Extended-spectrum beta-lactamase antibiotic resistance plasmids have diverse transfer rates and can be spread in the absence of selection

Horizontal gene transfer, mediated by conjugative plasmids, is one of the main drivers of the global spread of antibiotic resistance. However, the relative contributions of different factors that underlie this plasmid spread are unclear, particularly for clinically relevant plasmids harboring antibiotic resistance genes. Here, we analyze nosocomial outbreak-associated plasmids that reflect the most relevant Extended Spectrum Beta-Lactamase (ESBL) mediated drug resistance plasmids to i) quantify conjugative transfer dynamics, and ii) investigate why some plasmid-strain associations are more successful than others, in terms of bacterial fitness and plasmid spread. We show that, in the absence of antibiotics, clinical Escherichia coli strains natively associated with ESBL-plasmids conjugate efficiently with three distinct E. coli strains and one Salmonella enterica Serovar Typhimurium strain. In more than 40% of the in vitro mating populations, ESBL-plasmids were transferred to recipients, reaching final transconjugant frequencies of up to 1% within 23 hours. Variation of final transconjugant frequencies was better explained by variation in conjugative transfer efficiency than by variable clonal expansion of transconjugants. We also identified plasmid-specific genetic factors, such as the presence/absence of particular transfer genes, that influenced final transconjugant frequencies. Finally, we validated the plasmid spread in a mouse model for gut colonization, demonstrating qualitative correlation between plasmid spread in vitro and in vivo. This suggests a potential for predictive modelling of plasmid spread in the gut of animals and humans, based on in vitro testing. Altogether, this may allow straightforward identification of resistance plasmids with high spreading potential and to implement quarantine or decolonization procedures to restrict their spread.