Antibiotic breakdown by susceptible bacteria enhances the establishment of β-lactam resistant mutants

For a better understanding of the evolution of antibiotic resistance, it is imperative to study the factors that determine the initial establishment of mutant resistance alleles. In addition to the antibiotic concentration, the establishment of resistance alleles may be affected by interactions with the surrounding susceptible cells from which they derive, for instance via the release of nutrients or removal of the antibiotic. Here, we investigate the effects of social interactions with surrounding susceptible cells on the establishment of Escherichia coli mutants with increasing {beta}-lactamase activity (i.e. the capacity to hydrolyze {beta}-lactam antibiotics) from single cells under the exposure of the antibiotic cefotaxime on agar plates. We find that mutant establishment probability is increased in the presence of susceptible cells due to the active breakdown of the antibiotic, but the rate of breakdown by the susceptible strain is much higher than expected based on its low enzymatic activity. A detailed theoretical model suggests that this observation can be explained by cell filamentation causing delayed lysis. While susceptible cells may hamper the spread of higher-resistant {beta}-lactamase mutants at relatively high frequencies, our findings show that they could promote establishment during their emergence.