Table of Contents
- Introduction to Aspergillus flavus
- Habitat of Aspergillus flavus
- Morphology of Aspergillus flavus
- Cultural characteristics of Aspergillus flavus
- The life cycle of Aspergillus flavus
- Pathogenesis and Clinical manifestations of Aspergillus flavus
- Laboratory Diagnosis for Aspergillus flavus
- Treatment of Aspergillus flavus infections
- Prevention and Control
- References
Introduction to Aspergillus flavus
- Aspergillus flavus is the second most common Aspergillus species affecting humans, after Aspergillus fumigatus.
- It is a mildly pathogenic, saprophytic mold commonly responsible for diseases in plants such as grains, cereals, trees, and nuts.
- The mold causes opportunistic infections in crops, impacting both pre-harvest and post-harvest stages.
- Infections can occur before harvesting, during which the fungus remains dormant until harvest time, after which it triggers yellowing in the infected plant parts.
- Post-harvest, A. flavus can continue to infect crops stored in storage rooms.
- It is capable of producing mycotoxins, which can lead to poisoning in both humans and animals.
- In addition to plant infections, A. flavus can cause opportunistic infections such as aspergillosis in immunocompromised humans and animals.
Habitat of Aspergillus flavus
- Aspergillus flavus is globally widespread and commonly inhabits soil.
- In soil, it exists as conidia or sclerotia, while in plant tissues, it occurs as mycelia.
- It is hosted by cereal grains, legumes, and tree nuts.
- Being thermotolerant, it can survive on a wider range of surfaces compared to many other fungi.
- It thrives in high-moisture, hot, and humid environments.
- It grows at a minimum temperature of 12 °C (54 °F), a maximum temperature of 48 °C (118 °F), and has an optimal growth temperature of 37 °C (98.6 °F).
- The optimum moisture content for its growth is around 14%, although this varies with crop type—for starchy cereals, it grows at 13–13.2% moisture levels, while for soybeans, it grows at 11.5–11.9%.
- Sclerotia can survive in soil under severe environmental conditions, producing conidia and possibly ascospores (as per recent data), which can cause population surges under hot and drought conditions.
Morphology of Aspergillus flavus
- The morphology of Aspergillus flavus is complex and is classified based on sclerotia formation:
- Group I (L-strains) have sclerotia larger than 400 µm in diameter.
- Group II (S-strains) have sclerotia smaller than 400 µm in diameter.
- The fungus exhibits both sexual and asexual reproduction.
- Asexual reproduction produces conidial spores and sclerotia, while sexual reproduction produces sclerotia.
- Asexual spores (conidia) are produced from the phialides located on the conidiophore vesicles.
- Conidial spores form a thick mycelial mat visible to the naked eye and measure 3–6 µm in size.
- Conidiophores develop from hyphal threads, are colorless, and have a rough texture.
- Phialides arising from conidiophores can be uniseriate or biseriate in arrangement.
- Hyphae occur as thread-like septate branches forming a mycelium, with each septum containing hyaline.
- The hyphae are extremely small and not visible to the naked eye.
- Sexual reproduction results in ascospores contained within the sclerotia.
Cultural characteristics of Aspergillus flavus
Sabouraud Dextrose Agar
- On Sabouraud Dextrose Agar, Aspergillus flavus forms white, soft, velvety colonies that gradually turn yellowish-green due to the pigment of the conidial spores.
Potato Dextrose Agar
- On Potato Dextrose Agar, the conidia display a characteristic green color, and the sclerotia form deep brown masses.
Malt extract agar
- On Malt Extract Agar, the mycelia initially appear smooth and later change to olive green, with sclerotia that are colorless.
Czapek yeast agar
- On Czapek Yeast Agar, colonies develop after 7 days of incubation at 25 °C and 37 °C, appearing velutinous, grey-blue-green, with uniseriate conidial heads.
The life cycle of Aspergillus flavus
- Aspergillus flavus survives the winter in soil, existing as propagules on decaying matter in the form of mycelia or as a thick, hard mass of mycelia known as sclerotia.
- The sclerotia germinate, producing hyphae and asexual spores called conidia.
- Conidia disperse into the air and environment through insects (bugs) and wind-assisted pollination.
- When conidia land on grains and legumes, they infect them—entering through the silks of corn into the corn kernel.
- The fungus grows, producing conidiophores and conidia from the surface of the sclerotia.
- Some conidia land on damaged leaf surfaces previously fed on by insects, causing secondary inoculation.
- Other spores may be dispersed into the soil by rainwater, leading to infection of oil-rich plants such as peanuts and cotton seeds.
Pathogenesis and Clinical manifestations of Aspergillus flavus
Aspergillus flavus is pathogenic to both plants and animals, including humans. It infects damaged plants and causes opportunistic infections in immunocompromised hosts.
Human Aspect of Aspergillus flavus infection
- Conidial spores of A. flavus bind to the lung cell basal lamina, initiating invasive aspergillosis.
- This binding is facilitated by proteins such as fibronectin, laminin, type IV collagen, fibrinogen, complement, albumin, and surfactant proteins.
- The fungus has a strong attraction to fibrinogen proteins, which enhances its adhesion to the basal lamina.
Aspergillosis
- A. flavus is the second leading cause of invasive and non-invasive aspergillosis and the most common cause of superficial infections.
- Clinical forms include:
- Allergic: Extrinsic asthma, extrinsic allergic alveolitis, and allergic bronchopulmonary aspergillosis (ABPA).
- Pulmonary and extrapulmonary colonization.
- Invasive infections: Pulmonary and extrapulmonary aspergillosis.
- Other possible manifestations:
- Chronic granulomatous sinusitis
- Keratitis
- Cutaneous aspergillosis
- Wound infections
- Osteomyelitis following trauma and fungal inoculation
- Fungal sinusitis results from deposition of large spores in the upper respiratory tract.
- Secondary transmission may occur through:
- Infection of wounds
- Smoking contaminated plant materials such as tobacco or marijuana
- Nosocomial infections can occur during surgical procedures, especially in transplant patients.
Aflatoxicosis
- Caused by ingestion of aflatoxins produced by the fungus.
- Sclerotia forms produce aflatoxins, mainly B1 and B2.
- The S-strain produces aflatoxin G1 and G2, which are not commonly produced by A. flavus.
- The L-strain is more aggressive but produces fewer aflatoxins, fewer sclerotia, and more acidity.
- Aflatoxins are carcinogenic and cause aflatoxicosis, leading to:
- Vomiting
- Abdominal cramping and pain
- Pulmonary edema
- Hemorrhaging
- Disruption of food digestion
- Poor absorption and metabolism in the abdomen
- Severe liver damage and liver cancer
- Mental impairment
- Coma and death
Carcinogenic effects of aflatoxins
- Long-term exposure to aflatoxin B1 can cause cancer, mainly hepatocellular carcinoma (HCC), but also tumors in the kidney, lungs, and colon in both humans and animals.
- HCC is strongly linked to aflatoxin B1 ingestion.
- Individuals with chronic Hepatitis B or C have a higher risk, as these conditions synergistically interact with aflatoxin B1 to increase HCC development.
Plant pathologies
- Colonization of plants is aided by dispersion methods and damage caused by insects or herbivores.
- Damaged plant surfaces provide entry points for the fungus.
- Insects and wind disperse spores to damaged surfaces, where they remain dormant until harvest and storage.
- During storage, the fungus germinates and spreads to nearby crops.
- On plants, colonization appears as:
- Powdery yellowish-green spores on the upper surface.
- Reddish-gold spores on the lower surface.
- In grains and legumes, infections are usually localized to small areas with discoloration and dullness of affected parts.
- Growth is rapid, with colonies appearing downy or powdery in texture.
Aspergillus Ear Rot
- Caused by aflatoxins from A. flavus.
- Appears as powdery olive-green (yellow-green) mold on corn ears, turning brown as masses age.
- High aflatoxin levels are associated with discolored, shriveled kernels, often near the ear tip.
- Infection is favored by hot, dry conditions during pollination and grain fill.
- Yellow-brown silks are most susceptible to infection.
- Infection process:
- Spores land on silks.
- Germinate and rapidly grow down the silk.
- Colonize the surface of developing kernels.
- As plants mature and moisture levels drop, the fungus invades internal tissues and continues to grow until moisture falls below 15%.
Laboratory Diagnosis for Aspergillus flavus
Microscopic Examination
KOH wet mount:
- Reveals uncolored, thick-walled conidiophores with rough or pitted vesicles.
- Vesicles measure approximately 800–1200 µm in diameter and produce phialides.
- Phialides may be uniseriate, biseriate, or a combination of both.
- Conidia measure 250–450 µm with thin, rough walls.
Culture Observation
Sabouraud Dextrose Agar (SDA)
- Early growth (24–48 hours): Colonies are white with a soft, velvety surface.
- After 4 days: Colonies become raised with a floccose center.
- Upon sporulation: Colonies turn yellowish-green due to conidia pigmentation.
- Sclerotia: Initially white, turning brown after 6 days.
- Colony diameter: 55–70 mm.
Potato Dextrose Agar (PDA)
- Produces green conidia with a dominated colony appearance.
- Colony edges are plain and flat, while the center is raised with a wrinkled cerebriform pattern.
- Produces colorless or brown exudates.
- Sclerotia: Compact masses of hardened fungal mycelia, deep brown in color.
- Colonies surrounded by a white border with a pale inner side.
Malt Extract Agar (MEA)
- Colonies are white, varying in shape and size.
- Initial growth: Smooth white mycelia.
- Later stages: Olive and dark green conidia form.
- Sclerotia: White and deep brown with colorless exudates at the colony center.
Czapek Yeast Agar (CYA)
- White, flat mycelia with large raised tufts of white woolly mycelia.
- Colonies are dry with exudates but no sclerotia production.
- No pigmentation—colonies appear uncolored.
- Some isolates produce velutinous, grey-blue-green colonies with uniseriate conidial heads.
Aflatoxin Detection
- Thin Layer Chromatography (TLC) is used for detecting aflatoxins.
- Plates are observed under a fluorescent microscope for aflatoxin visualization.
Treatment of Aspergillus flavus infections
- Aspergillus flavus infections are generally treated with antifungal drugs such as amphotericin B, itraconazole, voriconazole, posaconazole, and caspofungin.
- In cases where resistance develops, combination therapy may be applied.
- Essential oils derived from plants such as Cinnamomum zeylanicum (cinnamon), Mentha piperita (peppermint), Ocimum basilicum (basil), Origanum vulgare (origanum), Teloxys ambrosioides (epazote), Syzygium aromaticum (clove), and Thymus vulgaris (thyme) can be used for crop storage (e.g., maize) to inhibit the growth of A. flavus.
- Thymol and o-methoxycinnamaldehyde have been shown to significantly reduce contamination of maize grains.
Prevention and Control
- Avoid exposure to fungal spores, especially if allergic to fungi.
- Use prophylactic treatment with amphotericin B and itraconazole to help prevent Aspergillus flavus infections.
- Remove mold-forming materials and discard any food exposed to fungal toxins before ingestion.
References
- Alessandro C. P., 2009. Differences in pathogenicity and clinical syndromes due to Aspergillus fumigatus and Aspergillus flavus. Medical Mycology Review, pp. S261–S270.
- Klich, M. A., 2007. Aspergillus flavus: the major producer of aflatoxin. Molecular Plant Pathology, DOI: 10.1111/j.1364-3703.2007.00436.x.
- Raymond, J., Steven, E., Screen, and Bijan Shams-Pirzadeh, 2000. Lack of host specialization in Aspergillus flavus. Applied and Environmental Microbiology, 66(1), pp. 320–324.
- Fausto, A., Marcio, L. R., and Carolina, C., 2019. The still underestimated problem of fungal disease worldwide. Frontiers in Microbiology, 2019/00214.
- McClenny, N. Laboratory detection and identification of Aspergillus species by microscopic observation and culture: The traditional approach. Medical Mycology, 43, pp. S125–S128.
- Hindawi. (2017). Aspergillus flavus [Online]. Available at: https://www.hindawi.com/journals/ijmicro/2017/5273893/
- Microbiology Research. Aspergillus flavus article. Available at: https://www.microbiologyresearch.org/content/journal/micro/10.1099/mic.0.2007/007641-0?crawler=true
- Iowa State University Extension and Outreach. (2012). Aspergillus ear rot and aflatoxin production. Available at: https://crops.extension.iastate.edu/cropnews/2012/08/aspergillus-ear-rot-and-aflatoxin-production
- ScienceDirect. Aspergillus flavus. Available at: https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/aspergillus-flavus
- Bugwood Wiki. Aspergillus flavus. Available at: https://wiki.bugwood.org/Aspergillus_flavus
- Biology Discussion. Aspergillus: Habitat, reproduction, and importance (Ascomycotina). Available at: http://www.biologydiscussion.com/fungi/aspergillus-habitat-reproduction-and-importance-ascomycotina/24000
- Bust Mold. Aspergillus flavus. Available at: https://www.bustmold.com/resources/mold-library/aspergillus-flavus/
- AGRIS FAO. Aspergillus flavus record. Available at: http://agris.fao.org/agris-search/search.do?recordID=QY870002588
