Selective Chromogenic Isolation Medium for Bacillus anthracis Developed

A novel chromogenic agar, which selects for and distinguishes Bacillus anthracis from related Bacillus species, specifically B. cereus and B. thuringiensis, has been developed.

IITRI scientists Drs. Bruce Gingras, Margaret Juergensmeyer and Robert Sherwood were responsible for testing the efficacy of the medium, which was developed by L. Restaino and E.W. Frampton of R & F Laboratories, West Chicago, IL. Findings were detailed in a poster presentation at the 4th Decontamination Commodity Area Conference (Decon 2002), held Oct. 21-24, 2002, in San Diego, CA.

The medium incorporates chromogenic substrates for detecting specific enzyme activities in Bacillus anthracis, B. cereus, and B. thuringiensis. The enzymes targeted by the chromogenic mediium are not present in other Bacillus species, allowing for specific isolation of these three Bacillus species.

Inclusion of inhibitory compounds into the medium prevents the growth of environmental contaminants. The use of proprietary chromogenic substrates, heretofore identified as X-IP and X-CP, allows for the differentiation of Bacillus anthracis from near-neighbors B. cereus and B. thuringiensis.

Six strains of B. cereus and B. thuringiensis plated on this chromogenic agar displayed teal colonies with a white rim after 24 hours of incubation, while 16 strains of Bacillus anthracis exhibited cream colonies (a negative colorimetric reaction). An additional 24 hours of incubation causes these cream colonies to develop a blue center to the colony, positively identifying them as Bacillus anthracis.

The selectivity of this medium allows for an accurate isolation of Bacillus anthracis from environmental samples, substantially lowering the number of false positive and false negative samples. A reduced isolation time will allow for a potentially contaminated location to be declared free from a biological hazard in a much more rapid fashion.