Response of bacterial pathogens to process upgrades and floc sizes in a full-scale landfill leachate treatment plant

Bacterial pathogens in wastewater environments pose serious public health risks, serving as reservoirs for antibiotic resistance genes (ARGs) and contributing to the global antimicrobial resistance (AMR) crisis. This study investigated how process upgrades and sludge aggregate sizes influence the distribution of bacterial pathogens and ARGs in a full-scale landfill leachate treatment plant (LLTP). Using 16S rRNA and metagenomic sequencing, we analyzed potential pathogens and ARG profiles during the progression from conventional nitrification-denitrification (CND) to partial nitrification-denitrification (PND). Results showed a notable increase in the relative abundance of potential pathogenic genera in large aggregates following the process upgrade, indicating structural and functional shifts in microbial communities. The average abundance of WHO-priority ARGs, including baeR, smeR, and adeL, was significantly higher at the PND phase, likely linked to the process upgrade. Additionally, the activity of mobile genetic elements (MGEs), particularly those involved in horizontal gene transfer and DNA repair, was enhanced at the PND phase, accelerating ARG dissemination. Importantly, the process upgrade reduced the relative abundance of Streptococcus pyogenes, a high-risk pathogen, suggesting improved pathogen control. This study provides critical insights into optimizing LLTP processes to mitigate AMR risks and improve public health safety.