Mercury and Organomercurial Resistances Determined by Plasmids in Pseudomonas

Abstract

Hg and organomercurial resistance determined by genes on 10 P. aeruginosa plasmids and 1 P. putida plasmid were studied with regard to the range of substrates and the range of inducers. The plasmidless strains were sensitive to growth inhibition by Hg2+ and did not volatilize Hg0 from Hg2+. A strain with plasmid RP1 (which does not confer resistance to Hg2+) similarly did not volatilize Hg. All 10 plasmids determine Hg resistance by way of an inducible enzyme system. Hg2+ was reduced to Hg0, which is insoluble in water and rapidly volatilizes from the growth medium. Plasmids pMG1, pMG2, R26, R933, R93-1 and pVS1 in P. aeruginosa and MER in P. putida conferred resistance to and the ability to volatilize Hg from Hg2+, but strains with these plasmids were sensitive to and could not volatilize Hg from the organomercurials methylmercury, ethylmercury, phenylmercury and thimerosal. These plasmids also conferred resistance to the organomercurials merbromin, p-hydroxymercuribenzoate, and fluorescein mercuric acetate. The other plasmids, FP2, R38, R3108 and pVS2, determined resistance to and decomposition of a range of organomercurials, including methylmercury, ethylmercury, phenylmercury and thimerosal. These plasmids also conferred resistance to the organomercurials merbromin, p-hydroxymercuribenzoate and fluorescein mercuric acetate by a mechanism not involving degradation. In all cases, organomercurial decomposition and mercury volatilization were induced by exposure to Hg2+ or organomercurials. The plasmids differed in the relative efficacy of inducers. Hg2+ resistance with strains that are organomercurial sensitive appeared to be induced preferentially by Hg2+ and only poorly by organomercurials to which the cells are sensitive. The organomercurials p-hydroxymercuribenzoate, merbromin and fluorescein mercuric acetate were strong gratuitous inducers but not substrates for the Hg2+ volatilization system. With strains resistant to phenylmercury and thimerosal, these organomercurials were both inducers and substrates.