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Colicin E2-tolerant (known as Cet2) Escherichia coli K-12 mutants overproduce an inner membrane protein, CreD, which is believed to cause the Cet2 phenotype. Here, we show that overproduction of CreD in a Cet2 strain results from hyperactivation of the CreBC two-component regulator, but CreD overproduction is not responsible for the Cet2 phenotype. Through microarray analysis and gene knockout and overexpression studies, we show that overexpression of another CreBC-regulated gene, yieJ (also known as cbrC), causes the Cet2 phenotype.

Objectives: The use of triclosan within various environments has been linked to the development of multiple drug resistance (MDR) through the increased expression of efflux pumps such as AcrAB-TolC. In this work, we investigate the effect of triclosan exposure in order to ascertain the response of two species to the presence of this widely used biocide., Methods: The transcriptomes of Salmonella enterica serovar Typhimurium SL1344 and Escherichia coli K-12 MG1655 after exposure to the MIC of triclosan (0.12 mg/L) were determined in microarray experiments. Phenotypic validation of the transcriptomic data included RT-PCR, ability to form a biofilm and motility assays., Results: Despite important differences in the triclosan-dependent transcriptomes of the two species, increased expression of efflux pump component genes was seen in both. Increased expression of soxS was observed in Salmonella Typhimurium, however, within E. coli, decreased expression was seen. Expression of fabBAGI in Salmonella Typhimurium was decreased, whereas in E. coli expression of fabABFH was increased. Increased expression of ompR and genes within this regulon (e.g. ompC, csgD and ssrA) was seen in the transcriptome of Salmonella Typhimurium. An unexpected response of E. coli was the differential expression of genes within operons involved in iron homeostasis; these included fhu, fep and ent., Conclusions: These data indicate that whilst a core response to triclosan exposure exists, the differential transcriptome of each species was different. This suggests that E. coli K-12 should not be considered the paradigm for the Enterobacteriaceae when exploring the effects of antimicrobial agents.

Successful pathogens must be able to protect themselves against reactive nitrogen species generated either as part of host defense mechanisms or as products of their own metabolism. The regulatory protein NsrR (a member of the Rrf2 family of transcription factors) plays key roles in this stress response. Microarray analysis revealed that NsrR represses nine operons encoding 20 genes in Escherichia coli MG1655, including the hmpA, ytfE, and ygbA genes that were previously shown to be regulated by NsrR. Novel NsrR targets revealed by this study include hcp-hcr (which were predicted in a recent bioinformatic study to be NsrR regulated) and the well-studied nrfA promoter that directs the expression of the periplasmic respiratory nitrite reductase. Conversely, transcription from the ydbC promoter is strongly activated by NsrR. Regulation of the nrf operon by NsrR is consistent with the ability of the periplasmic nitrite reductase to reduce nitric oxide and hence protect against reactive nitrogen species. Gel retardation assays were used to show that both FNR and NarL bind to the hcp promoter. The expression of hcp and the contiguous gene hcr is not induced by hydroxylamine. As hmpA and ytfE encode a nitric oxide reductase and a mechanism to repair iron-sulfur centers damaged by nitric oxide, the demonstration that hcp-hcr, hmpA, and ytfE are the three transcripts most tightly regulated by NsrR highlights the possibility that the hybrid cluster protein, HCP, might also be part of a defense mechanism against reactive nitrogen stress.

The gene expression profile of Escherichia coli K-12 MG1655 grown in minimal medium supplemented with elevated copper concentrations (as copper-glycine) has been analysed using whole-genome oligonucleotide microarrays. At 750 muM copper-glycine, the expression of both the cue and cus copper-export systems is evident. At near-lethal copper concentrations (2 mM copper-glycine), the expression of these two regulons increases significantly. Other regulons with increased transcription in response to elevated concentrations of copper-glycine include those for the superoxide stress response, iron homeostasis, and envelope stress. Furthermore, a variety of ORFs with decreased expression in response to increased copper-glycine has been identified, including the zinc ABC transporter and genes involved in the chemotactic response.