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, ciprofloxacin was added to biosolid compost feedstock to achieve
a total concentration of 19 ppm, approximately 5-fold higher than that normally detected by liquid chromatography-tandem
mass spectrometry (LC-MS/MS) (1 to 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 the 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
Escherichia coli (P 0.28). Regression analysis showed that growth of the susceptible strain of E. coli can be reduced given a suf-
ficiently high concentration of ciprofloxacin (P < 0.007), a result that is consistent with adsorption being the primary mechanism
of sequestration. While analytical methods detected biologically significant concentrations of ciprofloxacin in the materials
tested here, the culture-based methods were consistent with the materials having sufficient adsorptive capacity to prevent typical
concentrations of ciprofloxacin residues from selectively enriching populations of resistant bacteria.