Mercury and Organomercurial Resistances Determined by Plasmids in Staphylococcus aureus

Abstract

Penicillinase plasmids of S. aureus often contain genes conferring resistance to inorganic mercury (Hg2+) and the organomercurial phenylmercury acetate. The mechanism of resistance was the enzymatic hydrolysis of the organomercurial phenylmercury to benzene plus inorganic ionic mercury, which was then enzymatically reduced to metallic mercury (Hg0). The Hg0 was rapidly volatilized from the medium into the atmosphere. After the mercurial was degraded and the Hg was volatilized, the resistant cells were able to grow. These plasmids also conferred the ability to volatilize Hg from thimerosal, although the plasmid-bearing strains were equally as thimerosal sensitive as the S. aureus without plasmids. None of the plasmids conferred the ability to volatilize Hg from several other organomercurials: methylmercury, ethylmercury, p-hydroxymercuribenzoate, merbromin and fluorescein mercuric acetate. (Organomercurial resistance-conferring plasmids of Escherichia coli and Pseudomonas aeruginosa confer the ability to degrade 2 or 3 of these organomercurials). Although Hg was not volatilized from p-hydroxymercuribenzoate or fluorescein mercuric acetate, the plasmid-bearing strains were resistant to these organomercurials. The ability to volatilize mercury from Hg2+ and phenylmercury was inducible. The range of inducers included Hg2+, phenylmercury and several organomercurials that were not substrates for the degradation system. Hg-sensitive mutants were isolated from the parental plasmids pI258 and pII147. Thirty-one such Hg-sensitive strains fall into 3 classes: Hg-sensitive strains totally devoid of the phenylmercury hydrolase and Hg2+ reductase activities, mutants with normal hydrolase levels and no detectable reductase and mutants with essentially normal hydrolase levels and low and variable (5-25%) levels of reductase activities. The Hg-sensitive strains were also sensitive to phenylmercury, including those with the potential for hydrolase activity.