Loss of YhcB results in dysregulation of coordinated peptidoglycan, LPS and phospholipid synthesis during Escherichia coli cell growth.

The cell envelope is essential for viability in all domains of life. It retains enzymes and substrates within a confined space while providing a protective barrier to the external environment. Destabilising the envelope of bacterial pathogens is a common strategy employed by antimicrobial treatment. However, even in one of the best studied organisms, Escherichia coli, there remain gaps in our understanding of how the synthesis of the successive layers of the cell envelope are coordinated during growth and cell division. Here, we used a whole-genome phenotypic screen to identify mutants with a defective cell envelope. We report that loss of yhcB, a conserved gene of unknown function, results in loss of envelope stability, increased cell permeability and dysregulated control of cell size. Using whole genome transposon mutagenesis strategies, we report the comprehensive genetic interaction network of yhcB, revealing all genes with a synthetic negative and a synthetic positive relationship. These genes include those previously reported to have a role in cell envelope biogenesis. Surprisingly, we identified genes previously annotated as essential that became non-essential in a ΔyhcB background. Subsequent analyses suggest that YhcB functions at the junction of several envelope biosynthetic pathways coordinating the spatiotemporal growth of the cell, highlighting YhcB as an as yet unexplored antimicrobial target.

Interplay of cold shock protein E with an uncharacterized protein, YciF, lowers porin expression and enhances bile resistance in Salmonella Typhimurium.

Bacterial cold shock proteins (CSPs) function as RNA chaperones. To assess CSP's roles in the intracellular human pathogen Salmonella Typhimurium, we analyzed their expression in varied stress conditions. We found that cold shock protein E (cspE or STM14_0732) is up-regulated during bile salt-induced stress and that an S. Typhimurium strain lacking cspE (ΔcspE) displays dose-dependent sensitivity to bile salts, specifically to deoxycholate. We also found that an uncharacterized gene, yciF (STM14_2092), is up-regulated in response to bile stress in WT but not in the ΔcspE strain. Complementation with WT CspE, but not with a F30V CspE variant, abrogated the bile sensitivity of ΔcspE as did multicopy overexpression of yciF. Northern blotting experiments with rifampicin disclosed that the regulation of yciF expression is, most likely, due to the RNA-stabilizing activity of CspE. Importantly, electrophoretic mobility shift assays indicated that purified CspE, but not the F30V variant, directly binds yciF mRNA. We also observed that the extra-cytoplasmic stress-response (ESR) pathway is augmented in the bile-treated ΔcspE strain, as judged by induction of RpoE regulon genes (rpoE, degP, and rybB) and downstream ESR genes (hfq, rne, and PNPase). Moreover, the transcript levels of the porin genes, ompD, ompF, and ompC, were higher in bile salts-stressed ΔcspE and correlated with higher intracellular accumulation of the fluorescent DNA stain bisBenzimide H 33258, indicating greater cell permeability. In conclusion, our study has identified YciF, a CspE target involved in the regulation of porins and in countering bile stress in S. Typhimurium.

Salmonella Typhimurium encoded cold shock protein E is essential for motility and biofilm formation.

The ability of bacteria to form biofilms increases their survival under adverse environmental conditions. Biofilms have enormous medical and environmental impact; consequently, the factors that influence biofilm formation are an important area of study. In this investigation, the roles of two cold shock proteins (CSP) during biofilm formation were investigated in Salmonella Typhimurium, which is a major foodborne pathogen. Among all CSP transcripts studied, the expression of cspE (STM14_0732) was higher during biofilm growth. The cspE deletion strain (ΔcspE) did not form biofilms on a cholesterol coated glass surface; however, complementation with WT cspE, but not the F30V mutant, was able to rescue this phenotype. Transcript levels of other CSPs demonstrated up-regulation of cspA (STM14_4399) in ΔcspE. The cspA deletion strain (ΔcspA) did not affect biofilm formation; however, ΔcspEΔcspA exhibited higher biofilm formation compared to ΔcspE. Most likely, the higher cspA amounts in ΔcspE reduced biofilm formation, which was corroborated using cspA over-expression studies. Further functional studies revealed that ΔcspE and ΔcspEΔcspA exhibited slow swimming but no swarming motility. Although cspA over-expression did not affect motility, cspE complementation restored the swarming motility of ΔcspE. The transcript levels of the major genes involved in motility in ΔcspE demonstrated lower expression of the class III (fliC, motA, cheY), but not class I (flhD) or class II (fliA, fliL), flagellar regulon genes. Overall, this study has identified the interplay of two CSPs in regulating two biological processes: CspE is essential for motility in a CspA-independent manner whereas biofilm formation is CspA-dependent.