Jundishapur J Microbiol. 2013 December; 6(10): e8085. DOI: 10.5812/jjm.8085 Published online 2013 December 1. Research Article Antibiotic Resistance Pattern Among Gram Negative Mercury Resistant Bacteria Isolated From Contaminated Environments Nima Mirzaei 1,* , Hilda Rastegari 2 , Mehdi Kargar 2 1 Department of Microbiology, Science and Research Branch, Islamic Azad University, Tehran, IR Iran 2 Department of Microbiology, Jahrom Branch, Islamic Azad University, Jahrom, IR Iran *Corresponding author: Nima Mirzaei, Department of Microbiology, Science and Research Branch, Islamic Azad University, Tehran, IR Iran. Tel: +98-2188984446, Fax: +98-2188957409, E-mail: nm_mirzaei@yahoo.com. Received: September 6, 2012; Revised: February 3, 2013; Accepted: February 18, 2013 Background: Mercury is one of the most toxic heavy metals. Even a small amount of it is toxic for all living organisms. Some of bacteria have developed special resistance mechanisms against mercury, in addition to resistant to different antibiotics. These bacteria usually acquire Hg and antibiotic resistance genes via horizontal gene transfer in their habitat. Objectives: The aim of this study was isolation and identification of mercury resistant bacteria and evaluating the relation between increase of environmental levels of mercury and prevalence of antibiotic resistance among Gram negative Hg resistant bacteria. Materials and Methods: The samples were collected from water and sediments of Kor River. We evaluated amounts of mercury in the water and sediment samples and counted the number of bacteria in both Hg containing and non-Hg containing media. Antibiotic resistance pattern was studied using disk diffusion method for Hg resistant and Hg sensitive bacteria. Results: The frequencies of mercury resistant bacteria were between 35% for Pole Khan station and 2.5% for Doroodzan station. These stations were the most and the lowest mercury-contaminated areas of the Kor River respectively. Pseudomonas sp., Entrobacter sp., Escherichia coli, Klebsiella sp. And Serratia marcescens were isolated as mercury resistant bacteria. The highest level of antibiotic resistance was seen for ampicillin, tetracycline and sterptomycin. Antibiotic resistance frequencies among these bacteria were higher than mercury sensitive ones. Conclusions: Our results showed that Hg resistance genes in contaminated areas are exchanged between residing bacteria along with the antibiotic resistance genes and resulted in prevalence of antibiotic resistance among residing bacteria. Keywords: Multiple Antibiotic Resistance; Mercury; Tetracycline Implication for health policy/practice/research/medical education: The increase of environmental mercury pollution not only affects all living organisms but also results in spreading multiple antibiotic resistance among residing bacteria especially in Gram negative bacteria. This will be a serious public health problem in contaminated regions. Copyright © 2013, Ahvaz Jundishapur University of Medical Sciences; Published by Kowsar Corp. This is an open-access article distributed under the terms of the Cre- ative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. Background Nowadays public health authorities are concerned about toxic chemicals such as mercury which is found in the rivers and enters the human body through sea foods consumption (1). Naturally, mercury exists in the environment in small amounts; however its level has in- creased due to human activities (2). Mercury enters and aggregates in tissues, binds to the sulfhydryl groups of the enzymes and causes serious problems in humans (3). Consumption of methylmercury, the most toxic form of mercury, may cause Minamata disease which is distin- guishable by different neurological symptoms in human (4). Even small amounts of mercury are toxic for all living organisms; however some bacteria residing in contami- nated areas can exchange resistance genes due to contin- ual exposure to the toxic levels of mercury and become resistant to it (3). Resistance to mercury is controlled by a set of genes or- ganized in an operon named mer. mer operon consists of mer RTPABD genes. Regulation of gene expression in mer operon is controlled by merD and merR. merP and merT are involved in transporting Hg into the cytoplasm, merB degrades organic mercury and finally merA encodes Mer- curic reductase. This enzyme reduces Hg 2+ , one of the most toxic forms of mercury, to metallic mercury Hg 0 which is less toxic and not stable in the environment. So it plays the key role in mercury bioremediation process (5-7). Barkay et al. first reported bacterial resistance to mercury compounds in a clinically isolated Staphylococ- cus aureus (8). Ray et al. evaluated the ability of N 2 fixing bacteria (Bei- jerinckia and Azotobacter) to remove mercury. They report- ed that there is a direct relationship between increased use of herbicides and spreading resistant to mercury in