Aspergillus flavus: Биология, патогенность и токсины.

Species of fungus

Aspergillus flavus is a saprotrophic and pathogenic fungus with a cosmopolitan distribution. It is best known for its colonization of cereal grains, legumes, and tree nuts. Postharvest rot typically develops during harvest, storage, and/or transit. Its specific name flavus derives from the Latin meaning yellow, a reference to the frequently observed colour of the spores. A. flavus infections can occur while hosts are still in the field (preharvest), but often show no symptoms (dormancy) until postharvest storage or transport. In addition to causing preharvest and postharvest infections, many strains produce significant quantities of toxic compounds known as mycotoxins, which, when consumed, are toxic to mammals. A. flavus is also an opportunistic human and animal pathogen, causing aspergillosis in immunocompromised individuals.

Aspergillus flavus is found globally as a saprophyte in soils and causes disease on many important agriculture crops. Common hosts of the pathogen are cereal grains, legumes, and tree nuts. Specifically, A. flavus infection causes ear rot in corn and yellow mold in peanuts either before or after harvest. Hyphal growth usually occurs by thread like branching and produces mycelia. Hyphae are septate and hyaline. Once established, the mycelium secretes degradative enzymes or proteins which can break down complex nutrients (food). Individual hyphae strands are not typically seen by the unaided eye; however, conidia producing thick mycelial mats are often seen. The conidiospores are asexual spores produced by A. flavus during reproduction. The conidiophores of A. flavus are rough and colorless. Phialides are both uniseriate (arranged in one row) and biseriate. Sexual reproduction occurs between sexually compatible strains belonging to different vegetative compatibility groups. Aspergillus flavus is complex in its morphology and can be classified into two groups based on the size of sclerotia produced. Group I consists of L strains with sclerotia greater than 400 μm in diameter. Group II consists of S strains with sclerotia less than 400 μm in diameter. Both L and S strains can produce the two most common aflatoxins (B1 and B2). Unique to the S strains is the production of aflatoxin G1 and G2 which typically are not produced by A. flavus.

Aspergillus flavus overwinters in the soil and appears as propagules on decaying matter, either as mycelia or sclerotia. Sclerotia germinate to produce additional hyphae and asexual spores called conidia. These conidia are said to be the primary inoculum for A. flavus. The propagules in the soil, which are now conidia, are dispersed by wind and insects (such as stink bugs or lygus bugs). The conidia can land on and infect either grains or legumes. The spores enter the corn through the silks and thus infect the kernel. Conidiophores and conidia are produced in the spring from sclerotial surfaces. There is a secondary inoculum for A. flavus, which is conidia on leaf parts and leaves. A. flavus grows on leaves after damage by leaf feeding insects. Insects are said to be a source of inoculum and promote inoculum production.

Aspergillus flavus is unique in that it is a thermotolerant fungus, so can survive at temperatures that other fungi cannot. A. flavus can contribute to the storage rots, especially when the plant material is stored at high moisture levels. A. flavus grows and thrives in hot and humid climates. Moisture: A. flavus growth occurs at different moisture levels for different crops. For starchy cereals, growth occurs at 13.0–13.2%. For soybeans, growth occurs at 11.5–11.8%. For other crops, growth occurs at 14%.

To ensure grains and legumes remain free of A. flavus infection, certain conditions must be incorporated before, during, and after harvest. Moisture levels should be kept below 11.5%. Temperature in storage units should be kept as low as possible since the pathogen is unable to grow below 5 °C. The low temperature facilitates slower respiration and prevents moisture increase. Fumigants are used to decrease the occurrence and persistence of insects and mites, which aids the rapid growth of the pathogen. Sanitary practices including, removing old and unripe seeds, exclusion of damaged and broken seeds, and overall cleanliness assist in minimizing the colonization and spread of the pathogen. The yeast successfully competes with A. flavus for space and nutrients, ultimately limiting its growth. Essential oils of Glycyrrhiza glabra inhibit growth.

Aspergillus flavus strain AF36 is noncarcinogenic and aflatoxin free and is used as an active ingredient in pesticides. AF36 is a fungal antagonist and is applied as a commercial biocontrol to cotton and corn to reduce aflatoxin exposure. AF36 was initially isolated in Arizona and has also occurred in Texas. It is grown on sterile seeds which serve as the carrier and a source of nutrients. Following application and colonization and in the presence of high moisture, AF36 growing seeds outcompete aflatoxin producing strains of A. flavus. Nonaflatoxin spore dispersal is aided by wind and insects.

Aspergillus flavus infections will not always reduce crop yields alone; however, postharvest disease can reduce the total crop yield by 10 to 30%, and in developing countries that produce perishable crops, total loss can be greater than 30%. In grains and legumes, postharvest disease results in the production of mycotoxins. The amount of aflatoxins produced by A. flavus is affected by environmental factors. If other competitive fungal organisms are present on host plants, aflatoxin production is low. However, if noncompetitive fungal organisms are present on host plants, aflatoxin production can be quite high. The nature of the host is also an important factor in aflatoxin production. High A. flavus growth on soybean produces very little aflatoxin. High A. flavus growth aided by increased moisture content and warm temperatures on peanut, nutmeg, and peppers produces high concentrations of aflatoxins. A. flavus growth on spices produces low concentrations of aflatoxin as long as the spices remain dry. The deaths of ten conservationists present at the opening of a 15th century tomb in Kraków, Poland in the 1970s has been attributed to aflatoxins originating from A. flavus present in the tomb.

After the premature death of several Polish scientists following the 1973 opening of the tomb of the 15th century Polish King (and Lithuanian Grand Duke) Casimir IV Jagiellon, microbiologist Bolesław Smyk identified the presence of the fungus Aspergillus flavus in samples taken from the tomb, and media reports have suggested that the likely cause of the deaths were the aflatoxins produced by this fungus.