Antibiotic resistance and plasmid DNA contents of Streptococcus thermophilus strains isolated from Turkish yogurts

Thirty-four Streptococcus thermophilus strains were isolated from yogurt samples obtained from various villages in different regions of Turkey. These strains were examined for their antibiotic resistance patterns and plasmid carriage. The susceptibility of the isolates to 8 antibiotics (vancomycin, tetracycline, chloramphenicol, erythromycin, cephalothin, ampicillin-sulbactam, gentamicin and penicillin G) was determined by the disk diffusion method. Most strains of S. thermophilus were resistant to gentamicin (79%) and penicillin G (64%) and susceptible to chloramphenicol (94%) and tetracycline (88%). In plasmid DNA analyses, it was determined that 7 strains did not contain any plasmid DNA, while other strains included plasmid DNA ranging from 1 to 5. The molecular weights of these plasmids were found to be in various sizes from 1.88 to 19.89 kb. An interaction was observed between the resistance to antibiotics and the occurrence of plasmids in some S. thermophilus strains; for instance, some strains carrying only one or no plasmid were observed to be susceptible to most antibiotics. The resistance to most antibiotics was determined in 3 strains (B55.S, B56.S, B58.S) containing 5 plasmid DNAs. However, no correlation was observed between the resistance to antibiotics and the occurrence of plasmids in some strains.