Timed-release capsule method for coliform enumeration.

Abstract

Wax-coated capsules containing selective ingredients (brilliant green and oxgall) were added at the time of inoculation of most-probable-number media (modified lactose broths). The inhibitory ingredients gradually diffused from the capsules into the nonselective media, imparting selectivity to the media. Concentrations of brilliant green did not reach inhibitory levels until 2 or more h had elapsed, which permitted repair of some injured cells. Resuscitation of heat-injured Escherichia coli B cells occurred in the capsule-containing media, but not in conventional brilliant green bile 2% broth or violet red bile agar. No statistically significant differences were noted between coliform counts obtained on two groups of water samples by using the capsule, most-probable-number, membrane filtration, and pour plate methods. The capsule method could be used, however, as a combined presumptive and confirmed test for the examination of water. Improvements are needed to adapt the capsule method to the analysis of some categories of food.