Fatty acid salts promote antibiotic resistance development by modulating bacterial motility and antibiotic uptake

Wastewater treatment plants (WWTPs) are antibiotic resistance hotspots, driven by antibiotics and potentially exacerbated by non-selective constituents such as organic pollutants. Fatty acid salts (FAs) are abundant in WWTPs, originating from natural sources (e.g., medium- and long-chain FAs as soap, dairy products, oils) and external additions (e.g., short-chain FAs as carbon sources to enhance denitrification). The effects of FAs on driving resistance remain unclear. We investigated the effects of typical FAs (Sodium acetate-C2, Sodium decanoate-C10, Sodium stearate-C18) on antibiotic resistance development in activated sludge (AS) exposed to cefepime (CPM at 25 μg/L) for 10 days. Compared to CPM exposure alone, FAs addition (1 mg/L) caused higher phenotypic resistance to CPM under aerobic (35.9%-93.0%) and anoxic (53.8%-92.3%) conditions. Under aerobic conditions, sodium decanoate increased ARG abundance by 16.1%, showing the most significant enrichment. Six AS-isolated, G⁺ and G^- species demonstrated FAs-facilitated antibiotic resistance development. Notably, 1 mg/L sodium decanoate induced the highest increase in CPM resistance in E. coli by 11.7-fold in 40 days. On one hand, FAs reduced cell motility and increased cellular dispersal, thereby decreasing collective protection. On the other hand, FAs reduced EPS and increased membrane permeability, enhancing antibiotic uptake and selection pressure. The resistance-promoting effects of FAs extended to other antibiotics (ciprofloxacin and tetracycline) and across a broader range of FAs concentrations (1-100 mg/L). These findings provide critical insights into the significant risks posed by non-selective substances, such as FAs, in driving antibiotic resistance development across various environments, including but not limited to WWTPs.