Davis, Ian J; (2003) Characterisation of heavy metal and antibiotic-resistance genes in oral bacteria. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Antibiotic resistant bacteria pose a serious threat to human and animal health. Therefore the mechanisms that lead to the selection of resistant bacteria need to be investigated. One possible mechanism is that the silver and mercury in amalgam dental restorations may select for bacteria that contain heavy metal and antibiotic-resistance determinants leading to the spread of these resistances, particularly if they are contained on the same mobile genetic element. In this work the genetic basis for mercury and antibiotic resistance in Enterococcus faecium isolate 664.1H1 was investigated. This organism encodes transferable resistance to mercury, tetracycline and streptomycin. The mercuric reductase protein, MerA, encoded on the mer operon confers mercury resistance and the aadK gene product confers streptomycin resistance. Both determinants are encoded on a large plasmid, pMER1, which has homology to three broad host range conjugative plasmids from the Inc18 incompatibility group, including pRE25 isolated from Enterococcus faecalis RE25. The tetracycline resistance determinant was identified as tet(S) which is located upstream of sequence homologous to orf6, orf9, orf7 and orf8 of conjugative transposon Tn916. Filter mating studies show that pMER1 is capable of transfer independent of the tet(S) element into E. faecalis and Bacillus subtilis recipients. No transfer of the tet(S) element could be achieved in the absence of pMER1 suggesting there is interaction between these elements. This work also investigated whether silver resistant bacteria can be isolated from teeth. Two silver resistant Enterobacter cloacae isolates were isolated from infected teeth containing dental restorations. Both isolates are also resistant to ampicillin, erythromycin and clindamycin. silE, which is encoded on the silver resistance operon, has been sequenced from both isolates. This work provides further evidence of the link between antibiotic and heavy metal-resistance in bacteria. These findings also highlight the composite nature of both the mer operon and enterococcal mobile elements.

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