Role of arsenic and its resistance in nature.

Contamination of the environment with heavy metals has increased drastically over the last few decades. The heavy metals that are toxic include mercury, cadmium, arsenic, and selenium. Of these heavy metals, arsenic is one of the most important global environmental pollutants and is a persistent bioaccumulative carcinogen. It is a toxic metalloid that exists in two major inorganic forms: arsenate and arsenite. Arsenite disrupts enzymatic functions in cells, while arsenate behaves as a phosphate analog and interferes with phosphate uptake and utilization. Despite its toxicity, arsenic may be actively sequestered in plant and animal tissues. Various microbes interact with this metal and have shown resistance to arsenic exposure, and they appear to possess the ars operon for arsenic resistance consisting of three to five genes, i.e., arsRBC or arsRDABC, organized into a single transcriptional unit; some microbes even use it for respiration. Microbial interactions with metals may have several implications for the environment. Microbes may play a role in cycling of toxic heavy metals and in remediation of metal-contaminated sites. There is a correlation between tolerance to heavy metals and antibiotic resistance, a global problem currently threatening the treatment of infections in plants, animals, and humans. The purpose of this review is to highlight the nature and role of toxic arsenic in bacterial systems and to discuss the various genes responsible for this heavy-metal resistance in nature and the mechanisms to detoxify this element.