Occurrence, diversity, and persistence of antibiotic resistant enterococci in full-scale constructed wetlands treating urban wastewater in Sicily

Abstract

Enterococci isolated from different sites of an urban wastewater treatment plant (consisting of three horizontal subsurface flow constructed wetlands) were investigated. One-hundred-thirty isolates were identified at species level and tested for resistance to eleven antibiotics, by microdilution method, and their clonal relatedness was established by SmaI-PFGE analysis. Results highlighted the persistence of enterococcal population in all effluents and the dominance of E. faecium species. A high incidence of antibiotic resistance against erythromycin, chloramphenicol, rifampicin and ampicillin was observed, with 120 strains (93%) showing a multi-drug-resistance. Numerous pulso-types with a unique pattern were detected indicating a high diversity within enterococcal population. The recurrence of some pulso-types in different effluents was disclosed and, within the same pulso-types, different resistance patterns were observed. Comparing the MIC values of strains from inlet and outlet, different trends were observed, highlighting a certain variability among constructed wetlands in affecting the antibiotic resistance among enterococcal population.

Keywords: Antibiotic resistance; Clones; Constructed wetland; Enterococcus faecalis; Enterococcus faecium; Urban wastewater treatment plant.