Isolation and characterization of chromium (VI) tolerant bacteria from tannery effluents

The tannery industry is a potent environment polluting agent worldwide. Chromium (VI) is a major heavy metal in tannery effluents and their accumulation in soil and water is a serious environmental problem. This study investigates the capacity of indigenous bacteria isolated from tannery effluents for tolerance to chromium (VI).

The chromium tolerance of isolates assessed through both agar dilution and broth microdilution methods. Isolates were identified by morphological and biochemical analysis. The tolerance of isolates to cadmium, nickel, lead, and vanadium and also their multidrug-resistant (MDR) profile were determined. Then the top isolate was characterized via 16S rRNA sequencing and its growth temperature and pH were optimized. Finally, the kinetic of chromium biosorption and chromium removal efficiency was determined using a Nutrient broth medium and wastewater containing 20 mg/L chromium, respectively.

Of 32 screened chromium tolerant isolates, 14 isolates with higher chromium tolerance were selected for further study. 78.57% of isolates represented simultaneous MDR and Multi Heavy Metal tolerance (MHMT) phenotypes and MDR indices of 0.2-1 indicating their source from niches with high antibiotic contamination. However, there was no significant correlation between MDR and MHMT phenotypes among isolates. The top isolate was identified as Lactococcus lactis and showed optimal growth at pH 6 and 25 °C. The maximum chromium biosorption occurred at the end of the exponential phase upon optimized conditions and the approximate chromium removal efficiency of 52.5% was obtained.

The isolated bacteria specifically L. lactis after more evaluations, may show the potential for bioremediation of chromium from tannery effluents.