More than 750 isolates of faecal coliforms (>200 strains), enterococci (>200 strains) and pseudomonads (>340 strains) from three wastewater treatment plants (WTPs) and from four groundwater wells in the vicinity of leaking sewers were tested for resistance against 14 antibiotics. Most, or at least some, strains of the three bacterial groups, isolated from raw or treated sewage of the three WTPs, were resistant against penicillin G, ampicillin, vancomycin, erythromycin, triple sulfa and trimethoprim/sulfamethoxazole (SXT). Only a few strains of pseudomonads or faecal coliforms were resistant against some of the other tested antibiotics. The antibiotic resistances of pseudomonads, faecal coliforms and enterococci from groundwater varied to a higher extent. In contrast to the faecal coliforms and enterococci, most pseudomonads from all groundwater samples, including those from non-polluted groundwater, were additionally resistant against chloramphenicol and SXT. Pseudomonads from sewage and groundwater had more multiple antibiotic resistances than the faecal coliforms or the enterococci, and many pseudomonads from groundwater were resistant to more antibiotics than those from sewage. The pseudomonads from non-polluted groundwater were the most resistant isolates of all. The few surviving faecal coliforms in groundwater seemed to gain multiple antibiotic resistances, whereas the enterococci lost antibiotic resistances. Pseudomonads, and presumably, other autochthonous soil or groundwater bacteria, such as antibiotic-producing Actinomyces sp., seem to contribute significantly to the gene pool for acquisition of resistances against antibiotics in these environments.
The possible deterioration of the microbiological quality of groundwater due to leaky sewer systems was investigated in a medium‐sized city (Rastatt) in SW‐Germany. Specially constructed focus observation wells drilled close to leaky sewers have been compared with the background observation wells in urban and rural areas. The sampling was performed in March (shallow depth of groundwater) and in July and October (deep groundwater table). It included the measurement of ammonium ions, temperature, electric conductivity, oxygen, and the depth of the groundwater table. Coliforms, Escherichia coli, and enterococci were enumerated as indicator organisms for wastewater contamination. In addition, the total number of colony forming units was determined. Bacterial counts of coliforms and E. coli generally increased during the course of the investigation (March to October). This may have been caused either by a slightly increased groundwater temperature (higher activity rates) or by a higher proportion of sewage in the urban groundwater system as a result of the dry season (summer). The contamination with faecal indicator bacteria was greater adjacent to leakage locations. In addition, elevated concentrations of ammonium ions were observed at these areas. The results provide evidence that leaky sewer systems elevate faecal indicator concentrations in groundwater with associated potential health risk where such waters are used for potable water supply.
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