Unveiling antibiotic resistance dynamics in single and two-stage anaerobic digestion of dairy cow manure: Implications for environmental health

This study investigated antibiotic-resistant bacteria (ARB) survival during mesophilic anaerobic digestion (AD) of dairy cow manure with cefazolin (CEZ) antibiotic. Comparing single- and two-stage AD systems, we assessed ARB removal rates, biogas yield, volatile fatty acid concentration, and microbial populations. The results revealed that in the two-stage AD system, the methanogenic phase (MP) exhibited the highest removal rates of CEZ-resistant (CEZ-r) and oxytetracycline-resistant (OTC-r) bacteria at 9-16 % and 69-72 %, respectively. However, the MP had a higher proportion of ARB among the total culturable bacteria (24-30 %) compared to the acidogenic phase (AP) and single-stage AD. CEZ negatively impacted ARB removal rates and biogas production in both systems, with a more pronounced effect in single-stage AD. Biogas yield in the two-stage AD ranged from 337.7-385.6 mL/gVS, which was 10-20 % higher than that of the single-stage AD, regardless of CEZ addition. The reduction in biogas production due to CEZ was primarily attributed to the suppression of VFA production. As for microbial populations, the changes in the percentages of Firmicutes, Bacteroidetes, and Proteobacteria were likely related to the variations in ARB numbers, and the abundance of Methanosaetaceae significantly influenced biogas production.