Abstract
Escherichia coli sequence type 131 (ST131) is a successful pandemic clone associated with the spread of β-lactam and fluoroquinolone resistance. However, the reasons for the global dominance of this clone are largely unknown. It has been suggested that the success of ST131 E. coli is due to the resistance and virulence genes it possesses. Additionally, E. coli can decrease the permeability of its outer membrane through porin downregulation, contributing to β-lactam resistance. Aberrations in permeability correlate with decreased carbapenem susceptibility when the organism produces an extended-spectrum β-lactamase or plasmid-encoded AmpC. Altering the production of porins could impact the fitness, and therefore the success of ST131 E. coli. The experiments described in this dissertation sought to identify differences in the regulation of the OmpC and OmpF porins among different E. coli sequence types by comparing steady-state mRNA levels, mRNA half-life, small regulatory RNA (sRNA) expression, and protein production. It was hypothesized that the regulation of porin production in clinical isolates of E. coli would involve sRNAs and be dependent upon sequence type, impacting the emergence of carbapenem resistance through levels of porin production. Four main findings resulted from testing this hypothesis. (1) Levels of ompC mRNA were much higher than the corresponding levels of OmpC protein, and the variability in OmpC production between isolates correlated with sequence type. ST131 isolates produced the lowest levels of OmpC protein. (2) The ompC mRNA half-life was extended in ST131 isolates compared to non-ST131 isolates. However, the length of mRNA half-life did not correlate with steady-state ompC levels or the amount of OmpC produced. (3) Expression of the sRNA, MicC, was higher in ST131 isolates compared to non-ST131 isolates and correlated with OmpC protein levels in most isolates. (4) Conjugation and transformation experiments demonstrated that the level of porin and β-lactamase production were both important for the emergence of carbapenem resistance. Collectively, these findings indicate that OmpC regulation differs between ST131 and non-ST131 E. coli and involves the differential expression of micC. Moreover, these data suggest that physiological differences among different E. coli sequence types may contribute to the global success of ST131 E. coli.