| Summary: | Antibiotic-resistant bacteria present a serious public health threat and studies investigating the prevalence of antibiotic-resistant bacteria and the genes encoding for antibiotic resistance (GEAR) are essential. Such studies help us to understand how antibiotic-resistant bacteria and GEAR are obtained, transferred and maintained within a population. The aim of this study was to identify the prevalence and maintenance of antibiotic-resistant bacteria in the oral microbiota of children aged 4-6 years who had not previously taken antibiotics. Bacteria resistant to penicillin, ampicillin, tetracycline and erythromycin were widespread among the children studied. Tetracycline-resistant bacteria were found in 98% of the children sampled and the tetracycline resistance determinants most responsible for this resistance were tet(M) and tet(W). The tetracycline-resistant bacteria were maintained within 15 children over a period of twelve months. In three of these children the tet(M) gene was found to persist in a variety of genera and this was found to be contained within a Tn9/6-like element by Southern blot analysis. The tetracycline resistance determinant tet(S) was found for the first time in Streptococcus intermedius . It was found to be transferable to Enterococcus faecalis and other Streptococcus spp. by filter matings. The tet(S) gene was shown to be present on a novel Tn9/f5-like element designated Tn97r5S. The similarities between Tn916 and Tn976S included the presence of conjugative and excision and integration modules. However the tet(M) in Tn976 had been completely replaced by tet(S) in Tn9/f5S. The gene tet(32) was also found for the first time in Streptococcus parasanguinis and Eubacterium saburreum, previously it had been found in a human colonic bacterium K10. The upstream region of the oral tet(32) gene in E. saburreum 41.2T.2 was found to be more closely related to upstream sequences of tet(W) in Roseburia sp. A2-123, upstream sequences of tet(W) within TnB1230 (originally isolated from Butyrivibrio fibrisolvens ) and orf!4, orf25 and orf26 from Tn5397. This work provides further evidence that antibiotic-resistant bacteria are prevalent in the oral microbiota of children and that the genes responsible, especially those encoding resistance to tetracycline, can be maintained in the oral microbiota even when the children have not been directly exposed to antibiotics. These findings show that tetracycline resistance determinants can be mosaic in structure and can be easily transferred due to their containment within conjugative transposons. Such elements then undergo evolutionary changes, whereby recombination of the conjugative transposon modules result in new genetic elements.
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