Sublethal Exposure to Common Benzalkonium Chloride Leads to Antimicrobial Tolerance and Antibiotic Cross-Resistance in Commensal and Opportunistic Bacterial Species

The production and consumer use of disinfectants has substantially increased during the COVID-19 pandemic. Benzalkonium chloride (BAC) is a mixture of alkyl benzyl dimethyl ammonium chloride compounds and is the most common active ingredient in surface cleaning and disinfecting products. Accordingly, BAC compounds are routinely in contact with microorganisms in indoor environments, which may contribute to the development of antimicrobial tolerance and cross-resistance. To investigate the impact of BAC exposure on commensal and opportunistic bacteria of public health importance, we exposed Staphylococcus epidermidis, Corynebacterium xerosis, Staphylococcus aureus, Klebsiella pneumoniae, Escherichia coli, and Pseudomonas aeruginosa to a standard BAC mixture (BAC12–14), as well as purified BAC16. Minimum inhibitory concentrations (MICs) and antibiotic susceptibilities were determined before and after repeated exposure to sublethal BAC concentrations. MICs for Gram-negative bacteria were significantly higher than Gram-positive bacteria. Additionally, BAC12–14 MICs were significantly higher for opportunistic pathogens and BAC-tolerance was associated with antibiotic cross-resistance. These results suggest that common Gram-negative opportunistic pathogens are less sensitive to BAC-inhibition than commensal species and may preferentially develop antimicrobial tolerance upon repeated or prolonged exposure to BAC12–14. Reevaluating the formulation and concentration of BAC-containing products in efforts to limit the development of antimicrobial tolerance and antibiotic co-resistance is warranted.