Carbapenem-resistant bacteria over a wastewater treatment process: Carbapenem-resistant Enterobacteriaceae in untreated wastewater and intrinsically-resistant bacteria in final effluent

Abstract

Although urban wastewater treatment plants (UWWTPs) are important sites for antibiotic resistance elimination, their limitation in producing resistance-free effluents is recognized. Despite the critical importance of carbapenems to human health, the fate and diversity of carbapenem-resistant bacteria (CRB) in UWWTPs is still poorly understood. We analysed CRB from different treatment stages in a UWWTP applying UV-C radiation. After secondary treatment, bacteria grown in mFC medium reduced in 1.9-log units (p < 0.0001), while CRB counts reduction (0.2-log units) was not significant. UV-C reduced the abundance of total bacteria and CRB (1.8 and 2.4-log units, respectively, relative to values after secondary treatment). Yet after incubation in the dark, CRB increased (0.6-log units) in UV-treated samples. Albeit in low amounts, carbapenem-resistant Enterobacteriaceae (CRE) were detected in raw wastewater being absent from the final effluent. Thirty-four CRE isolates were identified as Citrobacter, Enterobacter, Leclercia and Lelliottia. These were multiresistant and yielded classes 1, 2 and 3 integrons (94%, 88% and 88%, respectively). In all CRE isolates, blaGES-5 was found in integrons. Three isolates were selected for whole-genome sequencing; in Citrobacter braaki (n = 2) blaGES-5 was part of class 3 integrons, while in Lelliotia sp. RWM.1 blaGES-5 was in a class 1 integron with a novel cassette array (blaOXA-10/aacA4-blaGES-5-blaBEl-1). These integrons were in contigs with high similarity with mobilizable plasmids. Genes encoding resistance to other antibiotics were detected in these isolates. In the final effluent, CRB were predominantly affiliated with Stenotrophomonas maltophilia.UV-C radiation significantly reduced the abundance and prevalence of CRB. Bacteria intrinsically-resistant to carbapenems were cultivated after all treatment stages, while CRE only in raw wastewater. In these samples, we detected CRE with blaGES-5, in integrons and plasmids. This raises concern as horizontal gene transfer may occur within these systems. Carbapenem resistance surveillance in UWWTPs is essential to implement mitigation measures in a timely manner.