Bacterial communities in residential wastewater treatment plants are physiologically adapted to high concentrations of quaternary ammonium compounds

Benzalkonium chloride (BAC) is a quaternary ammonium compound (QAC) widely used as the active ingredient of disinfectants. Its excessive discharge into wastewater is constant and in high concentrations, likely affecting the physiology of microbial communities. We compared the physiological community profile of activated sludge (AS) bacteria with and without prior in vitro exposure to a high concentration of BAC (10 mg L⁻¹). We measured the community functional diversity, carbon substrate multifunctionality, and the median effective concentration that inhibits carbon respiration (EC₅₀) using Biolog EcoPlates supplemented with a gradient of 0–50 mg L⁻¹ of BAC. Surprisingly, we did not find significant differences in the physiological parameters among treatments. Certain abundant bacteria, including Pseudomonas, could explain the community's tolerance to high concentrations of BAC. We suggest that bacterial communities in wastewater treatment plants’ AS are “naturally” adapted to BAC due to frequent and high-dose exposure. We highlight the need to understand better the effects of QACs in wastewater, their impact on the selection of tolerant groups, and the alteration in community metabolic profiles.