Demonstration of staphylococci with inducible macrolide-lincosamide-streptogramin B (MLSB) resistance in sewage and river water and of the capacity of anhydroerythromycin to induce MLSB

Staphylococci causing diseases in humans and animals are well described, whereas not very much is known about the staphylococci in natural ecosystems. Due to increased consumption of antibiotics, multiresistant species are released with excrements. Therefore, 1048 staphylococci from raw and treated sewage and from receiving water bodies were isolated, identified and tested for resistance against erythromycin, clindamycin, oxacillin and ciprofloxacin. More resistant staphylococci were present in raw sewage (33.8%) than in treated sewage (24.9%) or river water (10.9%). Of all isolates, 20.2% were resistant against the macrolide erythromycin which can induce cross-resistance against lincosamides and streptogramin B antibiotics (iMLS(B)). Erythromycin is metabolized to anhydroerythromycin and excreted with urine into sewage. The question arises whether anhydroerythromycin can also induce resistance against MLSB antibiotics in staphylococci. This was investigated with antibiotic susceptibility tests (D-tests) and macrodilution assays. Staphylococci with iMLS(B) phenotype in river water were more numerous (27.8%) than in treated sewage (18.9%). The most common MLSB resistance gene was ermC. Traces of erythromycin and anhydroerythromycin (1ngL(-1)) induced already resistance against clindamycin after only 10min exposure. This is reported for the first time and is relevant for risk assessment.