Effect of pH on cadmium toxicity and associated gene expression in Escherichia Coli

Abstract

Metals can exert toxic effects on microorganisms. It has been widely reported that lowering environmental pH reduces cadmium toxicity in bacteria. Understanding mechanisms by which pH mediates cadmium toxicity would be useful for minimizing cadmium toxicity in the environment and for gaining insight into the interactions between organic and inorganic components of life. We confirmed that cadmium was less toxic to Escherichia coli at pH 5 than at pH 7 in M9 minimal salts medium through growth curve analysis. To investigate cellular mechanisms by which lowering pH decreases cadmium toxicity, we used DNA microarrays to characterize global gene expression patterns in E. coli in response to cadmium exposure at moderately acidic (5) and neutral (7) values of pH. Increased expression of several stress response genes including hdeA, otsA, and yjbJ at pH 5 after only 5 minutes was observed and suggests that acidic pH more rapidly induces genes that confer cadmium resistance. Genes involved in transport were more highly expressed at pH 5 than at pH 7 in the presence of cadmium. Of the genes that showed an interaction between pH and cadmium effects, 46% encoded hypothetical proteins. Geochemical modeling software predicted that concentrations of both monovalent hydroxylated cadmium (CdOH+), previously implicated in the effect of pH on cadmium toxicity, and cadmium hydroxide chloride (CdOHCl) increased with pH; however, concentrations of both cadmium species were at least two orders of magnitude lower than concentrations of divalent ionic cadmium (Cd2+). Our data both demonstrate that transcriptional responses of E. coli to cadmium are affected by pH as well as provide insight into mechanisms by which pH mediates cadmium toxicity.