Community-level antimicrobial resistance in response to temporal gradients of antibiotic stress

Abstract

The development of antimicrobial resistance poses a global threat to human health. However, antimicrobial resistance has mainly been studied from a clinical perspective in single bacterial isolates. The purpose of this thesis was to investigate 1) how a resistance phenotype developed at the community level in a temporally variable environment and 2) how different evolutionary histories of bacterial community members influenced the community-scale resistance phenotype. Three treatment groups of bacteria with different evolutionary histories (ampicillin-evolved, ancestral, and mixed), were exposed to gradually increasing concentrations of ampicillin reaching a maximum concentration after 30 days, after which the ampicillin stress was removed representing a stress recovery phase. At the end of the ampicillin treatment phase and the recovery phase individual clones were randomly isolated from the samples and phenotyped for their level of ampicillin resistance.

Bacterial communities exposed to higher ampicillin concentrations had greater resistance than those exposed to lower ampicillin concentrations. The resistance of bacteria grown at low ampicillin concentrations increased during the recovery phase, but the resistance of the evolved treatment group grown at the highest ampicillin concentration decreased. Few differences were observed between the ancestral and mixed treatment groups. Overall, the level of resistance was positively correlated with higher ampicillin concentrations. The resistance of the evolved treatment group grown at the highest concentration of ampicillin decreased during the recovery phase, potentially indicating fitness costs of resistance. At low concentrations of ampicillin, resistance increased during the recovery phase, suggesting that resistance did not impose fitness costs. There were almost no differences between the ancestral treatment group and the mixed treatment group, which suggests that the evolved bacteria did not protect sensitive bacteria. Together these findings highlight how community level resistance can vary in response to temporally variable antibiotic concentrations and the role of evolutionary history in this resistance phenotype.