Effect of ceftiofur on mesophilic anaerobic digestion of dairy manure and the removal of the cephalosporin-resistance gene CMY-2
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Date
2019
Authors
Flores Orozco, Daniel
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Abstract
This research study evaluated the effect of ceftiofur (CEF), one of the most commonly used antibiotics on dairy farms, on the performance and stability of mesophilic anaerobic digestion of dairy manure, in terms of methane production, organic matter removal (COD, dCOD, TS, and VS) and intermediates (VFAs) profile. Additionally, this study also determined the fate of the antibiotic-resistance gene (ARG) cmy-2, a cephalosporin-resistance marker, in a mesophilic fed-batch anaerobic digester over a 600-day period and assessed how the presence of CEF alters the cmy-2 removal rate in 15-day batch anaerobic digestions. A semi-continuous lab-scale anaerobic digester was set up and operated under mesophilic conditions, with a hydraulic retention time of 30 days and fed with dairy manure from a commercial farm. The sludge of the digester was analyzed for cmy-2 levels and used as inoculum for batch experiments where different CEF concentrations (0.2-250 mg/L) were tested. The results indicated that low CEF concentrations (0.2 – 2 mg/L) do not have any significant effect on either methane production or the stability of the process. CEF concentrations of 10 mg/L can reduce methane productivity by more than 10% in anaerobic digesters operated at HRTs below 20 days. The presence of high CEF levels (50-250 mg/L) can cause a decrease in methane production of up to 60%, although the overall stability of the process is not compromised. The biochemical analyses suggested that hydrolytic microorganisms were the most affected by the presence of the antibiotic, while acetogens seemed to have temporarily slowed down, which was reflected in lower organic matter removal and slightly greater levels of butyrate and valerate accumulation during the anaerobic digestion process, respectively. Methanogens, on the other hand, were not affected, since no acetate or hydrogen accumulation was detected. Regarding the levels of the antibiotic resistance gene cmy-2, this study demonstrated that anaerobic digestions is a good alternative for the removal (> 90%) of the high cmy-2 levels found in dairy manures (>100 copies/ng DNA), and that the presence of ceftiofur (50-250 mg/L) does not substantially increase levels of the gene during 15-day batch anaerobic digestions.
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Keywords
Anaerobic digestion, Dairy manure, Ceftiofur, Antibiotic resistance genes, Methanogenesis