Toxic chromate reduction by resistant and sensitive bacteria isolated from tannery effluent contaminated soil

Bacterial strains ZA-6, W-61, KS-2 and KS-14 were isolated from agricultural soil irrigated with tannery effluents and subsequently identified by 16S rDNA sequencing as Stenotrophomonas maltophilia, Staphylococcus gallinarum, Pantoea sp. and Aeromonas sp., respectively. All isolates were examined for their resistance to hexavalent chromium and other heavy metal ions. The bacterial isolate S. maltophilia ZA-6 and S. gallinarum W-61 were resistant to 16.5 and 12.4 mM of potassium chromate, respectively, whereas Pantoea sp. KS-2 and Aeromonas sp. KS-14 were found to be sensitive to potassium chromate. S. maltophilia ZA-6 and S. gallinarum W-61 completely reduced 500 μM Cr6+ to Cr3+ within 56 h, while chromate-sensitive isolates Pantoea sp. KS-2 and Aeromonas sp. KS-14 exhibited poor chromate-reducing activity. Chromate reduction was severely affected in the presence of the metabolic inhibitors sodium cyanide and sodium azide. Sodium cyanide completely inhibited chromate reduction in each isolate, whereas 1 mM sodium azide and 10 mM sodium sulfate affected the inhibition of chromate reduction to varying degrees. The use of 1 mM 2,4-dinitrophenol, an uncoupling agent, stimulated the chromate reduction. The cell-free extract (CFE) of chromate-resistant isolates readily reduced Cr6+ to Cr3+, with that of S. gallinarum W-61 showing a Km value of 121.7 μM chromate and a Vmax of 1.12 μmol/min per milligram protein in the presence of NADH. The chromate-resistant isolates displayed lower Michealis–Menton constant (Km) values and higher maximum velocity (Vmax) than chromate-sensitive isolates. These results suggest that chromate resistance and reduction in these bacteria are related.