Trends in Bacterial Antibiotic Resistance in Soils Following Long-Term Application of Municipal Biosolids.

Biosolids from wastewater treatment plants can provide nutrient- and organic-rich fertilizers for agricultural soil amendments. Biosolids result when the products of biological degradation of organic materials settle from the water. While methods for subsequent processing of biosolids (e.g., anaerobic digestion, drying, composting) target reductions in pathogen levels, concerns remain about organic (e.g., antibiotics, endocrine-disrupting compounds) or inorganic (e.g., heavy metals) compounds that may survive treatment processes and that may in turn be applied to soils. Studies have suggested that trace levels of antibiotics in biosolids can contribute to the development of antibiotic resistance in amended soils which, if true, could increase the potential for transmission of resistant organisms into the fertilized crops and ultimately into the human food chain. To assess the potential risk associated with the long-term use of biosolids, our research team sampled biosolid-amended soils at the Hornsby Bend Biosolids Management Plant (Austin, TX), where municipal biosolids have been applied to fields annually since 2002. Soil cores collected from sites receiving either10, 20, or 30 dry tons of biosolids per acre per year were divided into 0-30, 30-60, and 60-90 cm depths. In the laboratory, selective enrichment for Gram-positive organisms was followed by assessment for high-level resistance to 16 antibiotics. Overall, Gram-positive isolates from the Hornsby Bend soils showed high levels of antibiotic resistance, with more than 70% of isolates resistant to lincomycin and more than 20% of isolates resistant to penicillin, vancomycin, or ciprofloxacin. However, these levels were similar to resistance in bacteria from control soils with no history of biosolid application. Slight trends in increased resistance in plots with highest biosolids application rates (30 tons per acre per year) were found for penicillin, erythromycin, and chloramphenicol, but these trends were not statistically significant. Significantly reduced overall antibiotic resistance in deeper soils was found for all sites regardless of biosolids application rate, suggesting minimal transport of antibiotics and/or resistant organisms from the surface into deeper soil layers. Additional analysis will assess ranges in antibiotic sensitivity and multiple-antibiotic resistance in individual isolates. Data from this study will aid in establishing the public health and environmental risk of long-term use of biosolids application for agricultural soil amendment.