Abstract
Uropathogenic Escherichia coli, the predominant causative agent of urinary tract infections, are becoming increasingly resistant to commonly utilized β-lactam antibiotics leading to higher medical costs, prolonged treatment from persistent infections, and increased mortality. This rising resistance is primarily caused by the dissemination of acquired resistance mechanisms, namely plasmid-encoded β-lactamases. These enzymes include extended spectrum β-lactamases, AmpC cephalosporinases, and serine-carbapanemases. Decreased drug influx due to outer membrane porin defects can confer resistance to the carbapenems when paired with the production of the ESBLs and AmpCs. In this study, the mRNA and protein expression of the plasmid-borne β-lactamases, CMY-2, CTX-M-14, CTX-M-15, and KPC and the protein production of the porins, OmpC and OmpF in E. coli clinical isolates were characterized. It was hypothesized that increased β-lactamase production and/or outer membrane porin aberrations would result in elevated β-lactam minimum inhibitory concentrations.
There were four main findings from the work presented in this study. (1) β-lactamase transcript and protein levels were positively correlated for the CTX-Ms and CMY-2 but not KPC. (2) β-lactamase protein production broadly correlated with non-susceptibility to cephalosporins ceftriaxone, cefoxitin, ceftazidime and cefepime but not the carbapenems. Except for the KPC producers, the isolates were largely susceptible to the carbapenems meropenem, ertapenem and imipenem. (3) KPC protein levels could not explain variability in the β-lactam MICs of the KPC producers. (4) Porin aberrations were associated with carbapenem non-susceptibility in five isolates: three non-carbapenemase producers and two carbapenemase producers. Two of the five isolates, both non-carbapenemase producers, had undetectable levels of OmpC and OmpF on western blot analysis. The other three isolates with detectable porins harbored identical SNPs and indels within ompC. Additionally, one of these three isolates had a 122 bp deletion within ompF while the other two had identical SNPs within ompF.
Together, these findings indicate that the β-lactamase mRNA-protein correlation is enzyme specific and subject to post-transcriptional regulatory factors. The strength of the association between β-lactamase protein levels and β-lactam MICs are also enzyme specific and dependent on other resistance mechanisms such as porin deficiencies.