Integrative review finds fitness consequences of ESBL and AmpC resistance plasmids in E. coli are assay-dependent

The global spread of resistance to third-generation cephalosporins (TGCs) in Escherichia coli limits therapeutic options and poses major challenges for human, animal, and environmental health. The spread of resistance genes, including those for extended-spectrum β-lactamases (ESBLs), AmpC-type β-lactamases, and carbapenemases, has been facilitated by horizontal gene transfer (HGT), often via conjugative plasmids. This plasmid-mediated mobilization has enabled rapid adaptation to front-line antibiotics across diverse bacterial populations and ecological niches. Here, we bring together an integrative synthesis of molecular mechanisms, genetic vehicles, and ecological dynamics of cephalosporin resistance in E. coli, alongside a PRISMA-guided quantitative synthesis of 40 studies that provide data on the fitness consequences of resistance plasmids. We have analyzed a total of 154 experimental observations to identify patterns related to plasmid host background, resistance gene family, and fitness-assay framework. Because multiple observations were frequently contributed by the same study, we accounted for hierarchical structure using mixed-effects models with Study_ID as a random intercept and evaluated key patterns in the full dataset and stratified by assay type (growth curves vs. head-to-head competition assays). Moreover, we found that fitness estimates were sensitive to assay type. For instance, head-to-head competition experiments captured a broader range of deviations from neutrality than growth curve assays, although the apparent difference in mean standardized fitness between assay types was attenuated after accounting for study-level clustering. Across the curated dataset, host-associated and resistance gene-family-associated signals were method-dependent: both were evident overall and in head-to-head competition assays, but were not retained in growth-curve-only subsets. Our analysis supports a context-dependent interpretation in which plasmid-host compatibility, resistance-gene context, ecological setting, and the measurement framework jointly shape the observed fitness consequences and dissemination potential of resistance plasmids across environments.