Fate of antibiotic resistance genes under different wastewater treatments and environmental conditions in an Algerian watershed

In recent decades, antibiotic resistance has become a major health threat. This study evaluates the efficiency of two wastewater treatment plants (WWTP), conventional activated sludge and advanced filtration-based Enviro-Septic, for removing antibiotic resistance genes (ARGs) and their prevalence in an Algerian watershed. Thirty-five wastewater and 122 river samples were collected. Sampling covered a 50 km transect, from a low-pollution site to a water reservoir, at six sites. The study analyzed different fecal indicators (E. coli (EC), spores of sulfite-reducing clostridia (SRC), somatic coliphages (SOMCPH)), CrAssphage (CrAssPH)), and three ARGs (bla_TEM, tetW, and sul1). Mean concentrations in raw sewage from the conventional and Enviro-Septic WWTPs were ∼7.1 and 6.4 log₁₀ (CFU/100 ml) for EC, 6.2 log₁₀ (PFU or CFU)/100 ml for SOMCPH and SRC in both treatments, and ∼7.5 and 5.2 for CrAssPH, respectively. The conventional WWTP achieved reductions of ∼4 log₁₀ for EC and SOMCPH, 3.5 log₁₀ for CrAssPH, and 1 log₁₀ for SRC. The Enviro-Septic system showed similar efficacy for EC and SRC but lower for SOMCPH (2.8 log₁₀) and CrAssPH (2.5 log₁₀). The mean concentrations (log₁₀ GC/100 ml) of ARGs in raw sewage of the conventional and the Enviro-Septic WWTP were 8.6 and 7.3 for tetW, 9.4 and 8.1 for sul1, 8.4 and 6.3 for bla_TEM, respectively. Both treatments achieved reductions of 2.9-3 log₁₀ for all ARGs. All river samples tested positive for the three ARGs, with lower concentrations at less fecally polluted sites, showing a reduction of up to 4 log₁₀. Strong correlations (p < 0.05) were observed between culturable indicators, CrAssPH, and ARGs (ρ 0.58-0.96), indicating a strong association between ARGs and human fecal contamination, although other environmental sources cannot be ruled out. This study provides insights into ARG dynamics and supports strategies to mitigate their spread, and protect public health.