Aims:  To assess the ability of five probiotic bacteria to bind aflatoxin B1 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 B1 (AFB1). 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 AFB1. Additionally, the role of teichoic acids in the AFB1 binding process was assessed. Lactobacillus reuteri strain NRRL14171 and Lactobacillus casei strain Shirota were the most efficient strains for binding AFB1. The stability of the AFB1–bacteria complex appears to be related to the binding ability of a particular strain; AFB1 binding was also pH‐dependent. Our results suggest that teichoic acids could be responsible for this ability. Conclusions:  Our results provide information concerning AFB1 binding by previously untested strains, leading to enhanced understanding of the mechanism by which probiotic bacteria bind AFB1. Significance and Impact of the Study:  Our results support the suggestion that some probiotic bacteria could prevent absorption of aflatoxin from the gastrointestinal tract.

SummaryDeletion of the spermidine synthase gene in the fungus Aspergillus nidulans results in a strain, ∆ ∆ ∆ ∆ spdA , which requires spermidine for growth and accumulates putrescine as the sole polyamine. Vegetative growth but not sporulation or sterigmatocystin production is observed when ∆ ∆ ∆ ∆ spdA is grown on media supplemented with 0.05-0.10 mM exogenous spermidine. Supplementation of ∆ ∆ ∆ ∆ spdA with ≥ ≥ ≥ ≥ 0.10 mM spermidine restores sterigmatocystin production and ≥ ≥ ≥ ≥ 0.50 mM spermidine produces a phenotype with denser asexual spore production and decreased radial hyphal growth compared with the wild type. ∆ ∆ ∆ ∆ spdA spores germinate in unsupplemented media but germ tube growth ceases after 8 h upon which time the spores swell to approximately three times their normal diameter. Hyphal growth is resumed upon addition of 1.0 mM spermidine. Suppression of a G protein signalling pathway could not force asexual sporulation and sterigmatocystin production in ∆ ∆ ∆ ∆ spdA strains grown in media lacking spermidine but could force both processes in ∆ ∆ ∆ ∆ spdA strains supplemented with 0.05 mM spermidine. These results show that increasing levels of spermidine are required for the transitions from (i) germ tube to hyphal growth and (ii) hyphal growth to tissue differentiation and secondary metabolism. Suppression of G protein signalling can over-ride the spermidine requirement for the latter but not the former transition.

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In previous communications the essential role of spermidine in Ustilago maydis was demonstrated by means of the disruption of the genes encoding ornithine decarboxylase (ODC) and spermidine synthase (SPE). However, the assignation of specific roles to each polyamine in different cellular functions was not possible because the spermidine added to satisfy the auxotrophic requirement of odc/spe double mutants is partly back converted into putrescine. In this study, we have approached this problem through the disruption of the gene-encoding polyamine oxidase (PAO), required for the conversion of spermidine into putrescine, and the construction of odc/pao double mutants that were unable to synthesize putrescine by either ornithine decarboxylation or retroconversion from spermidine. Phenotypic analysis of the mutants provided evidence that putrescine is only an intermediary in spermidine biosynthesis, and has no direct role in cell growth, dimorphic transition, or any other vital function of U. maydis. Nevertheless, our results show that putrescine may play a role in the protection of U. maydis against salt and osmotic stress, and possibly virulence. Evidence was also obtained that the retroconversion of spermidine into putrescine is not essential for U. maydis growth but may be important for its survival under natural conditions.