Increased protection afforded by the defined substrate technology Colilert system by its ability to detect Shigella β-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 β‐glucuronidase (β‐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 β‐glu. Shigellas were therefore collected from laboratories and utilities throughout the world to determine the incidence of β‐glu positivity by the widely used DST CS. The shigellas were diluted to low concentrations (between 1 and 10 100 ml‐1) to simulate a pollution event. The DST CS demonstrated a 71%β‐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.