Enumeration of Sulphite-Reducing Clostridia from Potable Water Supplies

The enumeration of sulphite-reducing Clostridia from water upon egg yolk-free tryptose-sulphite-cycloserine agar (TSC) was evaluated against the standard UK method of enumeration upon membrane-clostridial agar (MCA), with or without heat treatment of the sample. Using 280 samples of treated drinking water, raw groundwater, river water and sewage effluent, the recoveries of presumptive sulphite-reducing Clostridia were significantly higher on TSC than on MCA (both incubated at .37°C for 48 h), particularly from potable water samples where the system integrity had been breached. Clostridia were isolated from 26 of the 224 groundwater and drinking water samples, on 25 occasions with TSC but on only 3 occasions with MCA. From river water and sewage effluent samples, background growth was higher on TSC. Of 85 isolates from TSC, 92.9% were confirmed as Clostridia, whilst only 56.5% of 23 isolates from MCA confirmed, the remaining being Gram-positive rods and cocci and Gram-negative rods. Heat treatment of samples to 70°C for ten minutes prior to filtration did not affect counts on MCA, but significantly reduced both Clostridia and background counts on TSC. Clostridia isolated from compromised treated water supplies include Cloistridium perfringens, C. bgratii, C. difficile, C. limosum and C. subtgrminale.