ASSOCIATION OF MERCURY RESISTANCE WITH ANTIBIOTIC-RESISTANCE IN THE GRAM-NEGATIVE FECAL BACTERIA OF PRIMATES

Gram-negative fecal bacteria from three longitudinal Hg exposure exper iments and from two independent survey collections were examined for t heir carriage of the mercury resistance (mer) locus. The occurrence of antibiotic resistance was also assessed in both mercury-resistant (Hg -r) and mercury-susceptible (Hg-5) isolates from the same collections. The longitudinal studies involved exposure of the intestinal flora to Hg released from amalgam ''silver'' dental restorations in six monkey s. Hg-r strains were recovered before the installation of amalgams, an d frequently these became the dominant strains,while amalgams were ins talled, Such persistent Hg-r strains always carried the same mer locus throughout the experiments. In both the longitudinal and survey colle ctions, certain mer loci were preferentially associated with one genus , whereas other mer loci were recovered from many genera. In general, strains with any mer locus were more likely to be multiresistant than were strains without mer loci; this clustering tendency was also seen for antibiotic resistance genes. However, the association of antibioti c multiresistance with mer loci was not random; regardless of source, certain mer loci occurred in highly multiresistant strains (with as ma ny as seven antibiotic resistances), whereas other mer loci were found in strains without any antibiotic resistance. The majority of highly multiresistant Hg-r strains also carried genes characteristic of an in tegron, a novel genetic element which enables the formation of tandem arrays of antibiotic resistance genes. Hg-r strains lacking antibiotic resistance showed no evidence of integron components.