Presence of Antimicrobial-Resistant Bacteria and Resistance Genes in Soil Exposed to Wastewater Treatment Plant Effluent

Antimicrobial resistance (AMR) has become a world-wide health issue, and anthropogenic antibiotics entering the environment is cause for concern with regard to impacts on environmental bacteria. As water resources have become scarcer, reuse of wastewater treatment plant (WWTP) effluent has increased, creating a conduit for environmental antibiotic pollution. The aim of this study was to determine the impact of spray-irrigating effluent on the incidence of AMR in soil organisms in agricultural lands (Astronomy Site, Pennsylvania State University). This study performed culture work to assess resistance of Gram-negative and Gram-positive soil bacteria to four antibiotics (sulfamethoxazole, trimethoprim, ciprofloxacin, and ampicillin) and molecular work (qPCR) to quantify genes associated with AMR (sulI, sulII, ermB, and intI1). Compared to a control site, Gram-negative bacteria at the Astronomy Site appeared to have increased resistance to sulfamethoxazole and trimethoprim. Higher numbers of resistance genes by depth (below 35 cm) and by location were consistently observed at the Astronomy Site with copy numbers of some genes up to 106-fold higher than the control site. Increased quantities of sulI and intI1 in the top 0-5 cm of the soil profile appeared to be dependent upon the amount of effluent irrigation received, whereas the presence of sulII and ermB showed the opposite patterns. Overall, long-term reuse of WWTP effluent to spray irrigate cropped lands does appear to alter and possibly increase AMR in soil environments; however, additional work is necessary to determine potential impacts on human, wildlife, plant, and soil health.