Susceptibility to tetracycline and erythromycin of Lactobacillus paracasei strains isolated from traditional Italian fermented foods

The aim of this study was to evaluate the susceptibility of 197 isolates of Lactobacillus paracasei, isolated from Italian fermented products coming from different geographical areas, to tetracycline and erythromycin, two antimicrobials widely used in clinical and animal therapy. Isolation media were supplemented with antibiotics according to the microbiological breakpoints (BPs) defined by European Food Safety Authority (EFSA). Isolates were identified at the species level and were typed by rep-PCR using the (GTG)5 primer. A total of 121 genotypically different strains were detected and their phenotypic antimicrobial resistance to tetracycline and erythromycin was determined as the minimum inhibitory concentration (MIC) using the broth microdilution method. The presence of the genes ermB, ermC and tetL, tetM, tetS, tetW, in the phenotypically resistant isolates was investigated by PCR. Tetracycline induction of tetM expression on representative resistant strains, grown in medium either lacking or containing the antibiotic, was also analyzed by RT-PCR. the 121 tested strains, 77.7% were susceptible to tetracycline (MIC ≤ 4 µg/ml; BP = 4 µg/ml) and 94.2% to erythromycin (MIC ≤ 1 µg/ml; BP = 1 µg/ml). Overall, 27 strains appeared to be resistant to tetracycline (16 ≤ MIC ≤ 128 µg/ml ) (TetR), and 7 of them were also resistant to erythromycin (MIC ≥ 1024 µg/ml) (ErmR). The tetM and ermB genes were the most frequently detected in the TetR and/or ErmR strains. The tetM expression was induced by antibiotic addition to the growth medium. Our study confirmed that L. paracasei is quite sensitive to tetracycline and erythromycin, but the high level of resistance of ErmR strains suggested that acquired resistance took place. Further investigations are required to analyze whether the genes identified in L. paracasei isolates might be horizontally transferred to other species. Since “commensal” bacteria, which L. paracasei belongs to, may play an active role in the spreading of antibiotic resistance, a series of measures inspired from a principle of precaution should be taken before they are used as commercial starters or probiotic cultures in food products, complemented by a more prudent use of antibiotics in agriculture, veterinary, and human medicine.