HORIZONTAL SPREAD OF MER OPERONS AMONG GRAM-POSITIVE BACTERIA IN NATURAL ENVIRONMENTS

Horizontal dissemination of the genes responsible for resistance to to xic pollutants may play a key role in the adaptation of bacterial popu lations to environmental contaminants. However, the frequency and exte nt of gene dissemination in natural environments is not known. A natur al horizontal spread of two distinct mercury resistance (mer) operon v ariants, which occurred amongst diverse Bacillus and related species o ver wide geographical areas, is reported. One mer variant encodes a me rcuric reductase with a single N-terminal domain, whilst the other enc odes a reductase with a duplicated N-terminal domain. The strains cont aining the former mer operon types are sensitive to organomercurials, and are most common in the terrestrial mercury-resistant Bacillus popu lations studied in this work. The strains containing the latter operon types are resistant to organomercurials, and dominate in a Minamata B ay mercury-resistant Bacillus population, previously described in the literature. At least three distinct transposons (related to a class ii vancomycin-resistance transposon, Tn1546, from a clinical Enterococcu s strain) and conjugative plasmids are implicated as mediators of the spread of these mer operons.