Detection and characterisation of genes encoding antibiotic resistance in the cultivable oral microflora.

The emergence of antibiotic-resistant bacteria has become a major threat to public health. The increased use of antibiotics has selected for the dissemination of antibiotic resistance genes between organisms from different species and different genera. There is a large body of evidence that the indigenous microbiota can act as a reservoir of antibiotic-resistant bacteria. However little is known about the molecular basis for this in bacteria from the oral cavity. Therefore the aim of this work was to determine the prevalence of antibiotic-resistant bacteria and antibiotic resistance genes in the cultivable oral microbiota. Saliva and plaque samples were taken from each of 60 healthy adults who had not taken any antibiotics during the previous three months. Each sample was plated onto antibiotic-containing media to quantitate and identify antibiotic-resistant strains. All of the individuals harboured bacteria resistant to erythromycin, gentamicin, vancomycin and tetracycline. Only 4 individuals (7%) did not have any cultivable bacteria resistant to amoxycillin. Oral bacteria resistant to gentamicin were the most commonly isolated (constituting 23% of total cultivable oral bacteria) followed by erythromycin (18% of the total viable count), vancomycin (16% of the total viable count), tetracycline (10% of the total viable count) and amoxycillin (4% of the total viable count). Multiply-resistant bacteria were found with 55% of tetracycline-resistant isolates being resistant also to erythromycin and 6% resistant also to both amoxycillin and erythromycin. The most prevalent genes encoding tetracycline and erythromycin resistance were tet(M), tet(W), tet(0), and mef and erm(B) respectively. In some cases, tet(M) and ermB were contained within a Tn/5 5-like conjugative transposon and could be co-tranferred to Enterococcus faecalis. Finally the nature of the genetic support for one of the tet(W) genes, was determined and found to be flanked by two transposases belonging to two different families of insertion sequences (IS30 and IS256). This element was highly unstable in E. coli. This study showed that antibiotic-resistant bacteria and antibiotic resistance genes are present in the oral microbiota and that oral bacteria are likely to play an important role in the evolution and dissemination of antibiotic resistance genes