Investigation of the prevalence of Tn916/Tn916-like elements in oral streptococci

Streptococcus species are the most abundant genera in the oral microbiota. The prevalence of antibiotic resistant oral streptococci is on the increase and the most reported antibiotic resistance profile in oral streptococci include, among several antibiotics, resistance to β-lactams antibiotics, macrolides and tetracycline. Several studies have shown that majority of the antibiotic resistance genes that have been isolated in oral streptococci are usually associated with mobile genetic elements such as plasmids and conjugative transposons. In the current study, we used molecular based techniques to screen a total of 100 clinical oral streptococci strains isolated in Norway for the presence of Tn916/Tn916-like elements. We also investigated the diversity of these elements in oral streptococci by Restriction Fragment Length Polymorphism (RFLP) and DNA sequencing. It was found that a total of 34 strains were positive for tetracycline resistance gene, tet(M), and integrase gene (int) belonging to the Tn916-like family. The minimum inhibitory concentration (MIC) of these tetracycline-resistant oral streptococci ranged between 2-64 µg/mL. Most of the positive strains for tet(M) and int genes harbor the wild type Tn916 and a few harbor other Tn916-like elements suggesting high versatility of the Tn916 element in oral streptococci in Norway.