environmental conditions and produce spores. Group I consists of L strains with sclerotia greater than 400 μm in diameter and Group II consists of S strains with sclerotia less than 400 μm in diameter .
Aspergillus flavus reproduces both asexually and sexually. Asexual reproduction produces spores called conidia that are dispersed by wind or insects and can infect plants and animals. Sexual reproduction produces spores called ascospores that are contained within the sclerotia and can germinate under favorable conditions .
The conidia of Aspergillus flavus are spherical or oval, with a diameter of 3 to 6 µm and a rough or pitted surface. They are usually yellow-green in color, but can also be brown or black depending on the strain. The conidia are produced from specialized cells called phialides that are arranged on the tip of a stalk called conidiophore. The phialides can be either uniseriate (single row) or biseriate (double row) or both on the same conidiophore .
The conidiophore of Aspergillus flavus is hyaline (colorless) and rough-textured, with a swollen base called vesicle. The vesicle can be spherical, oval, or pyriform (pear-shaped), with a diameter of 800 to 1200 µm. The vesicle bears the phialides on its surface, forming a radiating structure called conidial head .
The hyphae of Aspergillus flavus are thread-like, septate (divided by cross-walls), and branched. They are also hyaline and have a diameter of 2 to 4 µm. The hyphae form a network called mycelium that can grow on various substrates such as soil, plant tissues, or organic matter .
The sclerotia of Aspergillus flavus are dark brown or black in color and have a diameter of 100 to 1000 µm. They are composed of compacted hyphae and can contain ascospores inside. The sclerotia can germinate to produce either conidia or ascospores depending on the environmental conditions .
The ascospores of Aspergillus flavus are oval or elliptical, with a diameter of 5 to 7 µm and a smooth surface. They are usually hyaline or light brown in color and are produced inside sac-like structures called asci. The asci are formed within the sclerotia and can release the ascospores when they rupture .
Aspergillus flavus is a fungus that can grow on various culture media, depending on the availability of nutrients, temperature, and pH. The cultural characteristics of A. flavus vary according to the type and composition of the medium, but some general features can be observed.
- On Sabouraud Dextrose Agar (SDA), A. flavus produces white, soft, velvety colonies that turn yellowish-green after 4 days of incubation at 25°C. The colonies are 55-70 mm in diameter and have a floccose center. The reverse side of the colony is reddish-brown due to the production of sclerotia, which are hard masses of mycelia that can survive in harsh conditions. Sclerotia are also produced on Potato Dextrose Agar (PDA) and Malt Extract Agar (MEA), but not on Czapek Yeast Agar (CYA).
- On PDA, A. flavus produces green conidia, which are asexual spores that disperse by air or insects. The conidia are produced on conidiophores, which are specialized hyphae that bear phialides, which are flask-shaped cells that produce conidia at their tips. The phialides can be arranged in one or two rows (uniseriate or biseriate) on the conidiophore vesicle, which is a swollen structure at the end of the conidiophore. The conidia are spherical, rough-walled, and 3-6 µm in diameter. The colonies on PDA are flat at the edges and raised at the center, with a wrinkled cerebriform pattern. They also produce exudates that are colorless or brown.
- On MEA, A. flavus produces smooth white mycelia that change to olive-green and dark green conidia after 7 days of incubation at 25°C. The sclerotia are white and deep brown, with colorless exudates at the center of the colonies.
- On CYA, A. flavus produces colonies after 7 days of incubation at 25°C and 37°C. The colonies are velutinous, grey-blue-green, and uniseriate conidial heads. No sclerotia or exudates are produced on this medium.
The cultural characteristics of A. flavus can be used to identify and differentiate it from other Aspergillus species, such as A. fumigatus, A. niger, and A. terreus. However, molecular methods such as PCR and sequencing are more reliable and accurate for identification and classification of A. flavus strains.
Aspergillus flavus is a fungus that can grow on various substrates, such as soil, plant debris, grains, legumes, and nuts. It can also infect animals and humans, causing diseases and producing toxins. The life cycle of A. flavus involves both asexual and sexual reproduction, as well as different forms of propagules that can survive and disperse in the environment.
Asexual reproduction is the most common mode of reproduction for A. flavus. It produces spores called conidia that are formed on specialized structures called conidiophores. Conidiophores are hyphae (filamentous cells) that emerge from the mycelium (the network of hyphae) and bear vesicles (swollen tips) that produce phialides (flask-shaped cells). Phialides can be arranged in one or two rows (uniseriate or biseriate) on the vesicle, and each phialide can produce several conidia. Conidia are spherical or oval, rough-walled, and yellow-green in color. They are typically 3 to 6 µm in diameter.
Conidia are the main source of infection and dissemination for A. flavus. They can be released from the conidiophores and carried by air currents, insects, or water to new substrates or hosts. When they land on a suitable surface, they can germinate and produce germ tubes that grow into new hyphae and mycelia. Conidia can also remain dormant for long periods of time until favorable conditions are met.
Sexual reproduction is less common but still possible for A. flavus. It produces sexual spores called ascospores that are formed inside sac-like structures called asci. Asci are contained within sclerotia, which are hard masses of compacted hyphae that serve as survival structures for the fungus. Sclerotia can vary in size, shape, and color depending on the strain of A. flavus. They can be white, yellow, brown, or black, and range from less than 1 mm to more than 5 mm in diameter.
Sexual reproduction occurs between two compatible strains of A. flavus that have different mating types (MAT1-1 or MAT1-2). When these strains are cultured together, they can exchange genetic material through a process called parasexuality, which involves fusion of hyphae, formation of diploid nuclei, and recombination of chromosomes. This results in the production of sclerotia that contain asci with ascospores. Ascospores are haploid and genetically diverse from their parents.
Ascospores are another source of infection and dissemination for A. flavus. They can be released from the sclerotia and dispersed by air currents, insects, or water to new substrates or hosts. When they land on a suitable surface, they can germinate and produce germ tubes that grow into new hyphae and mycelia. Ascospores can also remain dormant for long periods of time until favorable conditions are met.
Overwintering and colonization
The life cycle of A. flavus is influenced by environmental factors such as temperature, moisture, pH, oxygen, light, and nutrients. The fungus prefers warm and humid conditions for growth and sporulation, but it can also tolerate high temperatures and low water availability. The optimum temperature for growth is 37 °C (98.6 °F), but the fungus can grow between 12 °C (54 °F) and 48 °C (118 °F). The optimum moisture level for growth is 14%, but it can vary depending on the crop type.
The fungus overwinters in soil or plant debris as conidia or sclerotia. These propagules can survive harsh conditions such as cold, drought, or UV radiation. They can also colonize dead or decaying organic matter and break it down by secreting enzymes. This helps the fungus to obtain nutrients and recycle carbon and nitrogen in the ecosystem.
Infection and infestation
The fungus infects plants or animals by penetrating their tissues with its hyphae or by producing toxins that affect their metabolism or immune system. The fungus can infect seeds, roots, stems, leaves, flowers, fruits, nuts, or pods of various crops such as corn, peanuts, cotton, or nut trees. The fungus can also infect humans or animals by inhalation, ingestion, or wound contamination of conidia or sclerotia.
The infection process involves several steps:
- Attachment: The propagules attach to the surface of the host tissue by physical contact or by using adhesive substances.
- Germination: The propagules germinate and produce germ tubes that grow into hyphae.
- Penetration: The hyphae penetrate the host tissue by mechanical force or by secreting enzymes that degrade the cell wall or membrane.
- Colonization: The hyphae grow and branch inside the host tissue, forming a mycelium that consumes the host nutrients and causes damage or disease.
- Sporulation: The mycelium produces conidiophores and conidia on the surface of the host tissue, forming visible colonies or molds. The conidia can be dispersed to new hosts or substrates, completing the cycle.
The infection can be asymptomatic or symptomatic, depending on the host susceptibility, the fungal virulence, and the environmental conditions. The symptoms can include discoloration, rotting, wilting, shriveling, or aflatoxin contamination of the host tissue. Aflatoxins are toxic and carcinogenic compounds that can cause acute or chronic effects on humans or animals, such as vomiting, liver damage, liver cancer, or death.
Aspergillus flavus is a thermotolerant fungus that can grow in temperatures up to 48°C. It can infect cereals, grains, legumes, and nuts, producing aflatoxins that are carcinogenic and hepatotoxic. In humans, it can cause various types of aspergillosis, depending on the immune status and the site of infection .
Human Aspect of Aspergillus flavus infection
Aspergillus flavus can enter the human body through inhalation of airborne conidia, ingestion of contaminated food, or direct inoculation through wounds or surgery . The conidia can bind to the lung cell basal lamina and germinate into hyphae, causing invasive pulmonary aspergillosis. This condition is mainly seen in immunocompromised patients, such as those with hematologic malignancies, organ transplantation, chemotherapy, or corticosteroid therapy . The hyphae can invade the blood vessels and disseminate to other organs, such as the brain, heart, kidneys, liver, and skin . The symptoms of invasive aspergillosis include fever, cough, chest pain, hemoptysis, dyspnea, and weight loss . The mortality rate of invasive aspergillosis is high, ranging from 30% to 90% .
Aspergillus flavus can also cause non-invasive or chronic forms of aspergillosis, such as allergic bronchopulmonary aspergillosis (ABPA), aspergilloma (fungus ball), chronic pulmonary aspergillosis (CPA), and chronic granulomatous sinusitis . These conditions are more common in patients with underlying lung diseases, such as asthma, cystic fibrosis, tuberculosis, or bronchiectasis . The symptoms of these forms of aspergillosis include cough, wheezing, sputum production, hemoptysis, fatigue, and weight loss .
Aspergillus flavus can also affect other sites of the body, such as the eyes (keratitis), ears (otitis), skin (cutaneous aspergillosis), bones (osteomyelitis), and nails (onychomycosis) . These infections are usually caused by direct inoculation of the fungus through trauma or surgery . The symptoms of these infections vary depending on the site and severity of infection but may include pain, swelling, redness, discharge, ulceration, and loss of function .
Aflatoxicosis is a condition caused by ingestion of aflatoxins produced by Aspergillus flavus. Aflatoxins are potent toxins that can affect the liver, kidneys, immune system, and nervous system . Aflatoxins are mainly found in cereals, grains, legumes, and nuts that are stored in warm and humid conditions . Aflatoxicosis can occur in both humans and animals that consume contaminated food .
The symptoms of aflatoxicosis depend on the dose and duration of exposure to aflatoxins. Acute aflatoxicosis can cause vomiting, abdominal pain, pulmonary edema, hemorrhage, liver damage, coma, and death . Chronic aflatoxicosis can cause growth retardation, malnutrition, immunosuppression, liver cancer (hepatocellular carcinoma), and mental impairment .
Carcinogenic effects of aflatoxins
Aflatoxins are classified as group 1 carcinogens by the International Agency for Research on Cancer (IARC). Aflatoxin B1 is the most potent hepatocarcinogen among aflatoxins and can induce tumors mainly in the liver, but also in the kidneys, lungs, and colon in humans and animals . Aflatoxin B1 can cause DNA damage and mutations that activate oncogenes and inactivate tumor suppressor genes . Aflatoxin B1 can also interfere with the metabolism and detoxification of other carcinogens .
The risk of developing aflatoxin-associated hepatocellular carcinoma (HCC) is influenced by several factors, such as the dose and duration of exposure to aflatoxins, the genetic susceptibility of the individual, and the co-infection with hepatitis B or C viruses . Hepatitis B and C viruses can synergize with aflatoxin B1 to increase the risk of HCC by causing chronic inflammation, cirrhosis, and immunosuppression .
Aspergillus flavus is a ubiquitous mold that can cause a variety of infections in humans, ranging from allergic reactions to invasive and disseminated disease. The most common route of exposure is inhalation of airborne spores, which can colonize the respiratory tract and cause pulmonary aspergillosis. However, other modes of infection are possible, such as ingestion of contaminated food or water, direct inoculation through wounds or surgery, or dissemination from a primary site of infection to other organs.
The clinical manifestations and outcomes of Aspergillus flavus infection depend on several factors, such as the immune status of the host, the underlying conditions or comorbidities, the fungal load and virulence, and the antifungal susceptibility and resistance. The spectrum of aspergillosis can be broadly classified into four categories:
Allergic or eosinophilic disease: This occurs when the host mounts an exaggerated immune response to the fungal antigens, resulting in inflammation and tissue damage. Examples of allergic aspergillosis include allergic bronchopulmonary aspergillosis (ABPA), allergic fungal rhinosinusitis (AFRS), and allergic Aspergillus otitis externa (AAOE). These conditions are typically seen in patients with asthma, cystic fibrosis, chronic rhinosinusitis, or atopy. The symptoms may include wheezing, coughing, nasal congestion, discharge, or polyps, ear pain or discharge, and eosinophilia. The diagnosis is based on clinical criteria, serological tests for specific IgE and IgG antibodies against Aspergillus antigens, and radiological findings. The treatment involves corticosteroids to reduce inflammation and antifungal drugs to eradicate the fungus.
Pulmonary or extrapulmonary colonization: This occurs when the fungus grows on pre-existing lung cavities or bronchiectasis, without invading the surrounding tissues or causing systemic symptoms. Examples of pulmonary colonization include aspergilloma (fungus ball) and chronic pulmonary aspergillosis (CPA). These conditions are typically seen in patients with tuberculosis, sarcoidosis, COPD, or other chronic lung diseases. The symptoms may include hemoptysis, cough, weight loss, fatigue, and chest pain. The diagnosis is based on radiological findings of a mass or cavity with an air crescent sign or a pleural-based lesion, positive culture or histopathology of sputum or bronchoalveolar lavage (BAL) fluid, and serological tests for Aspergillus precipitins. The treatment involves surgery to remove the fungus ball or antifungal drugs to control the fungal growth.
Invasive infection: This occurs when the fungus invades the blood vessels and disseminates to other organs, causing tissue necrosis and organ failure. Examples of invasive infection include invasive pulmonary aspergillosis (IPA), invasive sinusitis, cerebral aspergillosis, endocarditis, osteomyelitis, and cutaneous aspergillosis. These conditions are typically seen in patients with severe immunosuppression, such as hematological malignancies, stem cell or solid organ transplantation, HIV/AIDS, neutropenia, or prolonged use of corticosteroids or immunosuppressive drugs. The symptoms may include fever, cough, dyspnea, chest pain, hemoptysis, sinus pain or discharge, headache, seizures, focal neurological deficits, skin lesions or ulcers, and signs of sepsis or multiorgan dysfunction. The diagnosis is based on radiological findings of nodules, cavities, wedge-shaped infarcts, halo sign (a ring of ground-glass opacity around a nodule), air crescent sign (a crescent-shaped area of air around a nodule), or reversed halo sign (a ring of consolidation around a ground-glass opacity), positive culture or histopathology of blood, sputum, BAL fluid, biopsy, or aspirate specimens, and molecular tests for Aspergillus DNA or galactomannan antigen. The treatment involves aggressive antifungal therapy with amphotericin B, itraconazole, voriconazole, posaconazole, or caspofungin, and supportive care for organ dysfunction. Surgery may be indicated for some cases of sinusitis, cerebral aspergillosis, or endocarditis.
Aflatoxicosis: This occurs when the host ingests food or water contaminated with aflatoxins, which are toxic metabolites produced by Aspergillus flavus and some other Aspergillus species. Aflatoxins can cause acute liver damage, liver cancer, and immunosuppression. They can also affect other organs, such as the kidney, lung, and colon. Aflatoxicosis is more common in tropical and subtropical regions, where food storage and preservation are poor. The symptoms may include vomiting, abdominal pain, pulmonary edema, hemorrhage, jaundice, hepatomegaly, and liver failure. The diagnosis is based on the detection of aflatoxins in blood, urine, or food samples, and the exclusion of other causes of liver disease. The treatment involves supportive care for liver dysfunction, and the prevention of further exposure to aflatoxins.
Aspergillus flavus is a fungal pathogen that can infect various crops, such as corn, peanuts, cottonseed, and tree nuts. It can cause pre-harvest and post-harvest diseases, as well as produce aflatoxins, which are toxic and carcinogenic compounds that can contaminate food and feed.
Some of the plant diseases caused by A. flavus are:
Aspergillus ear rot: It is a disease of corn that occurs when A. flavus infects the ears through the silk or wounds. It causes a powdery olive-green or yellow-green mold on the kernels, which may turn brown with age. The infected kernels may also be discolored, shriveled, or lightweight. The disease is favored by hot and dry conditions during pollination and grain fill, as well as insect damage and poor husk coverage.
Yellow mold of seedlings or aflaroot: It is a disease of peanuts that occurs when A. flavus infects the seeds or seedlings, causing yellowish-brown discoloration and rotting of the roots, hypocotyls, and cotyledons. The disease reduces seed germination, seedling emergence, and plant vigor. It is favored by high soil temperature and moisture, as well as mechanical injury or insect damage to the seeds or seedlings.
Cotton boll rot: It is a disease of cotton that occurs when A. flavus infects the bolls through wounds caused by insects, hail, or mechanical damage. It causes a soft rot of the boll tissues, accompanied by a greenish-yellow mold on the surface. The infected bolls may also produce aflatoxins, which can contaminate the cottonseed and affect its quality and value. The disease is favored by high temperature and humidity, as well as insect infestation and poor boll opening.
Tree nut infection: A. flavus can infect various tree nuts, such as almonds, pistachios, walnuts, pecans, and cashews, causing kernel discoloration, mold growth, and aflatoxin contamination. The fungus can enter the nuts through cracks in the shells or wounds caused by insects or mechanical damage. The infection can occur in the field or during storage, depending on the moisture content and temperature of the nuts. The infected nuts may have reduced quality, flavor, and market value.
Table 1: Summary of plant pathologies caused by Aspergillus flavus
|Corn||Aspergillus ear rot||Powdery olive-green or yellow-green mold on kernels; discolored, shriveled, or lightweight kernels||Hot and dry weather; insect damage; poor husk coverage||Reduced yield and quality; aflatoxin contamination|
|Peanuts||Yellow mold of seedlings or aflaroot||Yellowish-brown discoloration and rotting of roots, hypocotyls, and cotyledons||High soil temperature and moisture; mechanical injury or insect damage to seeds or seedlings||Reduced seed germination, seedling emergence, and plant vigor; aflatoxin contamination|
|Cotton||Cotton boll rot||Soft rot of boll tissues; greenish-yellow mold on boll surface||High temperature and humidity; insect infestation; poor boll opening||Reduced yield and quality; aflatoxin contamination of cottonseed|
|Tree nuts||Tree nut infection||Kernel discoloration; mold growth; aflatoxin contamination||Cracks in shells or wounds in nuts; high moisture content and temperature of nuts in field or storage||Reduced quality, flavor, and market value|
The laboratory diagnosis of Aspergillus flavus infection can be challenging, as the fungus is ubiquitous in the environment and can be isolated from non-invasive samples such as sputum or bronchoalveolar lavage (BAL) fluid. Therefore, a combination of clinical, radiological, microbiological, and immunological criteria is often required to establish a definitive diagnosis.
The microscopic examination of clinical specimens can reveal the presence of characteristic hyphae and conidia of Aspergillus flavus. The hyphae are septate, branched, and hyaline, with acute-angle branching. The conidia are produced on conidiophores that have rough or pitted vesicles with uniseriate or biseriate phialides. The conidia are spherical to ellipsoidal, 3 to 6 µm in diameter, with thin rough walls and a yellow-green color.
A KOH wet mount or a calcofluor white stain can be used to visualize the fungal elements under a light microscope. A lactophenol cotton blue stain can be used to enhance the contrast and morphology of the fungus. A periodic acid-Schiff (PAS) stain or a Grocott-Gomori methenamine silver (GMS) stain can be used to detect the fungus in tissue sections.
The culture of clinical specimens on appropriate media can help to identify the species of Aspergillus by observing the colony morphology and color. The most commonly used media are Sabouraud dextrose agar (SDA), potato dextrose agar (PDA), malt extract agar (MEA), and Czapek yeast agar (CYA). The incubation temperature is usually 25°C or 37°C, and the incubation time is usually 5 to 7 days.
The colony characteristics of Aspergillus flavus on different media are as follows :
- SDA: The colonies are initially white and velvety, then become yellowish-green and floccose with sporulation. Sclerotia may form at the center of the colonies, appearing white at first and then turning brown. The reverse side of the colonies may be yellow or red-brown.
- PDA: The colonies are green due to the conidia, with a flat margin and a raised center. Sclerotia are deep brown and exudates may be colorless or brown. The reverse side of the colonies may be pale or yellow-brown.
- MEA: The colonies are smooth and white at first, then become olive-green with sporulation. Sclerotia are colorless and exudates may be present at the center of the colonies. The reverse side of the colonies may be pale or yellow-brown.
- CYA: The colonies are white and flat with large tufted wool of white mycelia. No sclerotia or pigmentation are produced on this medium.
The molecular identification of Aspergillus flavus can be performed by polymerase chain reaction (PCR) amplification and sequencing of specific regions of the fungal genome, such as the internal transcribed spacer (ITS) region, the beta-tubulin gene, or the calmodulin gene. These methods can provide rapid and accurate identification of Aspergillus species at the sub-species level.
The detection of galactomannan antigen in serum or BAL fluid can be used as a non-invasive diagnostic tool for invasive aspergillosis caused by Aspergillus flavus. Galactomannan is a polysaccharide component of the fungal cell wall that is released during active infection. A commercial enzyme immunoassay (EIA) kit is available for galactomannan detection, with a sensitivity of 70% to 90% and a specificity of 80% to 95%. However, false-positive results may occur due to cross-reactivity with other fungi or bacterial products, or due to contamination from certain antibiotics or food products. Therefore, galactomannan testing should be interpreted in conjunction with other diagnostic criteria.
The detection of antibodies against Aspergillus flavus in serum can be useful for diagnosing allergic bronchopulmonary aspergillosis (ABPA) or chronic pulmonary aspergillosis (CPA). ABPA is a hypersensitivity reaction to Aspergillus antigens in patients with asthma or cystic fibrosis, while CPA is a chronic infection of the lungs with Aspergillus species in patients with underlying lung diseases.
The methods for antibody detection include immunodiffusion, enzyme-linked immunosorbent assay (ELISA), and immunoblotting. The most commonly used antigens are Aspergillus fumigatus or Aspergillus flavus extracts, which can cross-react with other Aspergillus species. The sensitivity and specificity of these methods vary depending on the antigen, the technique, and the clinical condition.
The presence of precipitating antibodies (IgG) against Aspergillus flavus can be demonstrated in 69% to 90% of patients with ABPA, but also in 10% of asthmatics without ABPA and in some cases of CPA. The presence of specific IgE antibodies against Aspergillus flavus can be detected in 80% to 100% of patients with ABPA, but also in 20% to 40% of asthmatics without ABPA and in some cases of CPA. Therefore, antibody testing should be interpreted in conjunction with other diagnostic criteria.
Aspergillus flavus can cause various types of infections in humans and animals, ranging from allergic reactions to invasive and disseminated disease. The treatment and prevention of these infections depend on the type and severity of the infection, the underlying host factors, and the susceptibility of the fungus to antifungal drugs.
The treatment options for different types of aspergillosis are summarized below:
Allergic bronchopulmonary aspergillosis (ABPA): This is a hypersensitivity reaction to Aspergillus antigens in the airways of patients with asthma or cystic fibrosis. The treatment consists of oral corticosteroids to reduce inflammation and oral antifungal drugs, such as itraconazole, to reduce fungal load. The duration of treatment may vary from 3 to 24 months, depending on the response and relapse rate.
Aspergilloma: This is a fungal mass or ball that grows in a pre-existing cavity in the lung, usually caused by tuberculosis or other chronic lung diseases. The treatment depends on the symptoms and the risk of bleeding. Asymptomatic patients may not require any treatment, while symptomatic patients may benefit from oral antifungal drugs, such as itraconazole or voriconazole, to reduce fungal growth and prevent hemoptysis. In some cases, surgical removal of the aspergilloma may be indicated, especially if there is a large cavity, severe bleeding, or poor response to antifungal therapy.
Chronic pulmonary aspergillosis (CPA): This is a progressive infection that causes lung damage and fibrosis in patients with underlying lung diseases or immunodeficiency. The treatment consists of long-term oral antifungal therapy, usually with itraconazole or voriconazole, for 6 to 12 months or longer. The goal of treatment is to improve symptoms, reduce fungal burden, and prevent disease progression. Surgical resection of the affected lung may be considered in selected cases.
Invasive aspergillosis (IA): This is a life-threatening infection that invades the blood vessels and disseminates to various organs, especially in immunocompromised patients. The treatment requires prompt initiation of intravenous antifungal therapy, usually with voriconazole as the first-line agent. Alternative agents include lipid formulations of amphotericin B, posaconazole, isavuconazole, caspofungin, and micafungin. The duration of treatment depends on the clinical response, the resolution of neutropenia or immunosuppression, and the clearance of fungal markers from the blood or other specimens.
The prevention of aspergillosis is challenging because Aspergillus spores are ubiquitous in the environment and difficult to avoid. However, some general measures may help reduce the risk of exposure and infection, especially in high-risk populations:
- Avoid areas with a lot of dust or mold, such as construction sites, excavation sites, compost piles, or decaying vegetation. If unavoidable, wear a N95 respirator or a face mask to filter out fungal spores.
- Avoid activities that involve close contact with soil or dust, such as gardening, yard work, or visiting wooded areas. If unavoidable, wear protective clothing, gloves, and shoes, and wash them after use. Clean any skin injuries well with soap and water if exposed to soil or dust.
- Avoid smoking or inhaling contaminated plant material, such as tobacco, marijuana, or hay.
- Avoid eating moldy food or food that has been stored in humid conditions. Discard any food that shows signs of fungal growth or contamination.
- For patients who are hospitalized or undergoing surgery, follow strict infection control practices to prevent nosocomial transmission of Aspergillus spores.
- For patients who are immunocompromised or have hematologic malignancies or hematopoietic stem cell transplantation (HSCT), consider prophylactic antifungal therapy with agents such as posaconazole, voriconazole, itraconazole, or micafungin . The choice and duration of prophylaxis depend on the level of risk and the local epidemiology of Aspergillus species.
- For patients who are at high risk for invasive aspergillosis but not eligible for prophylaxis, consider early diagnosis and preemptive therapy based on clinical signs and symptoms, radiological findings, and fungal markers such as galactomannan or beta-D-glucan .
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