Gama, J. A. | Search bioRxiv

bioRxiv2020

Cryptic β-lactamase evolution is driven by low β-lactam concentrations

Our current understanding of how low antibiotic concentrations shape the evolution of contemporary {beta}-lactamases is limited. Using the wide-spread carbapenemase OXA-48, we tested the long-standing hypothesis that selective compartments with low antibiotic concentrations cause standing genetic diversity that could act as a gateway to develop clinical resistance. Here, we subjected Escherichia coli expressing blaOXA-48, on a clinical plasmid, to experimental evolution at sub-minimum inhibitory concentrations (sub-MIC) of ceftazidime. We identified and characterized seven single variants of OXA-48. Susceptibility profiles and dose-response curves showed that they increased resistance only marginally. However, in competition experiments at sub-MIC of ceftazidime, they showed strong selectable fitness benefits. Increased resistance was also reflected in elevated catalytic efficiencies towards ceftazidime. These changes are likely caused by enhanced flexibility of the {Omega}- and {beta}5-{beta}6 loops. In conclusion, low-level concentrations of {beta}-lactams can drive the evolution of {beta}-lactamases through cryptic phenotypes which may act as stepping-stones towards clinical resistance.

evolutionary biology↗

bioRxiv2020

Piggybacking on niche-adaptation reduces the cost of multidrug resistance plasmids

The persistence of plasmids in bacterial populations represents a puzzling evolutionary problem with serious clinical implications due to their role in the ongoing antibiotic resistance crisis. Recently, major advancements have been made towards resolving this "plasmid paradox" but mainly in a non-clinical context. Here we propose an additional explanation for the maintenance of multidrug resistance (MDR) plasmids in clinical Escherichia coli strains. After co-evolving two MDR plasmids encoding last resort carbapenem resistance with an extraintestinal pathogenic E. coli strain, we observed that chromosomal media adaptive mutations in the global regulatory systems CCR (Carbon Catabolite Repression) and ArcAB (Aerobic Respiration Control) pleiotropically mitigated the costs of both plasmids. Mechanistically, cost reductions were due to a net downregulation of plasmid gene expression. Our results suggest that global chromosomal transcriptional re-wiring during bacterial niche-adaptation may facilitate plasmid maintenance.

microbiology↗