Mercury and organomercurial resistance in bacteria isolated from freshwater fish of wetland fisheries around Calcutta

Abstract

Mercury-resistant bacteria belonging to the genera Bacillus, Escherichia, Klebsiella, Micrococcus, Pseudomonas, Salmonella, Sarcina, Shigella, Staphylococcus and Streptococcus were isolated from gills and guts of fresh water fish collected from wetland fisheries around Calcutta, India, contaminated with mercury compounds. The total number of bacteria, as well as Hg-resistant bacteria, were always higher in guts than gills. Bottom-dwelling fish contained higher number of bacteria, including Hg-resistant bacteria, than surface and middle water dwelling fish. They belonged either to narrow-spectrum or to broad-spectrum Hg-resistant groups and they also possessed other heavy metal and antibiotic resistant properties. In the presence of toxic levels of HgCl 2, phenylmercuric acetate (PMA) and methylmercuric chloride (MMC), the lag in growth of the bacterial strains gradually increased with increasing concentration of Hg-compounds. Narrow-spectrum Hg-resistant bacterial strains volatilized only HgCl 2 from the liquid medium in the range of 64–89%, whereas the broad-spectrum group exhibited a high level of HgCl 2 (80–94%), PMA (72–84%) and MMC (64–80%) volatilizing capacity with inducible mercuric reductase and organomercurial lyase enzyme activities in their cell-free extracts. Cell-free extracts prepared from narrow-spectrum Hg-resistant bacterial strains induced by HgCl 2 exhibited Hg +2-dependent NADPH oxidation, indicating the presence of only mercuric reductase enzyme.