Large-Scale Analysis of Fitness Cost of tet(X4)-Positive Plasmids in Escherichia coli

Abstract

Tigecycline is one of important antimicrobial agents for the treatment of infections caused by multidrug-resistant (MDR) Gram-negative bacteria. However, the emergence and prevalence of plasmid-mediated tigecycline resistance gene <i>tet</i>(X4) are threatening human and animal health. Fitness cost elicited by resistance plasmids is a key factor affecting the maintenance and transmission of antibiotic resistance genes (ARGs) in the host. A comparative analysis of the fitness cost of different types of <i>tet</i>(X4)-positive plasmids is helpful to understand and predict the prevalence of dominant plasmids. In this study, we performed a large-scale analysis of fitness cost of <i>tet</i>(X4)-positive plasmids origin from clinical isolates. These plasmids were successfully electroporated into a reference strain <i>Escherichia coli</i> TOP10, and a series of transformants carrying the <i>tet</i>(X) gene were obtained. The effects of <i>tet</i>(X4)-positive plasmids on the growth rate, plasmid stability, relative fitness, biofilm formation, and virulence in a <i>Galleria mellonella</i> model were evaluated. Consequently, we found that these plasmids resulted in varying degrees of fitness cost on TOP10, including delayed bacterial growth and attenuated virulence. Out of these plasmids, <i>tet</i>(X4)-harboring IncFII plasmids showed the lowest fitness cost on the host. Furthermore, by means of experimental evolution in the presence of commonly used drugs in clinic, the fitness cost of <i>tet</i>(X4)-positive plasmids was substantially alleviated, accompanied by increased plasmid stability. Collectively, our data reveal the differential fitness cost caused by different types of <i>tet</i>(X4)-positive plasmids and suggest that the wide use of tetracycline antibiotics may promote the evolution of plasmids.