Effects of Hg2+, CH3-Hg+, and temperature on the expression of mercury resistance genes in environmental bacteria

Abstract

Twenty different bacterial isolates obtained from a mercury-contaminated site in Oak Ridge, Tenn., were grown on plate count agar amended with 25 μg of Hg2+ or 3 μg of CH3-Hg+ (R-Hg+) per ml. The total cellular RNA was extracted from each isolate by an acid-guanidine-thiocyanate-phenol-chloroform method. The transcripts of merA and merB were detected and quantitated by Northern (RNA) hybridization. A qualitative assay of mercuric reductase was used to confirm the enzyme activity. Low temperature (4°C) with the presence of Hg2+ (25 μg/ml) significantly increased the net merA transcripts of mid-log-phase cells of six environmental isolates. The net merA transcript production by 18 of the isolates increased when they were grown on 50% plate count broth with 15 μg of Hg2+ per ml, but only 8 isolates showed increased production of merB transcripts. The MICs of Hg2+ and R-Hg+ for 10 methyl mercury-resistant isolates ranged from 45 to 110 μg of Hg2+ and 0.6 to 4.5 μg of R-Hg+ per ml. R-Hg+ was able to induce the expression of merB in 70% of methyl mercury-resistant strains.