Key role of teichoic acids on aflatoxin B binding by probiotic bacteria.

AIMS: To assess the ability of five probiotic bacteria to bind aflatoxin B(1) and to determine the key role of teichoic acids in the binding mechanism. METHODS AND RESULTS: The strains were incubated in aqueous solutions containing aflatoxin B(1) (AFB(1)). The amount of free toxin was quantified by HPLC. Stability of the bacteria-aflatoxin complex was evaluated by repeated washes with buffer. In order to understand the binding process, protoplasts, spheroplasts and cell wall components of two strains were analysed to assess their capacity to bind AFB(1). Additionally, the role of teichoic acids in the AFB(1) binding process was assessed. Lactobacillus reuteri strain NRRL14171 and Lactobacillus casei strain Shirota were the most efficient strains for binding AFB(1). The stability of the AFB(1)-bacteria complex appears to be related to the binding ability of a particular strain; AFB(1) binding was also pH-dependent. Our results suggest that teichoic acids could be responsible for this ability. CONCLUSIONS: Our results provide information concerning AFB(1) binding by previously untested strains, leading to enhanced understanding of the mechanism by which probiotic bacteria bind AFB(1). SIGNIFICANCE AND IMPACT OF THE STUDY: Our results support the suggestion that some probiotic bacteria could prevent absorption of aflatoxin from the gastrointestinal tract.

Aflatoxin synthesis in corn fields in Guanajuato, Mexico.

Aflatoxin contamination of corn is an important problem internationally, particularly in tropical and subtropical conditions that favor infection and synthesis by Aspergillus. Environmental conditions (drought) and agronomic practices i.e. N fertilization have been reported as favorable to aflatoxin synthesis in the field. This study was undertaken to investigate whether the contamination of corn commonly observed in stored conditions in this important corn producing region of Mexico known as "El Bajio" is related to infection by Aspergillus under field conditions. Results using three corn hybrids of recognized susceptibility to infection showed that corn ears artificially inoculated in the field with a toxigenic strain of Aspergillus parasiticus presented a low content of aflatoxin ranging from 13.6 to 24.7 microg Kg(-1). No significant differences were observed between the hybrids tested. Similarly, N fertilization practices, 260 Kg N ha(-1), applied at sowing did not have an effect on the extent of the contamination observed of 6.2 and 19.3 mg of aflatoxin kg(-1) in natural infected and inoculated samples with A. parasiticus NRRL 2999, respectively. Our data suggest that the cases of aflatoxin contamination of corn in this part of Mexico are not related to infection occurring during the crops growing period but most probably to poor storage conditions of corn.

Correlation between the regulation of sterigmatocystin biosynthesis and asexual and sexual sporulation in Emericella nidulans.

We analyzed the regulation of sterigmatocystin biosynthesis in wild type and mutant strains of Emericella nidulans (= Aspergillus nidulans). A positive correlation between both asexual and sexual sporulation and synthesis of the mycotoxin was observed. Those conditions which favored sporulation stimulated sterigmatocystin formation, and vice versa. Both processes were stimulated by light in a veA+ genetic background. In contrast, they were inhibited by diaminobutanone, an inhibitor of ornithine decarboxylase. The effect of this inhibitor was partially reverted by putrescine addition. Partial supplementation of specific requirements to auxotrophic mutants allowed normal vegetative growth, but interfered with asexual sporulation and sterigmatocystin biosynthesis. Synthesis of the mycotoxin was neither affected in a brlA mutant or in developmental mutants blocked at later steps in sporulation. As in wild type strain, diaminobutanone inhibited sterigmatocystin biosynthesis and cleisthotecia formation in the brlA mutant, and its effect was reverted by addition of putrescine. The inhibitor also affected the transcription of brlA. Our results indicate that sporulation and the synthesis of sterigmatocystin are co-regulated at a step previous to the brlA execution point.

Relationship between aflatoxin biosynthesis and sporulation in Aspergillus parasiticus.

Regulation of aflatoxin biosynthesis during differentiation of Aspergillus parasiticus was analyzed by using a drug that inhibits the development of the fungus and mutants affected in sporulation. Diaminobutanone, a competitive inhibitor of ornithine decarboxylase, repressed spore germination. If added after spore germination had occurred, it blocked sporulation completely and suppressed aflatoxin biosynthesis, but was only partially inhibitory of mycelial growth. Putrescine partially counteracted the inhibitory effect of the drug on both sporulation and aflatoxin biosynthesis. Analysis of mutants affected in sporulation confirmed the existence of a relationship between sporulation and aflatoxin formation. A nonsporulating mutant was unable to synthesize aflatoxins. In a sectorial mutant, the sporulating sector synthesized aflatoxins normally, whereas the asporogenous sector was unable to do so. It is suggested that regulation of aflatoxin biosynthesis is correlated with the sporulation process.