Planktonic cells of Staphylococcus and Bacillus species capable of faster chromium reduction in short incubation times as compared to their biofilms

Chromium is a carcinogenic toxicant widely used in many industries. The concentration of chromium is increasing in various areas of the world causing threat to living beings. Bioremediation is an inexpensive and eco-friendly approach to detoxify such contaminants. Many bacteria can live both as free cells (planktonic) and as biofilms (sessile), and the pattern of chromium reduction is different in these live forms. In the present study, chromium reduction by planktonic cells and biofilms of bacteria was determined in a comparative manner. Chromium-resistant bacteria capable of biofilm formation were isolated from contaminated soil and wastewater. The bacteria were characterized morphologically and biochemically, and identified by 16S rRNA gene sequencing. Cr(VI) reduction by planktonic cells and biofilms was determined over different periods of incubation. The wastewater isolates showed significant resistance against Cr(VI). Minimum inhibitory concentration (MIC) of Cr (VI) was found to be 900 μg ml−1 for Staphylococcus simulans KW1 and Staphylococcus hominis KW2. Whereas, Bacillus cereus KW4 and Staphylococcus equorum KS1 showed MIC values of 800 μg ml−1 and 850 μg ml−1 respectively. Among all the isolates, maximum biofilm formation was shown by Staphylococcus equorum KS1 both qualitatively and quantitatively. Planktonic cells of these bacteria were more efficient in Cr(VI) reduction as compared to their biofilms in 24 h. Maximum Cr(VI) reduction in planktonic form was shown by Staphylococcus simulans KW1 (27.9 μg ml−1, 55%), whereas in biofilm mode of growth the maximum Cr(VI) reduction was shown by Staphylococcus equorum KS1 (4.6 μg ml−1, 9.2%). The results show that for faster Cr(VI) reduction in wastewater the planktonic form of these bacteria is more suited.