Antibiotic Resistance in Lactobacillus reuteri and Lactobacillus plantarum

Lactic acid bacteria (LAB) may act as reservoirs of antibiotic resistance genes that can be transferred via the food chain or within the gastrointestinal tract to pathogenic bacteria. This thesis provides data required for assessing the potential risk of using antibiotic resistant strains of the LAB species Lactobacillus reuteri and Lactobacillus plantarum as food processing aids or probiotics. Knowledge of the distributions of antibiotic minimum inhibitory concentrations (MICs) for a species is needed when using a phenotypic method to differentiate strains with acquired resistance from susceptible strains or strains with intrinsic resistance. Controlled and standardised conditions are required for antibiotic susceptibility testing of LAB, as demonstrated here during evaluation of the Etest and broth microdilution MIC determination methods used. Inoculum size and incubation time were varied during broth microdilution testing of the susceptibilities of 35 LAB strains to six antibiotics. An increase in either parameter resulted in elevated MICs for all species. Antibiotic susceptibility profiles were determined for 56 L. reuteri and 121 L. plantarum strains that differed by source and spatial and temporal origin. MIC data obtained with the Etest and the broth microdilution methods corresponded well with each other. All L. plantarum strains were susceptible to ampicillin, gentamicin, erythromycin and clindamycin, and intrinsically resistant to streptomycin. Acquired resistance to tetracycline was associated with plasmid-bound tet(M). Lactobacillus reuteri strains had acquired resistance to tetracycline (n=28), ampicillin (n=14), erythromycin/clindamycin (n=6) and chloramphenicol (n=1). This resistance was attributed to mutational pbp genes for ampicillin and to added tet(W), erm and cat(TC) genes for the antibiotics inhibiting protein synthesis. Genetic relatedness was observed among L. reuteri strains with high MICs for both ampicillin and tetracycline and among strains with high MICs for both erythromycin and clindamycin. The majority of the antibiotic resistant L. reuteri strains carried the resistance genes on plasmids. Traits of putative transfer machineries adjacent to both plasmidand chromosome-located resistance genes were demonstrated. Lactobacillus reuteri as a donor of resistance genes in the human gut was investigated by studying the transferability of the tetracycline resistance gene tet(W) to faecal enterococci, bifidobacteria and lactobacilli. No gene transfer was demonstrated under the conditions tested.