Soil's Role in Restoring Ecosystem Services

March 06 - 09, 2014

Biosolids commonly contain antibiotic residues that could accumulate and persist in the soil after land-application. There is growing concern about the long-term effects of these antibiotics on the soil microbial community, and the potential for transmission of resistance traits from soil to human and livestock pathogens. Composting has been shown to significantly reduce the concentration of antibiotics in manure but it is not clear what affect antibiotics may have on the microbial community in the compost, and therefore the soil.

We used two biological assays to determine if the synthetic antibiotic ciprofloxacin selects for and amplifies ciprofloxacin-resistant bacteria during the composting process. A third biological assay measured the capacity of the compost to adsorb ciprofloxacin. Samples of biosolids were treated with 20 mg kg^-1 ciprofloxacin, placed in mesh bags attached to steel cables, and buried in aerated compost bays. Each week for four weeks a set of bags (treated and untreated control) was removed and analyzed.

Our results show that the ciprofloxacin had no selective effect on resistant populations of bacteria during or after composting and that the compost is capable of adsorbing and effectively neutralizing this antibiotic. We recommend that similar methods be used to study the biological effects of other common antibiotics during the compost process. It would also be prudent to use these or similar methods to study the biological activity of antibiotics in Class B biosolids that are routinely land-applied but not composted.