Relationship Between the Persistence of mer Operon Sequences in Escherichia coli and Their Resistance to Mercury Imtiyaz Murtaza, Amit Dutt,* Arif Ali Gene Expression Laboratory, Department of Biosciences, Jamia Millia Islamia, New Delhi-110 025, India Received: 25 May 2001 / Accepted: 27 June 2001 Abstract. Studies related to geographic distribution of E. coli carrying mer operon sequences were carried out on the Indian subcontinent. Out of the 80 E. coli isolates, collected from five geographically distinct regions of India, 68 were found to be resistant to one or the other heavy metal used in the study. Among these isolates, 36 were found to be resistant to the inorganic form (HgCl 2 ) and only 5 to resist both the inorganic and organic forms of mercury. Colony hybridization studies revealed 35 isolates out of 68 to hybridize with the probe. Interestingly, some of the mercury-sensitive isolates (Hg s ), especially from the Dal Lake, were found positive in hybridization studies. These findings, supported by mercury volatilization studies, indicate the presence of nonfunctional/vestigial mer sequences in the isolates collected from different environments. On the other hand, few of the mercury-resistant isolates (Hg r ) from the Yamuna River did not show any sign of hybridization. Further, volatilization studies also indicated an alternate mode of resistance mechanism operating in them. The studies demonstrate that the mer operon sequences share very high homology among the E. coli isolates collected from different geographical locations, and this metal resistance may be a genetic character that arose from a common ancestral background. Heavy metal resistance has been reported to be an im- portant characteristic of bacteria found near the polluted sites that include water bodies and landfills. Various metal-resistant bacteria have been previously reported, and the most frequently encountered heavy metal resis- tance among them is that for mercury, which has been detected in a wide range of bacterial genera [6, 9]. Bacteria have evolved a variety of means of resistance to different forms of mercury. A widely employed mecha- nism of bacterial resistance to mercurial compounds is the reduction of Hg 2 to its volatile metallic form, Hg 0 [10]. The biotransformation is mediated by mercuric reductase, an inducible NADPH-dependent, flavin-con- taining disulfide oxido-reductase enzyme. The gene en- coding mercuric reductase (mer A), together with genes coding for Hg 2 transport and regulatory functions, com- prises a narrow spectrum mer operon. However, mer B gene, if found associated with all the above genes, makes it a broad-spectrum mer operon [18]. The mer B gene product called organomercurial lyase cleaves the mercu- ric ion from the organic moiety, allowing subsequent reduction of Hg 2 to Hg 0 by mercuric reductase. It has been reported that DNA sequences of mercury resistance genes are 80 –90% identical when compared in several different Gram-negative systems [11]. Available data also indicate that plasmid-encoded resistance to mercury is as common as antibiotic resistance [19, 21]. In India it is estimated that about 180 tons of mer- cury salts are discharged into the environment annually [20]. In view of the toxicity of mercury and the harmful effects that it inflicts upon the biological community, there is a need to decrease the mercury load in water bodies, particularly in the river system. Despite the grav- ity of the problem, none of the river systems in India has been scanned for the occurrence of the mercury-resistant bacteria except for a few studies that have been carried out on the coastal regions [15]. The present study was carried out to evaluate the resistance offered by several multimetal-resistant E. coli isolates towards mercury and * Present address: Division of Cancer Research, 26/403, Cul- mannstrasse, Department of Pathology, University of Zurich, CH-8006, Zurich, Switzerland. Correspondence to: A. Ali; email: arif.bi@jmi.ernet.in CURRENT MICROBIOLOGY Vol. 44 (2002), pp. 178 –183 DOI: 10.1007/s00284-001-0085-6 Current Microbiology An International Journal © Springer-Verlag New York Inc. 2002