Bile tolerance and its effect on antibiotic susceptibility of probiotic Lactobacillus candidates

Lactobacilli have been crucial for the production of fermented products for centuries. They are also members of the mutualistic microbiota present in the human gastrointestinal and urogenital tract. Recently, increasing attention has been given to their probiotic, health-promoting capacities. Many human intervention studies demonstrating health effects have been published. However, as not all studies resulted in positive outcomes, scientific interest arose regarding the precise mechanisms of action of probiotics. Many reported mechanistic studies have addressed mainly the host responses, with less attention being focused on the specificities of the bacterial partners, notwithstanding the completion of Lactobacillus genome sequencing projects, and increasing possibilities of genomics-based and dedicated mutant analyses. In this emerging and highly interdisciplinary field, microbiologists are facing the challenge of molecular characterization of probiotic traits. This review addresses the advances in the understanding of the probiotic-host interaction with a focus on the molecular microbiology of lactobacilli. Insight into the molecules and genes involved should contribute to a more judicious application of probiotic lactobacilli and to improved screening of novel potential probiotics.

Bacteria used as probiotics or in starter cultures may serve as hosts of antibiotic resistance genes, which can be transferred to pathogenic bacteria. Before launching a starter culture or a probiotic product into the market, it is therefore important to verify that the single bacterial isolates (strains) do not contain transferable resistance genes. A study has been undertaken to establish the levels of susceptibility of Lactobacillus spp. to various antimicrobial agents. This is a prerequisite for differentiating putative transferable resistance from natural resistance. A selection of 62 strains has been screened with the use of the Etest (ABBiodisk, Stockholm, Sweden) for their susceptibility to 25 antimicrobial agents. The strains belonged to the following species: Lactobacillus plantarum/pentosus, L. rhamnosus, L. paracasei, L. sakei, L. curvatus and species of the L. acidophilus group: L. johnsonii, L. crispatus, L. gasseri, and L. acidophilus. The results from the Etests have shown that the level of susceptibility to the antimicrobial agents is species-dependent. For the following antimicrobial agents, susceptibility varied several folds between species: vancomycin, teicoplanin, tetracycline, norfloxacin, ciprofloxacin, fusidic acid, and clindamycin. The differences between the species were more subtle for the rest of the tested antimicrobial agents. On the basis of the result, it was possible to suggest minimal inhibition concentrations (MICs) for the individual Lactobacillus species to be used as a microbiological breakpoint when screening strains for transferable resistance genes.

Over the last 50 years, human life expectancy and quality of life have increased dramatically due to improvements in nutrition and the use of antibiotics in the fight against infectious diseases. However, the heyday of antibiotic treatment is on the wane due to the appearance and spread of resistance among harmful microorganisms. At present, there is great concern that commensal bacterial populations from food and the gastrointestinal tract (GIT) of humans and animals, such as lactic acid bacteria (LAB) and bifidobacteria, could act as a reservoir for antibiotic resistance genes. Resistances could ultimately be transferred to human pathogenic and opportunistic bacteria hampering the treatment of infections. LAB species have traditionally been used as starter cultures in the production of fermented feed and foodstuffs. Further, LAB and bifidobacteria are normal inhabitants of the GIT where they are known to exert health-promoting effects, and selected strains are currently been used as probiotics. Antibiotic resistance genes carried by LAB and bifidobacteria can be transferred to human pathogenic bacteria either during food manufacture or during passage through the GIT. The aim of this review is to address well-stated and recent knowledge on antibiotic resistance in typical LAB and bifidobacteria species. Therefore, the commonest antibiotic resistance profiles, the distinction between intrinsic and atypical resistances, and some of the genetic determinants already discovered will all be discussed.