Deciphering the key factors for heavy metal resistance in gram-negative bacteria

Abstract

Heavy metals (HMs) are versatile elements of nature with five times higher atomic weight and density than water. HMs are ubiquitous in nature due to the industrial, domestic, agricultural, medical and technological applications. These are toxic at trace levels and therefore attract more and more interest for their least bioaccumulation and thus high persistence in the environment. Among HMs, arsenic, cadmium, chromium, lead and mercury rank as priority metals that are of public health significance and ecological concern. Interestingly, bacteria have been found as efficient tool for heavy metal degradation as well as resistance. Several bacteria have been reported for the HM accumulation which has been controlled by the metal resistance gene, carried on genome or in plasmid. In nature, Gram-negative bacteria are dependent on plant-derived simple carbon (C) compounds. In HMs abundant flora and fauna, they survive by different cellular mechanisms like metal sorption, mineralization, uptake and accumulation, extracellular precipitation, enzymatic mechanisms for oxidation or reduction to a less toxic form and efflux of heavy metals from the cells to adapt in HM stresses. Hence, here we focus on the mechanism of microbial interaction with these heavy metals which can open the new horizon for the exploitation of Gram-negative bacteria and their gene pool as HM remediator agents, biological indicator and plant growth promoters.