Survey of antibiotic resistance traits in strains of Lactobacillus casei/paracasei/rhamnosus

In this study the occurrence of antibiotic resistance (AR) traits was evaluated in 184 lactic acid bacteria (LAB) strains belonging to the species Lactobacillus casei/paracasei/rhamnosus and isolated from various sources. The strains were tested for resistance at the cut-off values fixed by the European Food Safety Authority (EFSA) for the antibiotics ampicillin, chloramphenicol, clindamycin, gentamicin, erythromycin, kanamycin, streptomycin and tetracycline, in order to distinguish resistant from susceptible strains. The strains that were not inhibited at the cut-off concentrations for one or more antibiotics, namely 27 L. paracasei and 50 L. rhamnosus strains, were further examined for minimum inhibitory concentration (MIC) and presence of acquired genes encoding resistance to the specific antibiotics by PCR assays. A minority of these strains exhibited MIC values that indicated a potentially acquired AR for ampicillin (one L. paracasei strain), clindamycin (two L. paracasei and one L. rhamnosus strains), gentamicin (two L. rhamnosus strains) and tetracycline (two L. paracasei strains and one L. rhamnosus strain); however, the genetic determinants responsible for resistance could not be identified. This study highlighted a low frequency of AR phenotypes and the absence of the most frequently acquired AR genes in the L. casei/paracasei/rhamnosus strains examined, thus evidencing a low risk related to AR dissemination by these bacteria.