ADAPTATION IN ACIDIC ENVIRONMENT POTENTIATES PATHOGENICITY IN BACTERIA

Microbes adapt to altering environmental stress with molecular and phenotypic changes favored by natural selection. For example, acidic stress appears in various natural and anthropogenic contexts. However, the rate, trade-offs, and genetic mechanisms of adaptation under persistent acidic stress remain poorly understood. More importantly, whether the adaptive results can alter the opportunity of host infection, or the difficulty of host treatment remains largely unexplored. To address these questions, we experimentally evolved Escherichia coli to acidic stress as a model system. Specifically, we propagated 12 replicate populations of E. coli for ~1,900 generations in Luria-Bertani media with two pH values (acidic: 4.5; neutral: 7.0) with daily transfers and shaking at 37°C. The evolved populations in the acidic environment adapted rapidly, sustaining a significant improvement in competitive fitness by (55.8 ± 7.3 %), while no significant difference was found in the neutral environment (-0.9 ± 1.5 %). Metagenomic sequencing of evolved populations revealed nonsynonymous and structural mutations enriched in the genes rho, rpoC, yfcD and rlmE for acidic environments. This result suggests the adaptation to acidic stress may involve the change of transcription and translation machineries or the enzymatic ability of dephosphorylation. Strikingly, adaptation in the acidic environment uniquely resulted in the emergence of population-level biofilm formation and collateral resistance to translation-targeting antibiotics. Together, our results demonstrate that environments can strongly shape the speed of microbial adaptation and reveal how pathogenicity can evolve as a byproduct of the adaptation to stressful environments.