INCREASED PROTECTION AFFORDED BY THE DEFINED SUBSTRATE TECHNOLOGY COLILERT SYSTEM BY ITS ABILITY TO DETECT SHIGELLA BETA-GLUCURONIDASE

The Defined Substrate Technology Colilert System (DST CS), which simultaneously detects total coliforms and Escherichia coli from a primary water sample, has been approved for use in the United States and other countries. The test determines the presence of E. coli in water by detection of beta-glucuronidase (beta-glu), an enzyme found in more than 95% of this species. In contrast, the elevated temperature lactose fermentation test, known as the 'faecal coliform' test, shows a false-negative rate of 15% and a false-positive rate of 15%. In a recent study of oxidant-damaged E. coli it was observed that Shigella spp. could produce a positive beta-glu. Shigellas were therefore collected from laboratories and utilities throughout the world to determine the incidence of beta-glu positivity by the widely used DST CS. The shigellas were diluted to low concentrations (between 1 and 10100 ml-l) to simulate a pollution event. The DST CS demonstrated a 71% beta-glu positive rate. In comparison, less than 2% of shigellas gave a positive faecal coliform test. Because shigellosis is primarily a water-borne disease, the ability of the DST CS to detect this genus increases the public health protection afforded by this method.