Adding a complex microbial agent twice to the composting of laying-hen manure promoted doxycycline degradation with a low risk on spreading tetracycline resistance genes

Poultry manure is a reservoir for antibiotics and antibiotic resistance genes and composting is an effective biological treatment for manure. This study explored the effect of using two methods of adding a complex microbial agent to the composting of laying-hen manure on doxycycline degradation and tetracycline resistance genes elimination. The results showed that incorporating a complex microbial agent at 0.8% (w/w) on the 0ᵗʰ and 11th day (group MT2) effectively degraded doxycycline with a final degradation rate of 46.83 ± 0.55%. The half-life of doxycycline in this group was 21.90 ± 0.00 days and was significantly lower than that of group MT1 (1.6% (w/w) complex microbial agent added on the 0ᵗʰ day) and group DT (compost without complex microbial agent). But there was no significant difference in the final degradation rate of doxycycline between group DT and group MT1. The addictive with the complex microbial agent changed the microbial community structure. Bacteroidetes, Firmicutes and Proteobacteria were the dominant phyla during composting. Aerococcus, Desemzia, Facklamia, Lactobacillus, Streptococcus, and Trichococcus were the bacteria related to the degradation of doxycycline. Moreover, the incorporation of a complex microbial agent could decrease the risk on spreading tetracycline resistance genes. The single addition promoted the elimination of tetM, whose possible hosts were Enterococcus, Lactobacillus, Staphylococcus, and Trichococcus. Adding the complex microbial agent twice promoted the elimination of tetX, which was related to the low abundance of Chryseobacterium, Flavobacterium and Neptunomonas in group MT2. Redundancy analysis showed that the bacterial community, residual doxycycline and physiochemical properties have a potential effect on the variation in tetracycline resistance genes levels. Overall, adding the complex microbial agent twice is an effective measure to degrade doxycycline.