Multiple antibiotic resistances of Enterococcus isolates from raw or sand-filtered sewage

Fifty antibiotic-resistant Enterococcus strains were isolated from raw sewage of a wastewater treatment plant and from the same sewage after trickling through a 25-cm sand column, which retained >99% of the initial population. All 50 Enterococcus isolates were resistant against triple sulfa and trimethoprim/sulfamethoxazole and none were resistant against vancomycin. Most of the isolates from raw sewage were resistant to more antibiotics than the isolates from sand column effluent. One Enterococcus isolate from raw sewage (no. 61) and one Enterococcus isolate from sand column effluent (no. 95) had ten antibiotic resistances each. Isolate no. 95 maintained its resistances in the absence of antibiotics during the whole study. It was compared with isolate no. 70, which was one of the isolates, being resistant only against the two sulfonamides. Phenotypically and biochemically, the two organisms were strains of Enterococcus faecalis. Sequence analysis of partical 16S rDNA allowed alignment of isolate no. 95 as a strain of Enterococcus faecium and of isolate no. 70 as a strain of E. faecalis. E. faecium strain no. 95 carried at least six different plasmids, whereas for E. faecalis strain no. 70, no discrete plasmid band was seen on the gels.