Impact of biosolids amendment and wastewater effluent irrigation on enteric antibiotic-resistant bacteria - a greenhouse study.

Reuse of wastewater effluent and biosolids in agriculture is essential to sustainable water and nutrient resource management practices. Wastewater and biosolids, however, are reportedly the recipients, reservoirs, and sources of antibiotic-resistant enteric pathogens. While decay rates of fecal bacterial indicators in soil are frequently studied, very few studies have reported on the persistence of the antibiotic-resistant sub-populations. Little is known about how multi-drug resistance phenotypes of enteric bacteria in agricultural soil change over time. In this study, germinated carrot seeds were planted in soil that received biosolids amendment and/or wastewater effluent irrigation in a greenhouse setting. We quantified total and antibiotic-resistant fecal bacterial indicators ( and enterococci) weekly in soil and total at harvest (day 77) on carrots. Antibiotic susceptibility of 121 and 110 enterococci collected isolates were determined. or enterococci were not recovered from the soil without biosolids amendment regardless of the irrigation water source. After biosolids amendment, soil and enterococci concentrations increased more than 3 log CFU/g-TS within the first week, declined slowly over time, but stayed above the detection limit (0.39 CFU/g-TS) over the entirety of the study. No statistical difference was found between effluent wastewater or water irrigation in soil total and antibiotic-resistant and enterococci concentrations or carrots levels. Soil antibiotic-resistant and enterococci decayed significantly faster than total and enterococci. Moreover, the prevalence of multi-drug resistant (resistance to three or more antibiotics) declined significantly over time, while almost all collected enterococci isolates showed multi-drug resistance phenotypes. At harvest, were present on carrots; the majority of which were resistant to ampicillin. The survival of antibiotic-resistant enteric bacteria in soil and on harvested carrots indicates there are transmission risks associated with biosolids amendment use in root crops.