The Toxicity And Its Underlying Mechanisms Of PFOS And PFOA To Escherichia Coli

ABSTRACTPerfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are two widely used polyfluorinated compounds (PFCs) and are persistent in the environment. Many studies have reported the toxic effects of PFOS and PFOA, however, the investigation on the toxicity of PFOS or PFOA to bacteria is still very limited. Bacteria, as decomposers in the ecosystem, are ubiquitous in the environment. Therefore, investigations about toxic effects to bacteria are helpful for evaluating the environmental risks of PFCs. This study systematically investigated their toxicities and the underlying mechanisms to Escherichia coli (E.coli). Some valuable research results are obtained.(1) The exposures to PFOS and PFOA showed remarkable toxic effects. The growth inhibition ratio and mortality of E.coli were dose-dependent. The determined 3 h 50% inhibitory concentration of PFOS and PFOA were 374±3 and 10.6±1.0 mg L-1, respectively.(2) To analyze the effects of PFOS and PFOA to E.coli, we observed the bacterial cells by using transmission electron microscopy and measured the zeta potential, cell surface hydrophobicity (CSH), membrane fluidity, and compositions of cell membrane fatty acids. The results indicated the damage of cell membrane. The zeta potential decreased at the low doses and gradually increased with the exposure concentration further increased. The CSH increased at the low doses and then decreased with the further increase in the exposure concentrations. With increasing exposure concentrations, the calculated ratio of saturated fatty acids to unsaturated fatty acids exhibited a decrease and the membrane fluidity displayed an increase.(3) To investigate the oxidative damage of E.coli exposed to PFOS and PFOA, we measured contents of some biochemical indicators and carried out cell growth inhibition experiments with ROS scavenger addition. The increases in the contents of intracellular reactive oxygen species, superoxide dismutase, and malondialdehyde indicated the increment of oxidative stress induced by the PFOS or PFOA in the bacterial cells.(4) Genes involved in cell division, membrane synthesis, oxidative stress, and DNA damage of the exposed cells were up or down expressed after the exposure to PFOS and PFOA, indicating the DNA damage by the PFOS and PFOA. PFOS down regulated the expression levels of genes controlling the membrane synthesis, while PFOA up expressed the genes controlling oxidative stress resistance.(5) To investigate the effects of PFOS and PFOA to the biological macromolecules and metabolites, we measured the bioaccumulations, performed Fourier transform infrared subtraction spectrum analysis, and analyzed metabolites of the bacteria exposed to PFOS or PFOA. The exposure to PFOS or PFOA induced generations of unsaturated carbon products, lipids, proteins, and RNA in the bacterial cells, and interfered the metabolic pathways and altered the composition of metabolites of E. coli.(6) The toxicities of PFOS and PFOA to E. coli were therefore ascribed to the membrane disruption, oxidative stress, and DNA damage induced cell inactivation and/or death. The dominating toxicity mechanisms of PFOS and PFOA were attributed to the membrane disruption and the oxidative stress, respectively.