The composition and persistence of faecal coliforms and enterococcal populations in sewage treatment plants

Abstract

Aims: The changes in structure and composition of faecal coliforms and enterococcal populations in sewage from different treatment plants, and the elimination of vancomycin-and erythromycin-resistant enterococci (VRE and ERE, respectively) in these treatment plants was analysed to determine any selective reduction. Methods and Results: Faecal coliforms, enterococci, VRE, ERE and spores of sulphite-reducing bacteria were enumerated using standard methods. Samples were enriched where necessary in order to isolate antibiotic resistant strains. The structure and composition of these bacterial populations were determined by biochemical fingerprinting and clustering analysis. High diversity and similarity indexes were detected among all the bacterial populations in raw and treated sewage, independently of their origin and the treatment processes employed. Antibiotic resistant strains were detected in all sewage tested and no selective reduction was observed. Conclusions: The faecal coliforms and enterococci populations did not differ in the sewage samples studied. The vancomycin and erythromycin resistances of the enterococcal populations were similar in the sewage samples. Resistance to both antibiotics persisted after the treatment process independently of raw sewage flow, faecal origin or size of the human population contributing to sewage. However, sewage of mixed origin (human and animal) presented a lower similarity index for the two bacterial populations compared with that of the other human sewage analysed. Significance and Impact of the Study: Although a significant reduction in bacterial populations was observed, the persistence of VRE and ERE strains in the same proportions in sewage suggests that there is no selective elimination of bacterial populations during the treatment processes. The ability of antibiotic resistance strains to survive sewage treatment systems should be considered in certain water reuse programmes.

Distribution and persistence of fecal bacterial populations in liquid and dewatered sludge from a biological treatment plant

“…The reutilization of treated sewage or sludge could carry a possible health risk if these antibiotic-resistant bacterial populations persist. The persistence of these VRE and ERE strains in treated sewage has already been demonstrated in previous studies (Vilanova et al, 2004). However, their accumulation or persistence in sludge of the treatment plants is poorly described.…”

“…It has been demonstrated that FC and ENT populations have high similarity in composition and structure between raw and treated sewage. Moreover, the predominant species of both bacterial groups are consequently the same (Vilanova et al, 2004) for both kind of sewages. In contrast, a different proportion of these bacterial indicators has been reported when comparing sludge with respect to sewage (Chauret et al, 1999).…”

“…Clusters were constituted by isolates which presented a correlation coefficient of PhP-profiles higher than 0.975. FC and ENT isolates were identified by comparing their biochemical PhP-profiles obtained in the present study with those which were identified in previous studies (Blanch et al, 2003;Vilanova et al, 2004). Then, species identification of the FC representative isolates was performed using the API 20E gallery following the manufacturer's instructions and database profiling (bioMérieux, La Balme, France).…”

“…However, it has been reported that in some treatment plants, the proportions of fecal coliforms (FC), enterococci (ENT) (Hill and Sobsey, 1998) and even in some cases spores of sulphite-reducing bacteria (SRB) (Vilanova et al, 2004) remained identical in treated sewage. It has been demonstrated that FC and ENT populations have high similarity in composition and structure between raw and treated sewage.…”

Determination of the Efficiency of Treated Sludge as a Fertilizer

“…Among the processes being carried out in sewage treatment plants to reduce the pathogens are the removal of bacteria by inactivation, grazing by ciliated protozoa and adsorption to sludge solids and/or encapsulation within sludge flocks followed by sedimentation (Bitton 1994). Some sewage treatment plants have reported the presence of faecal coliforms (FC), enterococci (ENT) and spores of sulphite-reducing bacteria (SRB) in treated sludge (Hill and Sobsey 1998;Vilanova et al 2004). …”