Environ Exp Biol (2017) 15: 161–171

Isolation, screening and identification of the best selenium-reducing bacteria and study on the sorption mechanism

Abstract

Selenium is an essential dietary trace element. However, it is toxic at micro-molar concentrations. Natural sources and anthropogenic activities have resulted in raised levels of selenium. Methods of remediation of polluted areas includevarious physico-chemical techniques, which are expensive and are not effective to efficiently reduce metal and metalloid concentrations. Nevertheless, it is well known that a large number of microorganisms can aerobically reduce selenite or selenate to Se (0). For this reason, microorganisms could be potentially used for bioremediation of polluted sites, as they have developed broad-range resistance mechanisms, such as oxidation, reduction, volatilization and methylation which mediate the natural cycling of these oxyanions and cause biological transformation of selenium oxyanions from one form to another. Among available technologies bacterial reduction of selenate to elemental Se (Se 0) has potential because elemental selenium is insoluble and has the lowest toxicity for biological systems. Moreover, it can be removed by some techniques such as filtration. It is worth noting that isolation of selenate-reducing bacteria has been reported from many different environments. Therefore, the aims of this research were isolation and identification of selenate-resistant bacteria, which may be applied for bioremediation purposes. In the first stage, serial dilutions of samples were cultured on modified Luria-Bertani agar medium (plus 0.2 mM sodium selenate). In order to limit the number of isolated bacteria and to achieve to the most and best resistant isolates, resistance threshold was studied by minimum inhibitory concentration, minimum bactericidal concentration and disc diffusion methods at 32.5 to 1200 mM. Of the isolated bacteria, one isolate from more resistant bacteria was selected and used to perform comprehensive studies. Biosorption and metal removal efficiency of the best isolate were investigated using metabolically active and inactive biomasses (using autoclave and drying by oven). Identification was performed using morphological (e.g. microscopic shape and colony features on agar medium) and biochemical tests (e.g. catalase/oxidase activity, utilization of maltose/sucrose and urea) and molecular methods. Among the isolates, only one isolate had high resistance to selenate; it showed minimum inhibitory concentration and minimum bactericidal concentration equal to 1200 mM and >1200 mM, respectively. Biosorption studies indicated that among different treatment methods, resting cells treated by autoclaving have the highest biosorption capacity (1550.3 mg g^–1) and metal removal efficiency (12.76%). Finally, the isolate was identified as a species of the Klebsiella genus. Overall, comparison of this study with other published work (especially reported results from Iranian studies) showed that this isolate has potential for industrial application on environment cleanup activities.