Comprehensive Study of Antibiotic Resistance in Enterococcus spp.: Comparison of Influents and Effluents of Wastewater Treatment Plants

<b>Background/Objectives</b>: The spread of antibiotic resistance, particularly through <i>Enterococcus</i> spp., in wastewater treatment plants (WWTPs) poses significant public health risks. Given that research on antibiotic-resistant enterococci and their antibiotic-resistance genes in aquatic environments is limited, we evaluated the role of <i>Enterococcus</i> spp. in WWTPs by comparing the antibiotic resistance rates, gene prevalence, biofilm formation, and residual antibiotics in the influent and effluent using culture-based methods. <b>Methods</b>: In 2022, influent and effluent samples were collected from 11 WWTPs in South Korea. Overall, 804 <i>Enterococcus</i> strains were isolated, and their resistance to 16 antibiotics was assessed using the microdilution method. <b>Results</b>: High resistance to tetracycline, ciprofloxacin, kanamycin, and erythromycin was observed. However, no significant differences in the overall resistance rates and biofilm formation were observed between the influent and effluent. Rates of resistance to ampicillin, ciprofloxacin, and gentamicin, as well as the prevalence of the <i>tetM</i> and <i>qnrS</i> genes, increased in the effluent, whereas resistance rates to chloramphenicol, florfenicol, erythromycin, and tylosin tartrate, along with the prevalence of the <i>optrA</i> gene, decreased. <i>E. faecium</i>, <i>E. hirae</i>, and <i>E. faecalis</i> were the dominant species, with <i>E. faecalis</i> exhibiting the highest resistance. <b>Conclusions</b>: Our results suggest that WWTPs do not effectively reduce the rates of resistant <i>Enterococcus</i> spp., indicating the need for continuous monitoring and improvement of the treatment process to mitigate the environmental release of antibiotic-resistant bacteria.