Effect of mercuric chloride at sub-lethal concentrations on the survivability of Escherichia coli cells challenged by N-ethyl-N-nitrosourea or ultraviolet light

This project investigated the protective effect of mercuric chloride at sub-lethal concentrations against the cytotoxicity of genotoxic agents on bacterial cells. Inorganic mercury, a heavy metal, is well known to be highly toxic to both eukaryotic and prokaryotic cells. However, we found that mercuric chloride at sub-lethal concentrations significantly increased the survivability of Escherichia coli (E. coli) cells treated by lethal dosage of DNA-damaging agents such us N-ethyl-N-nitrosourea (ENU) or ultraviolet (UV) light irradiation. Mercuric chloride at sub-lethal concentrations remarkably increased the induction of the SOS response in E. coli cells that were treated by UV light. In addition, the recA or lexA deficient mutation in E. coli abolished the protective effect of mercuric chloride against the cytotoxity of UV light irradiation. These findings suggest that mercuric chloride may protect E. coli cells from the cytotoxic effect of genotoxic agents through inducing the SOS response. Furthermore, cDNA microarray revealed that mercuric chloride increased the expression of genes involved in macromolecule synthesis, DNA repair, the SOS pathway, and cell division. Therefore, mercuric chloride at sub-lethal concentrations seemed to allow cells to better deal with stressful situations by up-regulating the expression of genes critical in cell growth, repair and adaptation. This research helps elucidate the self-salvaging mechanism(s) developed by bacterial cells to deal with the cytotoxicity of chemical or physical genotoxic agents (ENU or UV light), and explores the paradoxical protective effect of mercuric chloride on bacterial cells.