Abstract: Bifidobacterium longum is a commonly used probiotic in food industry, and its drug resistance is related to food safety issues. The objective of this study was to formulate antibiotic resistance determination criteria for B. longum and to explore the key genes of antibiotic resistance through functional gene analysis. The minimum inhibitory concentrations of 52 B. longum strains towards tetracycline, erythromycin, clindamycin, ampicillin, chloramphenicol, and vancomycin were determined with reference to ISO standard methods (ISO 10932). The species-specific microbiological cut-off values of these six antibiotics of B. longum were evaluated by Turnidge (T) and Kronvall (K) statistical methods and analyzed the antibiotic resistance mechanism from the molecular genetic basis. The results showed that the species-specific microbiological cut-off values for tetracycline, erythromycin, clindamycin, ampicillin, chloramphenicol, and vancomycin of B. longum were 8, 8, 0.25, 8/2(T/K), 8 and 2 μg/mL respectively, and the drug resistance rates were 28.85%, 25%, 28.85%, 3.85%/7.69%(T/K), 0 and 19.23%. Functional gene and comparative genomic analysis revealed that tet(W) and erm(X) both located on mobile genetic elements were the main resistance genes for tetracycline resistance and erythromycin and clindamycin resistancein B.longum.Phylogeneticanalysis indicated that the tet(W) and erm(X) gene sequences of different phenotypic resistance strains possessed significant variation. This study has certain guiding significance and reference value for the safety application of B. longum in food and the development of evaluation criteria for probiotics resistance.
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