Frequency of Vancomycin Resistant Enterococci Strains in Sewage Treatment Plants in Isfahan during 2018 and 2019

Enterococci are members of human and animal gastrointestinal tract normal microbiota that enter the environment via sewage and can survive for a long period of time. These bacteria have the ability to acquire resistance to different classes of antibiotics and play an important role in spread of vancomycin resistance among Gram positive cocci. The aim of this study was to assess the presence of different enterococcal species in sewage treatment plants (STPs) in Isfahan.

During 2018 and 2019 sampling was carried out from incoming raw sewage, sludge and effluent of 2 sewage treatment plants in Isfahan. Samples were prepared and cultured on M-Enterococcus agar supplemented with vancomycin and 2,3,5-triphenyltetrazolium chloride. Selected colonies were identified as enterococcus genus using common biochemical tests and at the species level by multiplex-PCR assay using specific primers. Antibiotic resistance of strains to penicillin, erythromycin, ciprofloxacin, rifampin, tetracycline, nitrofurantoin, chloramphenicol, linezolid and quinupristin-dalfopristin was determined according to the guidelines of CLSI. The presence of vanA-vanG genes among the strains was assessed using multiplex-PCR assay.

A total of 80 vancomycin resistant enterococci (VRE) strains were identified in which 96 and 4% were Enterococcus faecium and Enterococcus faecalis, respectively. All strains showed resistance to penicillin, erythromycin and ciprofloxacin, and none of the strains were resistant to linezolid and quinupristin-dalfopristin. Moreover, 5 antibiotic resistance patterns were identified among the isolates and all strains were resistant to at least 4 antibiotics. All VRE strains were only positive for vanA gene and the other resistance genes were not detected.

The high prevalence of multiresistant VRE strains in effluent of STPs in Isfahan indicating the low efficiency of sewage treatment process for removing of antibiotic resistant bacteria