| The mer loci of mercury resistant (HgR) Gram-negative bacterial isolates from the Primate Amalgam Collection were characterized and grouped into nine phylogenetic mer locus types. These nine mer loci demonstrated considerable polymorphism, supporting the view that the mer operon is a genetic mosaic and has a predominance of insertions/deletions of functional genes immediately before and after the merA gene. The highly variable region 5′ of merA was investigated. Variation at the DNA level in the merC gene residing there was greater than adjacent genes, however, the consequences of this variability for the predicted structure of the MerC protein were limited and, with two exceptions, putative functional elements (metal-binding ligands and transmembrane domains) were strongly conserved.; Using PCR and Southern hybridization, multi-antibiotic resistant (AbR) mercury resistant (Hg) and sensitive (HgS) primate fecal isolates (N = 151), exposed to mercury from dental amalgams, were assayed for the mer (HgR) locus, the integron, and physical linkage of the two. Southern hybridization with class-specific integrase gene probes revealed seven strains carrying class 1 integrase genes (intI1). No class 2, 3 and 4 intI genes were found in any HgR strains and no evidence of these four intI genes was found in HgS strains regardless of the number of phenotypic antibiotic resistances. PCR primers were used to amplify the array of integrated AbR gene cassettes found in integrons. Thirty-nine percent of HgR-AbR strains examined, had evidence of tandem antibiotic resistance cassettes, and only 1% of HgS-AbR strains were positive for these cassettes. Close linkage of HgR loci to multi-antibiotic resistant integrons was demonstrated using extra long PCR amplifications. Of 41 HgR-AbR strains tested for linkages between the merA gene of the HgR locus and integron genes, aadA1, sul1 or tniA; 56% were positive for at least two linkages. Fifty-two percent of these HgR-AbR strains demonstrating linkage also produced amplicands indicative of tandem antibiotic resistance cassettes. Of 47 HgR-AbR strains characterized which were isolated during or after amalgam placement, 16 demonstrated linkage. Thus, it appears that one-third of HgR bacteria exposed to the amalgam dental restorations carry HgR loci physically linked to multi-antibiotic resistant integrons. |
