Natural rpoS mutations contribute to the phenotypic heterogeneity of clonal populations in Escherichia coli O157:H7

Authors: Carter, Michelle; Louie, Jacqueline; Huynh, Steven; Parker, Craig

Submitted to: Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/26/2014
Publication Date: 6/9/2014
Citation: Carter, M.Q., Louie, J.W., Huynh, S., Parker, C. 2014. Natural rpoS mutations contribute to the phenotypic heterogeneity of clonal populations in Escherichia coli O157:H7. Food Microbiology. 44:108-118.

Interpretive Summary: We discovered various natural mutations in the gene encoding the global transcriptional regulator RpoS in curli-producing variants of the 2006 spinach-associated Escherichia coli O157:H7 outbreak strains. All mutations led to the production of a non-functional RpoS in curli-producing variants. Consequently, curli-producing variants displayed defective survival fitness under stressful conditions such as low pH, high temperature, or following the exposure to oxidative stress. However, this loss of survival fitness in curli-producing variants was compensated largely by the enhanced metabolic activities under the nutrient-limited conditions. Presence of subpopulations with distinct physiological properties would be beneficial mostly for the pathogen population as a whole because some variants are better equipped to survive under a particular condition or to adapt rapidly to a new environment. Therefore, ability to produce genetic variants, especially those carrying multiple altered phenotypes, would be more likely to improve fitness of a given pathogen population. Our study supports that the mutability of E. coli O157:H7 promotes the emergence of hyper-virulent strains including the 2006 spinach-associated outbreak strains.

Technical Abstract: We previously reported the distinct acid resistance between the curli-producing (C+) and curli-deficient (C-) variants of E. coli O157:H7, although the curli fimbriae were not associated with this intra-strain phenotypic divergence. Here we investigated the underlying molecular mechanism by examining the DNA sequences encoding the common transcriptional regulators of curli biogenesis and acid resistance. rpoS null mutations were detected in all eight C+ variants of the 2006 spinach-associated outbreak strains, whereas a wild-type rpoS was detected in all corresponding C- variants. Mutations included the single base insertion or deletion, transversion, or large deletion. Consistent with the role of RpoS, C+ variants were also much more sensitive to oxidative stress, osmotic stress, and heat shock compared with their corresponding C- variants. This loss of general stress fitness in C+ variants appeared to be the consequence of rpoS mutation solely since the stress resistances in C+ variants could be restored by a functional rpoS. Comparative transcriptomic analyses revealed a large number of differentially expressed genes between the curli variants derived from the same strain, characterized by the enhanced expression in C+ variants of genes related to cell metabolism, but a marked decrease in the transcription of genes related to stress fitness. Considering the low level curli production in the C+ variants of the 2006 spinach-associated outbreak strains, our data support that rpoS mutation is an adaptive mutation under nutrient-limited conditions, but not a result of selection for curli fimbriae.