Discipline: AMR & environment; Keywords: antibiotic resistance gene, cephapirin, dairy manure slurry.
Up to 90% of administrated antibiotics are eliminated from the animal body through the faeces or urine, implying that manure generated from animal production represents a major route of antibiotic transfer to the environment. The presence of antibiotics, even at very low concentrations, can contribute to emergence of antibiotic resistance genes (ARG) and selection of antibiotic resistant bacteria. Many studies have demonstrated that land application of untreated, antibiotic-laden manure can substantially enhance the abundance of ARG in agricultural soils. These ARG can be transferred to animals and humans through drinking water or the food chain, leading to decreased effectiveness of subsequent antibiotic therapies. Therefore, it is vital to develop cost-effective methods to degrade antibiotics in manure before land application to mitigate the dissemination of antibiotic resistance. Manure management practices such as composting and anaerobic digestion could effectively increase degradation of antibiotics and reduce the prevalence of ARG. However, high maintenance and operation costs, as well as high technical requirements, limit implementation of composting or anaerobic digester operations on many farms. Factors, such as temperature, pH, and CO2 and O2 levels, likely explain the effect of composting and anaerobic digestion on persistence of antibiotics and ARG. In the present study the objective of Dr M.M. Li and co-workers was to investigate two of these factors, temperature and pH, and evaluate their independent effects on antibiotic and ARG persistence. The results of their study were published in the Journal of Dairy Science, Volume 103 of 2020, page 2877 to 2882, with the title: Increasing temperature and pH can facilitate reductions of cephapirin and antibiotic resistance genes in dairy manure slurries.
The effect of various temperatures and initial pH shocks on the persistence of a cephalosporin antibiotic and ARG in dairy manure slurries was studied. Faeces and urine were collected from 5 healthy dairy cows administered with cephapirin (cephalosporin antibiotic) at dry-off via within udder infusion and were mixed with sterile water to generate manure slurries. Then in a 28 day incubation study, dairy manure slurries were either continuously exposed to one of three temperatures (10, 35 and 55°C) or received various initial pH (5, 7, 9, and 12) shocks.
The results showed that cephapirin was detected in the initial samples and on day one following all treatments, but it was undetectable thereafter. This indicates that cephapirin can be rapidly degraded irrespective of temperature and pH treatments. However, degradation was greater on day 1 with the 35°C and 55°C environments compared with the 10°C environment. Increasing pH beyond neutral also accelerated degradation as cephapirin concentrations were lower on day one after initial alkaline adjustments (pH 9 and 12) than after neutral and acidic adjustments (pH 7 and 5). No significant effect of temperature or initial pH was observed on abundances of a β-lactam ARG, cfxA, and a tetracycline ARG, tet(W), implying that bacteria that encoded cfxA or tet(W) genes were not sensitive to temperature or pH in dairy manure slurries. However, abundances of a macrolide ARG, mefA, were decreased in the 35oC and 55oC environments and also following exposure to a strong alkaline shock (pH 12).
The results suggest that increasing temperature or pH during storage of dairy manure slurries could be used together with other on-farm practices that are tailored to reduce the transfer of ARG from manure to the environment following land application.