Silver compounds are used as antimicrobial agents in medicine and bacteria that develop resistance to silver cations (Ag) pose problems similar to those of antibiotic-resistant bacteria. The first set of Ag resistance genes () was from plasmid pMG101, now assigned to the IncHI incompatibility group. Questions of whether genes are unique to pMG101 or are more widely found, and whether they are associated with a specific incompatibility group or occur in many plasmid groups and on bacterial chromosomes were addressed. genes were identified in five IncH plasmids, but not in plasmids of the IncP incompatibility group. Three genes (, and ) from these plasmids were PCR-amplified, cloned, sequenced and compared to those of pMG101. Differences of 0–50 nt per kb of sequence were found. Predicted gene products were 0–6% different in amino acid sequence, but the differences did not alter residues thought to be involved in protein function (see supplementary data at http://mic.sgmjournals.org or http://www.uic.edu/depts/mcmi/individual/gupta/index.htm). For representative IncH plasmid R476b and pMG101 the effects of Ag exposure on resistance levels were measured by growth. The inducibility of , and gene expression after Ag exposure was studied by reverse transcriptase (RT)-PCR. Silver resistance increased after Ag exposure for strains carrying plasmid R476b. and expression from R476b was inducible after Ag exposure and was constitutive and high from pMG101. The mRNA levels for the regulatory gene was constitutive for both pMG101 and R476b. Close homologues for from pMG101 are clustered on the chromosomes of strains K-12 and O157:H7, without contiguous and homologues. Insertion deletions of the K-12 chromosomal homologues for and gave Ag hypersensitivity for growth. The homologue knockout was complemented back to wild-type resistance by the same gene cloned on a plasmid. Homologues of genes have also been identified on other enterobacterial genomes.


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