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

[摘要] Metals can exert toxic effects on microorganisms. It has been widely reportedthat lowering environmental pH reduces cadmium toxicity in bacteria. Understandingmechanisms by which pH mediates cadmium toxicity would be useful for minimizingcadmium toxicity in the environment and for gaining insight into the interactions betweenorganic and inorganic components of life. We confirmed that cadmium was less toxic toEscherichia coli at pH 5 than at pH 7 in M9 minimal salts medium through growth curveanalysis. To investigate cellular mechanisms by which lowering pH decreases cadmiumtoxicity, 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 ofpH. Increased expression of several stress response genes including hdeA, otsA, and yjbJat pH 5 after only 5 minutes was observed and suggests that acidic pH more rapidlyinduces genes that confer cadmium resistance. Genes involved in transport were morehighly expressed at pH 5 than at pH 7 in the presence of cadmium. Of the genes thatshowed an interaction between pH and cadmium effects, 46% encoded hypotheticalproteins. Geochemical modeling software predicted that concentrations of bothmonovalent hydroxylated cadmium (CdOH+), previously implicated in the effect of pHon cadmium toxicity, and cadmium hydroxide chloride (CdOHCl) increased with pH;however, concentrations of both cadmium species were at least two orders of magnitudelower than concentrations of divalent ionic cadmium (Cd2+). Our data both demonstratethat transcriptional responses of E. coli to cadmium are affected by pH as well as provideinsight into mechanisms by which pH mediates cadmium toxicity.

[发布机构] University of Wisconsin

[效力级别] cadmium

[关键词] microorganisms;cadmium;toxicity;gene expression;pH factor