Biosolids and livestock manure are valuable high-carbon soil amendments, but they commonly contain antibiotic residues that might persist after land application. While composting reduces the concentration of extractable antibiotics in these materials, if the starting concentration is sufficiently high then remaining residues could impact microbial communities in the compost and soil to which these materials are applied. To examine this issue we spiked biosolids compost feedstock with ciprofloxacin at a concentration (19 ppm), approximately 5-fold higher than normally detected by LC-MS/MS (1-3.5 ppm). This feedstock was placed into mesh bags that were buried in aerated compost bays. Once a week a set of bags was removed and analyzed (treated and untreated, three replicates of each; 4 weeks). Addition of ciprofloxacin had no effect on recovery of resistant bacteria at any time point (P = 0.86), and a separate bioassay showed that aqueous extractions from materials with an estimated 59 ppm ciprofloxacin had no effect on the growth of a susceptible strain of E. coli (P = 0.28). Regression analysis showed that growth of the susceptible strain was diminished when compost was spiked with a wide range of ciprofloxacin (0-160 ppm; P<0.007), consistent with adsorption as the primary mechanism of antibiotic sequestration. Because bioassays reflect the bioavailability of residues whereas analytical assays do not, we recommend that similar bioassays be incorporated into studies of other antibiotic residues to better assess the risk that these residues pose for proliferating resistant populations of bacteria.
Youngquist, Caitlin firstname.lastname@example.org University of Wyoming
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