# Consequences of benzalkonium chloride tolerance for selection dynamics and de novo resistance evolution driven by antibiotics

## TL;DR

Bacteria that become tolerant to the biocide BAC may have altered responses to antibiotics, influencing how they evolve resistance.

## Contribution

The study reveals how BAC tolerance affects bacterial selection dynamics and resistance evolution to antibiotics.

## Key findings

- BAC-tolerant E. coli outcompetes its ancestor under low ciprofloxacin concentrations.
- BAC-tolerant strains show reduced ability to evolve resistance to certain antibiotics.
- Evolvability differences are partly explained by epistatic interactions involving ompF mutations.

## Abstract

Biocides are used in large amounts in industrial, medical, and domestic settings. Benzalkonium chloride (BAC) is a commonly used biocide, for which previous research revealed that Escherichia coli can rapidly adapt to tolerate BAC-disinfection, with consequences for antibiotic susceptibility. However, the consequences of BAC tolerance for selection dynamics and resistance evolution to antibiotics remain unknown. Here, we investigated the effect of BAC tolerance in E. coli on its response upon challenge with different antibiotics. Competition assays showed that subinhibitory concentrations of ciprofloxacin—but not ampicillin, colistin and gentamicin—select for the BAC-tolerant strain over the BAC-sensitive ancestor at a minimal selective concentration of 0.0013–0.0022 µg∙mL−1. In contrast, the BAC-sensitive ancestor was more likely to evolve resistance to ciprofloxacin, colistin and gentamicin than the BAC-tolerant strain when adapted to higher concentrations of antibiotics in a serial transfer laboratory evolution experiment. The observed difference in the evolvability of resistance to ciprofloxacin was partly explained by an epistatic interaction between the mutations conferring BAC tolerance and a knockout mutation in ompF encoding for the outer membrane porin F. Taken together, these findings suggest that BAC tolerance can be stabilized in environments containing low concentrations of ciprofloxacin, while it also constrains evolutionary pathways towards antibiotic resistance.