Safe food microbes

The overwhelming use of antibiotics for about six decades has been determining an increasing development of antibiotic resistance bacteria to conventional antibiotics. In particular, the food chain has been considered the main route for the introduction of antibiotic resistant (AR) bacteria in the human gastrointestinal tract where AR genes may be transferred to pathogen and opportunistic bacteria. In this context, the European Food Safety Authority (EFSA) demands the absence of acquired antibiotic resistance in microorganisms used in food and feed and establishes in FEEDAP technical guidance (EFSA 2012) the criteria for assessing the safety of AR bacteria. Lactic acid bacteria (LAB), because of their large use in fermented food, may represent a potential concern since they may act as AR gene reservoir. In this study, the Minimum Inhibitory Concentrations (MICs) of a collection of 189 LAB strains (69 Streptococcus thermophilus, 52 Lactococcus lactis, 63 and 8 Lactobacillus and Bifidum strains, respectively), used as added starter cultures and/or probiotics in foods, were determined and compared to the epidemiological cut-off values established by EFSA. Approximately 40% of strains showed a resistant phenotype to at least one out of the nine antibiotics (ampicillin, vancomycin, gentamycin, kanamycin, streptomycin, erythromycin, clindamycin, tetracycline and chloramphenicol) under study. As far as streptococci, a high proportion of strains (72%) showed MIC values for kanamycin equal or higher than the cut-off value. Similarly, the 49% of lactococci showed MIC values for streptomycin equal or higher than the breakpoint value, suggesting in these cases a possible revision of the established cut-off values. In addition, the MIC distributions of tetracycline in streptococci and clindamycin in lactococci were bimodal suggesting the potential presence of acquired AR genes in some strains, though in these species the overall incidence of AR strains was low. The distribution of antibiotic resistance was different among different Lactobacillus species and the number of strains evaluated for each species was insufficient to propose cut-off values. However, remarkable AR phenotypes were observed for kanamycin, tetracycline and clindamycin. Furthermore, in order to clarify the genetic basis of the acquired resistances, whole genome sequencing was carried out. First results obtained from genome sequencing of ten microbial strains indicated generally a discrete correlation between phenotype and genotype, with the exception of kanamycin and streptomycin probably due to the occurrence of genomic mutations in indigenous genes. On the whole, the data obtained can give a contribution to the knowledge of antibiotic resistance of LAB and can be used to revise and improve the established cut-off values.