Study of R hodococcus sp. strain NAU‐1 exposed to arsenite

Introduction

Microorganisms play an essential role in shaping the natural environment. They have evolved specific metabolic pathways allowing them to utilize a wide range of substrates, many of which are toxic to higher organisms. Through the conversion of both anthropogenic and naturally occurring pollutants to less toxic products, such microorganisms affect widespread natural bioremediation. An important toxic compound is arsenic, a metalloid that primarily exists in two redox states: the reduced form, arsenite (As^III), and the oxidized form, arsenate (As^V). As^III is more toxic to most of the organisms, as it is more soluble and mobile than arsenate (Jackson et al., ). Inorganic arsenic species are classified as potent human carcinogens. The US Environmental Protection Agency (EPA) has reduced the maximum contaminant level (MCL) for arsenic in drinking water to 10 μg l⁻¹ (Agency USEP, ); however, the groundwater arsenic concentration in some areas of India and Bangladesh has exceeded to an alarming level of 2000 μg l⁻¹ (Tripathi et al., ).

As^III interferes with sulfhydryl groups in amino acids and dithiols (glutaredoxin). The enzymes which generate cellular energy in glycolysis [phosphofructokinase (PFK), hexokinase and glyceraldehyde 3‐phosphate] and citric acid cycle [pyruvate dehydrogenase (PDH)] are also severely affected by As^III (Mandal and Suzuki, ; Ralph, ). As^V, a phosphate analogue, can interfere with phosphate uptake and oxidative phosphorylation by binding to the Fo/F1 ATP synthase, thereby inhibiting ATP production. Exposure to arsenicals either in vitro or in vivo in model organisms caused the induction of heat shock proteins (Hsp), superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), glutathione peroxidase enzymes (Hughes, ).

Microorganisms cope with the toxic effects of arsenic by: (i) minimizing the uptake of arsenate through the system for phosphate uptake, (ii) increasing the level of antioxidants to reduce the effect of reactive oxygen species and (iii) using arsenic detoxification pathway, the ars operon (Ahmann et al., ; Ji and Silver, ; Mukhopadhyay et al., ). All these studies suggest that arsenite resistance in bacteria involves multiple factors. Therefore, the aim of this study was to isolate an arsenite‐tolerant microorganism and understand the metabolic perturbations involved during As^III tolerance. Our results clearly demonstrated that in Rhodococcus sp. strain NAU‐1, ars operon, antioxidant system and...