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

A procedure involving the use of citrate-buffered lactose broth (pH 6.5) containing an analogue of a beta-galactoside (4-chloro-2-cyclopentylphenyl beta-D-galactopyranoside) has been developed for the enrichment of Shigella in competition with a 100-fold higher population of Escherichia coli. The system makes use of the beta-galactosidase activity of E. coli which hydrolyzes the phenolic derivative of beta-galactoside to galactose and an aglycone moiety (4-chloro-2-cyclopentylphenol) which is toxic to E. coli but is tolerated by Shigella. The procedure is particularly effective in the enrichment of S. sonnei and S. flexneri; S. dynsenteriae and S. boydii are enriched to a lesser extent.