Molecular adaptations in cadmium and lead resistant environmental species

Kepenek, Eda Şeyma
Environmental pollution caused by heavy metal exposure has detrimental effects on human health. For the reclamation of polluted areas, bacteria have been used in remediation. For those bioremediation attempts, finding the appropriate bacterial strains is an important issue. In their environment, bacteria may exist close to the source of heavy metals. Sudden exposure to high concentrations of the heavy metal may trigger quick response mechanisms to ensure survival. On the other hand, bacteria may live at a distance to the source and thus, experience the gradual increase in the heavy metal concentrations. The gradual increase in the concentration of the heavy metal may cause changes accumulating in time to ensure survival as well. In both, acute or gradual exposure situations, bacteria resist heavy metals by using several resistance mechanisms. In our study, we investigated whether there are differences between molecular profiles of the cells that are acclimated to heavy metals upon either acute or gradual exposure. We measured the changes in molecular profile by using ATR-FTIR spectroscopy and unsupervised chemometric analysis methods (PCA, HCA). We chose cadmium and lead as heavy metals, and we have studied the molecular profiles of Brevundimonas, Gordonia, and Microbacterium bacterial genera isolated from a freshwater source. In a laboratory environment, after extended exposure bacteria can be acclimated to live and grow in the existence of heavy metals. We allow the bacteria to mimic the situation of being close or away from the source of heavy metal by using two different acclimation procedures (acute vs. gradual). We acclimated bacterial strains to grow at 2 to 4 times higher concentrations than their corresponding minimum inhibitory concentrations. We then measured the molecular alterations caused by the nature of acclimation upon acute or gradual exposure. Results of the IR spectral analysis indicated substantial changes in structure and composition of bacterial macromolecules. Changes in the membrane exhibited itself in decreases of fatty acids amounts and as well as protein to lipid ratios in acclimated groups. The decrease in total protein concentrations that we measured perhaps hints inhibition of several anabolic pathways in all acclimated groups. Furthermore, total protein concentrations in acutely acclimated bacteria were significantly lower than that of gradually acclimated ones. A remarkable increase in exopolymer production was detected in both acutely and gradually heavy metal exposed groups. Exopolymer production, being one of the heavy metal resistance mechanisms in bacteria, appeared to play a central role in survival upon supra lethal exposures. Moreover, bacteria under acute cadmium exposure cadmium produced significantly higher exopolysaccharide than under gradual. On the contrary, gradually acclimated strains produced significantly higher exopolysaccharide than those of acutely acclimated ones under lead exposure. Results of PCA and HCA analyzes showed clearcut discriminations between acclimated bacterial strains as acute vs. gradual exposure and control for both of the metals. This work contributes novel insights into the potential role of IR spectroscopy in the molecular characterization of bacteria acclimated to different stress conditions. Furthermore, our study showed that we may use information about the exopolymer production and amount as background information to find appropriate bacterial strains in bioremediation studies.