Environmental factors induced macrolide resistance genes in composts consisting of erythromycin fermentation residue, cattle manure, and maize straw.

With the growing concerns about antibiotic resistance, it is more and more important to prevent the environmental pollution caused by antibiotic fermentation residues. In this study, composted erythromycin fermentation residue (EFR) with the mixture of cattle manure and maize straw at ratios of 0:10 (CK), 1:10 (T1), and 3:10 (T2) explores the effects on physicochemical characteristics, mobile genetic elements (MGEs), and antibiotic resistance genes (ARGs). Results reflected that the addition of EFR reduced the carbon/nitrogen ratio of each compost and improved the piles' temperature, which promoted the composting process. However, the contents of Na+, SO42-, and erythromycin were also significantly increased. After 30 days of composting, the degradation rates of erythromycin in CK, T1, and T2 were 72.7%, 20.3%, and 37.1%, respectively. Meanwhile, the total positive rates for 26 detected ARGs in T1 and T2 were 65.4%, whereas that of CK was only 23.1%. Further analysis revealed that ARGs responsible for ribosomal protection, such as ermF, ermT, and erm(35), dominated the composts of T1 and T2, and most were correlated with IS613, electrical conductivity (EC), nitrogen, and Zn2+. Above all, adding EFR helps to improve the nutritional value of composts, but the risks in soil salinization and ARG enrichment caused by high EC and erythromycin content should be further investigated and eliminated.