Genome-wide transcriptional response of chemostat-cultured Escherichia coli to zinc

Zinc is an essential trace metal ion for growth, but an excess of Zn is toxic and microorganisms express diverse resistance mechanisms. To understand global bacterial responses to excess Zn, we conducted transcriptome profiling experiments comparing Escherichia coli MG1655 grown under control conditions and cells grown with a toxic, sublethal ZnSO4 concentration (0.2 mM). Cultures were grown in a defined medium lacking inorganic phosphate, permitting maximum Zn bioavailability, and in glycerol-limited chemostats at a constant growth rate and pH. Sixty-four genes were significantly up-regulated by Zn stress, including genes known to be involved in Zn tolerance, particularly zntA, zraP, and hydG. Microarray transcriptome profiling was confirmed by real-time PCR determinations of cusF (involved in Ag and Cu efflux), ais (an Al-inducible gene), asr (encoding an acid shock-inducible periplasmic protein), cpxP (a periplasmic chaperone gene), and basR. Five upregulated genes, basR and basS [encoding a sensor-regulator implicated in Salmonella in Fe(III) sensing and antibiotic resistance], fliM (flagellar synthesis), and ycdM and yibD (both with unknown functions), are important for growth resistance to zinc, since mutants with mutations in these genes exhibited zinc sensitivity in liquid media and on metal gradient plates. Fifty-eight genes were significantly down-regulated by Zn stress; notably, several of these genes were involved in protection against acid stress. Since the mdt operon (encoding a multidrug resistance pump) was also up-regulated, these findings have important implications for understanding not only Zn homeostasis but also how bacterial antibiotic resistance is modulated by metal ions.