Relationship between tetracycline antibiotic susceptibility and genotype in oral cavity Lactobacilli clinical isolates.

Background: Antibiotic resistance, is often conferred by the presence of antibiotic resistance genes. This study aimed to investigate the relationship between tetracycline resistance (Tet-R) and genotype in 31 Lactobacillus isolates from caries-active patients.

Methods: The tetracycline susceptibility of Lactobacillus isolates was determined using the agar spot test and the genetic characteristics associated with tetracycline resistance using whole-genome sequencing (WGS).

Results: The minimum inhibitory concentration (MIC) values of most isolates were equal to or lower than the breakpoint MIC values. Four strains that were phenotypically more sensitive ( L. fermentum B09, S23 and L. rhamonsus B17) or more resistant ( L. plantarum B43) than other isolates to tetracycline were subjected to conduct whole-genome sequencing in order to detect the tetracycline resistance genes. The results revealed that the most common Tet-R genes in Lactobacillus strains were tet T, tet W, tet O and tet L. In addition, tet PB, tcr 3 and otr A were detected for the first time. There were distinct Tet-R gene mutations in Lactobacillus isolates. Overall, the mean expression values of Tet-R-mutated genes in L. plantarum B43 were elevated, and the relative expression levels of tet T and tet W genes in L. rhamonsus B17 L. fermentum B09 and S23 were decreased relative to reference strains.

Conclusion: The results of this study indicate that Lactobacillus isolates from saliva of caries-active patients do not present considerable tetracycline resistance reservoirs. However, genetic compounds associated with tetracycline resistance were identified by whole-genome sequencing, providing meaningful insights into tetracycline resistance mechanisms.