title: Amelioration of the Fitness Costs of Antibiotic Resistance Due To Reduced Outer Membrane Permeability by Upregulation of Alternative Porins.
abstract: The fitness cost of antibiotic resistance is a key parameter in determining the evolutionary success of resistant bacteria. Studies of the effect of antibiotic resistance on bacterial fitness are heavily biased toward target alterations. Here we investigated how the costs in the form of a severely impaired growth rate associated with resistance due to absence of two major outer membrane porins can be genetically compensated. We performed an evolution experiment with 16 lineages of a double mutant of Escherichia coli with the ompCF genes deleted, and reduced fitness and increased resistance to different classes of antibiotics, including the carbapenems ertapenem and meropenem. After serial passage for only 250 generations, the relative growth rate increased from 0.85 to 0.99 (susceptible wild type set to 1.0). Compensation of the costs followed two different adaptive pathways where upregulation of expression of alternative porins bypassed the need for functional OmpCF porins. The first compensatory mechanism involved mutations in the phoR and pstS genes, causing constitutive high-level expression of the PhoE porin. The second mechanism involved mutations in the hfq and chiX genes that disrupted Hfq-dependent small RNA regulation, causing overexpression of the ChiP porin. Although susceptibility was restored in compensated mutants with PhoE overexpression, evolved mutants with high ChiP expression maintained the resistance phenotype. Our findings may explain why porin composition is often altered in resistant clinical isolates and provide new insights into how bypass mechanisms may allow genetic adaptation to a common multidrug resistance mechanism.
authors.affiliation: Department of Medical Biochemistry and Microbiology, Uppsala University, Uppsala, Sweden.
authors.forename: Michael
authors.initials: M
authors.lastname: Knopp
authors.affiliation: Department of Medical Biochemistry and Microbiology, Uppsala University, Uppsala, Sweden dan.andersson@imbim.uu.se.
authors.forename: Dan I
authors.initials: DI
authors.lastname: Andersson
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001425:Bacterial Outer Membrane Proteins / Q000235:genetics / Q000378:metabolism; D015780:Carbapenems / Q000494:pharmacology; D002463:Cell Membrane Permeability; D004352:Drug Resistance, Microbial / Q000235:genetics; D004926:Escherichia coli / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D019143:Evolution, Molecular; D056084:Genetic Fitness; D008826:Microbial Sensitivity Tests; D018272:Porins / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation; D015854:Up-Regulation; D001618:beta-Lactamases / Q000235:genetics
chemical_list: D000900:Anti-Bacterial Agents; D001425:Bacterial Outer Membrane Proteins; D015780:Carbapenems; D018272:Porins; D001618:beta-Lactamases
keywords: Escherichia coli; antibiotic resistance; compensatory evolution; fitness cost; porin