The Evolution of Antibiotic Resistance in Experimental Populations of Bacteria

Abstract

Antibiotic resistance is a major threat to public health. Understanding how it evolves, and the genes that underlie resistance, is the main goal of my Ph.D. research. After a resistance mutation arises, it’s fate within a pathogen population will be etermined in part by its fitness: mutations that suffer little or no fitness cost are more likely to persist in the absence of antibiotic treatment. My research centers on understanding this process better by gaining knowledge about the spectrum of fitness effects associated with antibiotic resistance mutations. Using a meta-analysis framework I find that, across a range of antibiotics and pathogens, on average single resistance mutations exhibit fitness costs in the absence of drug, however, there are instances of cost-free mutations. To evaluate the conditions leading to the persistence of resistance in the absence of antibiotic, I use experimental evolution of the opportunistic pathogen Pseudomonas aeruginosa and the antibiotic ciprofloxacin to investigate the phenotypic and genetic differences associated with constant and fluctuating drug treatment. I find that fluctuating drug treatment leads to the evolution of cost-free resistance. At the genetic level, cost-free resistance is the result of second-site mutations that compensate for the fitness cost associated with ciprofloxacin-resistance mutations. Further examination of the resistance mutations shows a lack of epistatic interactions between co-occurring mutations that confer resistance within a single isolate. To investigate the repeatability of the genetic causes of resistance, I execute a second evolution experiment using multiple clinical strains of P. aeruginosa adapting to a constant ciprofloxacin selective pressure. I find a remarkable lack of parallel evolution at the genomic level both within and between different P. aeruginosa strains. I have shown that antibiotic resistance is costly, and that these costs can be ameliorated by second-site mutations that readily arise over short time scales. Additionally, different strains of the same bacteria can gain resistance through a diverse set of genetic mutations. On an applied level these results are not positive; combating resistance evolution will be difficult because pathogens can easily compensate fitness costs of resistance, and resistance itself can be gained via a large number of genetic targets.