Fate of Antibiotic Resistance Genes and Virulence Genes in Enterococci During Anaerobic Digestion Process of Thermal Hydrolyzed Sludge

Abstract

Municipal sludge contains large amounts of enterococci, which can harbor antibiotic resistance genes (ARGs) and virulence genes (VGs). ARG- and VG-containing enterococci therefore present potential resistance and virulence and, as a consequence, represent a significant health risk to humans. Therefore, the resistance phenotype of enterococci and the prevalence of ARGs and VGs in the enterococci isolated from the mesophilic (40℃) and thermophilic (55℃) anaerobic digestion of thermal hydrolyzed sludge was investigated. Results showed that the enterococci isolated from thermal hydrolyzed sludge showed significantly higher resistance to azithromycin than that to spiramycin and tetracycline. Thermophilic anaerobic digestion resulted in a greater reduction of enterococci abundance (by two orders of magnitude and one order of magnitude, respectively), and a greater reduction in the antibiotic resistance rates of the enterococci. However, thermophilic digestion can promote the expression of tetracycline resistance genes in the enterococci. Furthermore, both mesophilic and thermophilic digestion can facilitate horizontal genes transfer (HGT) between enterococci, which might result in an increase in the occurrence of double- or multiple-resistance. Mesophilic digestion reduced the prevalence of co-occurring ARGs and VGs in enterococci, while thermophilic digestion had the opposite effect. This research improves understanding of the occurrence and fate of ARGs and VGs in potential pathogens during the treatment of municipal sludge.