Han‐Tsung Wang

Aflatoxin production by isolates ofAspergillus flavus varies, ranging from highly toxigenic to completely atoxigenic. Several mechanisms have been identified which regulate aflatoxin production including medium carbon source and oxidative stress. In recent studies, aflatoxin production has been implicated in partially ameliorating oxidative stress inA. flavus. To better understand the role of aflatoxin production in oxidative stress responses, a selection of toxigenic and atoxigenic isolates ofA. flavus with moderate to high oxidative stress tolerance were exposed to increasing concentrations of H₂O₂ in both aflatoxin-conducive and non-conducive media. Mycelial mats were collected for global transcriptome sequencing followed by differential expression, functional prediction, and weighted co-expression analyses. Oxidative stress and medium carbon source had a significant effect on the expression of several secondary metabolite gene clusters including those for aflatoxin, aflatrem, aflavarin, cyclopiazonic acid, and kojic acid. Atoxigenic biological control isolates showed less differential expression under stress than other atoxigenic isolates suggesting expression profiles may be useful in screening. Increasing stress also resulted in regulation of SakA/Hog1 and MpkA MAP kinase signalling pathways pointing to their potential roles in regulating oxidative stress responses. Their expression was also influenced by medium carbon source. These results suggest that aflatoxin production along with that of other mycotoxins may occur as part of a concerted coping mechanism for oxidative stress and its effects in the environment. This mechanism is also regulated by availability of simple sugars and glycolytic compounds for their biosynthesis.

BACKGROUND: Zearalenone (ZEA) is an estrogenic mycotoxin produced by several Fusarium species and frequently contaminates cereals used for food or animal feed. This study attempted to select lactic acid bacteria (LAB) with high esterase activity from the digestive tract, with the goal of using these bacteria for ZEA detoxification.RESULTS: No ZEA activity‐related biotransformation products were observed in three isolates (B1, B2 and D10) during incubation in the presence of ZEA. All three LAB strains were Lactobacillus plantarum, but the API 50 CHL results suggested that the three isolates were different strains. Increased esterase activity was associated with an increase in cell growth, and the ZEA‐detoxifying capabilities of isolates rely on the concentration of bacteria in the culture medium. The lipolytic activity and ZEA removal assay indicated that ZEA degradation by the supernatant fraction was dependent on esterase activity; the supernatant of B2 strain showed the highest ZEA degradation ability and did not release the binding ZEA back into the medium. The D10 strain showed fast ZEA binding ability during the late log phase but began to release the bound ZEA back into the medium after the early stationary phase. All isolates showed good acid and bile salt tolerance ability but all strains showed low adhesion ability to epithelial cells.CONCLUSION: Based on the ZEA removal characterization and ability of the isolates, it is suggested that the isolates could be applied to ZEA detoxification of contaminated feed, but the with the requirement of high cell number for ZEA binding and limited degradation time before absorption of ZEA in the digestive tract. © 2018 Society of Chemical Industry