The Colifast Early Warning System, based on measuring β-galactosidase activity (2 h method), was evaluated for monitoring the level of faecal contamination in the upper tidal Thames. Two trials were performed, one following heavy rain in November 2000, the next during a dry and sunny period in July 2001. In general the b-galactosidase activity and the two coliform reference methods (recovery following membrane filtration with membrane lauryl sulphate broth (MLSB) and Colilert™ Quantitray) were comparable. However, in several samples in July the β-galactosidase activity seemed to overestimate the number of culturable coliforms, suggesting that the rapid enzymatic method detected β-galactosidase produced by other bacterial sources, such as Aeromonas spp. or Vibrio spp., or nonculturable coliforms. The latter could be attributed to sunlight-induced injury. Nevertheless, the rapid method based on β-galactosidase activity gave an estimate of the level of culturable coliforms, which did not differ from both coliform reference methods by more than one log. Monitoring of β-galactosidase activity in river water samples using the Colifast Analyser may therefore be useful as an early warning indicator of faecal contamination.

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