Improved procedure of selective enrichment of Shigella in the presence of Escherichia coli by use of 4-chloro-2-cyclopentylphenyl beta-D-galactopyranoside.

A previously described procedure for the selective enrichmen of Shigella in competition with E. coli has been modified and tested with a total of 48 strains of the four Shigella species. The new enrichment medium consists of 1,5-strength trypticase soy broth, 1 mM 4-chloro-2-cyclopentylphenyl beta-D-galactopyranoside (CPPG), 0.25% lactose, and 0.1 M citrate buffer (pH 6.2). In competition with a 1000-fold higher population of E. coli than Shigella, 42 of 48 strains from the four species of Shigella were selectively enriched by the new method. Different lots of CPPG did not appear to affect the performance of the medium.