Extensive use of chromium (Cr) and arsenic (As) based preservatives from the leather tanning industry in Pakistan has had a deleterious effect on the soils surrounding production facilities. Bacteria have been shown to be an active component in the geochemical cycling of both Cr and As, but it is unknown how these compounds affect microbial community composition or the prevalence and form of metal resistance. Therefore, we sought to understand the effects that long-term exposure to As and Cr had on the diversity and structure of soil microbial communities. Soils from three spatially isolated tanning facilities in the Punjab province of Pakistan were analyzed. The structure, diversity and abundance of microbial 16S rRNA genes were highly influenced by the concentration and presence of hexavalent chromium (Cr (VI)) and arsenic. When compared to control soils, contaminated soils were dominated by Proteobacteria while Actinobacteria and Acidobacteria (which are generally abundant in pristine soils) were minor components of the bacterial community. Shifts in community composition were significant and revealed that Cr (VI)-containing soils were more similar to each other than to As contaminated soils lacking Cr (VI). Diversity of the arsenic resistance genes, ars B and ACR3 were also determined. Results showed that ACR3 becomes less diverse as arsenic concentrations increase with a single OTU dominating at the highest concentration. Chronic exposure to either Cr or As not only alters the composition of the soil bacterial community in general, but affects the arsenic resistant individuals in different ways.

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Three chromium-resistant bacteria Bacillus pumilus-S4, Pseudomonas doudoroffii-S5 and Exiguobacterium-S8 were isolated from chromium-contaminated wastewater/soil and could resist very high concentrations of potassium chromate in Luria agar (up to 25 mg ml -1 ) and acetate minimal medium (2 mg ml -1 ). The strains showed growth at diverse pH and temperatures and could resist multiple heavy metals. Pseudomonas doudoroffii-S5 reduced (8.27 mg hexavalent chromium 24 h -1 ) at a lower initial potassium chromate concentration (100 lg ml -1 ), but overall more chromate (28.4 mg hexavalent chromium 24 h -1 ) was reduced at a higher initial concentration (1,000 lg ml -1 ). The addition of various heavy metals (zinc sulphate, copper sulphate, and manganese sulphate at 50 lg ml -1 ) in the chromium reduction media did not significantly affect the hexavalent chromium reduction potential of these isolates. The chromium removal/detoxification potential of these strains increased when used in conjunction with hydrophytes Eichornia crassipes and Pistia stratiotes. Interestingly, the whole process runs automatically with less energy input, that is, the bacterial strains support the growth of plant while in turn the plant releases exudates that help bacterial growth.

Bioremediation refers to the use of microorganisms to reduce or eliminate contaminants from water and soil. In the current research, different bacterial strains were screened for their chromate and arsenate reduction potential. For the removal of arsenic, eight arsenic resistant bacterial strains AsK03, AsK04, AsK06, AsK07, AsK08, AsK09, AsK15 and AsK18 and for chromium removal, eight chromium resistant bacterial strains CrK02, CrK08, CrK12, CrK14, CrK16, CrK19, CrK20 and CrK21 were isolated and selected, respectively, from several contaminated soil and water samples taken from tanneries located in Kasur. Resistance to chromium and arsenic was shown by all the strains on nutrient agar at preliminary concentration of 500 µg ml-1 . The maximum tolerable concentration (MTC) of these isolates was also studied. It was found that for arsenic resistance, two strains AsK04 and AsK09 had highest MTC of 100 mg ml-1 , AsK18 had 75 mg ml-1 , AsK03, AsK06 and AsK15 had 50 mg ml-1 and AsK07 and AsK08 had 45 mg ml-1 of Na2HAsO4 concentration. Similarly, chromium resistant strains were also checked for their MTC against K2CrO4. Here MTC of CrK16 and CrK19 were highest (75 mg ml-1 ), CrK08 and CrK12 were 50 mg ml-1 , CrK02 and CrK20 were 25 mg ml-1 , CrK14 and CrK21 had least MTC of 7.5 mg ml-1 . Fifteen of the bacterial strains were genetically identified by sequencing of 16S rRNA gene. BLAST analysis revealed that the seven strains are homologous to genus Bacillus.