Antibiotic resistance begets more resistance: chromosomal resistance mutations mitigate fitness costs conferred by multi-resistant clinical plasmids

Abstract

Plasmids are the primary vectors of horizontal transfer of antibiotic resistance genes among bacteria. Previous studies have shown that the spread and maintenance of plasmids among bacterial populations depend on the genetic makeup of both the plasmid and the host bacterium. Antibiotic resistance can also be acquired through mutations in the bacterial chromosome, which not only confer resistance but also result in changes in bacterial physiology and typically a reduction in fitness. However, it is unclear whether chromosomal resistance mutations affect the interaction between plasmids and the host bacteria. To address this question, we introduced 13 clinical plasmids into a susceptible Escherichia coli strain and three different congenic mutants that were resistant to nitrofurantoin (Δ nfsAB ), ciprofloxacin ( gyrA, S83L), and streptomycin ( rpsL, K42N) and determined how the plasmids affected the exponential growth rates of the host in glucose minimal media. We find that though plasmids confer costs on the susceptible strains, those costs are fully mitigated in the three resistant mutants. In several cases, this results in a competitive advantage of the resistant strains over the susceptible strain when both carry the same plasmid and are grown in the absence of antibiotics. Our results suggest that bacteria carrying chromosomal mutations for antibiotic resistance could be a better reservoir for resistance plasmids, thereby driving the evolution of multi-drug resistance. IMPORTANCE Plasmids have led to the rampant spread of antibiotic resistance genes globally. Plasmids often carry antibiotic resistance genes and other genes needed for its maintenance and spread, which typically confer a fitness cost on the host cell observed as a reduced growth rate. Resistance is also acquired via chromosomal mutations, and similar to plasmids they also reduce bacterial fitness. However, we do not know whether resistance mutations affect the bacterial ability to carry plasmids. Here, we introduced 13 multi-resistant clinical plasmids into a susceptible and three different resistant E. coli strains and found that most of these plasmids do confer fitness cost on susceptible cells, but these costs disappear in the resistant strains which often lead to fitness advantage for the resistant strains in the absence of antibiotic selection. Our results imply that already resistant bacteria are a more favorable reservoir for multi-resistant plasmids, promoting the ascendance of multi-resistant bacteria.

Non-Canonical Aspects of Antibiotics and Antibiotic Resistance

“…Some mutations leading to AR have a rather unexpected effect on the fitness cost of carrying resistance plasmids: mutations causing nitrofurantoin, ciprofloxacin, and streptomycin resistance in E. coli mitigate the costs of bearing multi-resistance plasmids isolated from clinical strains [ 180 ]. Should this be proven in actual clinical environments, it could indicate that mutations can foster AR by providing better hosts for extrachromosomal elements.…”

“…Beta-lactam resistance in S. pneumoniae has similar features: while mutations are the likely source of individual changes in PBPs resulting in decreased susceptibility to penicillins, current clinical isolates acquire these changes through transformation followed by recombination in a particular order to gain clinically relevant resistance [179]. Some mutations leading to AR have a rather unexpected effect on the fitness cost of carrying resistance plasmids: mutations causing nitrofurantoin, ciprofloxacin, and streptomycin resistance in E. coli mitigate the costs of bearing multi-resistance plasmids isolated from clinical strains [180]. Should this be proven in actual clinical environments, it could indicate that mutations can foster AR by providing better hosts for extrachromosomal elements.…”