Biology and Control of Human Pathogens on Fresh Produce
Location: Produce Safety and Microbiology Research
Title: Evolutionary silence of the acid chaperone protein HdeB in enterohemorrhagic Escherichia coli O157:H7
Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 20, 2011
Publication Date: January 1, 2012
Citation: Carter, M.Q., Louie, J.W., Fagerquist, C.K., Sultan, O., Miller, W.G., Mandrell, R.E. 2012. Evolutionary silence of the acid chaperone protein HdeB in enterohemorrhagic Escherichia coli O157:H7. Applied and Environmental Microbiology. 78:1004-1014.
Interpretive Summary: Some periplasmic proteins in Escherichia coli and other enteric bacteria protect other proteins under stressful environmental conditions (chaperones). We studied two proteins that protect acid-induced damage by binding to the denatured proteins and assisting in refolding them into active forms. These two proteins are critical in both E. coli and Shigella spp for cell survival at low pH (pH<3). We studied the role of these two proteins in various enterohemorrhagic E. coli (EHEC) strains exposed to low pH (pH 2.0). The proteins were important for cells of an E. coli O145 strain associated with a 2010 lettuce-associated outbreak, because when we removed the proteins, the cells died off quickly. An intriguing result, however, was that removal of expression of one of the proteins did not affect the acid survival of E. coli O157:H7 cells grown at either 28°C or 37°C, and removal of the other protein led to only a slight decrease in survival, and only at 37°C. These results indicated that, at least in E. coli O157:H7, neither protein was important in cell survival at acidic conditions where they were critical in other E. coli. We determined that the gene for one of the proteins had a start codon point mutation and this suggested a mechanism for natural silencing of the gene. Our study indicates that E. coli O157:H7 must have evolved mechanisms for surviving acidic conditions independent of the two chaperones; the mechanisms result in O157 being more acid resistant than other types of E. coli.
Periplasmic chaperones HdeA and HdeB are known to be important for cell survival at low pH (pH<3) in E. coli and Shigella spp. Here we investigated the roles of these two acid chaperones in survival of various enterohemorrhagic E. coli (EHEC) following exposure to pH 2.0. Similar to K-12 strains, the acid protections conferred by HdeA and HdeB in EHEC O145 were significant. Depletion of HdeA and HdeB led to more than 2- and 3-log reduction in survival for cells grown at 37°C and 28°C, respectively. However, this protection was much less in E. coli O157:H7 strains. Deletion of hdeB did not affect the acid survival of cells grown at either temperature, and deletion of hdeA led to less than 2-fold decrease in survival, but only for cells grown at 37°C. Sequence analysis of hdeAB revealed a point mutation at the putative start codon of the hdeB gene in all 26 E. coli O157:H7 strains analyzed, which shifted the start codon ATG to ATA. This mutation correlated with the lack of HdeB in E. coli O157:H7, however, the plasmid-borne O157-hdeB was able to restore partially the acid resistance in an E. coli O145 hdeAB deletion mutant, suggesting the potential function of O157-HdeB as an acid chaperone. We conclude that E. coli O157:H7 have evolved mechanisms of acid survival independent of the HdeA/B chaperones, and are more acid resistant than non-pathogenic K-12, and enteropathogenic E. coli O55:H7. These results suggest a divergent evolution of acid resistance mechanisms within E. coli species.