Selective inhibition of Escherichia coli in the presence of Salmonella typhimurium by phenethyl beta-D-galactopyranoside.

Phenethyl beta-d-galactopyranoside (PEG) was hydrolyzed by the beta-galactosidase of Escherichia coli to form the toxic product phenethyl alcohol. Salmonella typhimurium did not hydrolyze PEG. In mixed culture, the ratio of S. typhimurium to E. coli was increased by growing the organisms in lactose broth containing 2.5% PEG. The high concentration of PEG required for inhibition of E. coli can be attributed to inadequate cell permeability rather than to prevention of beta-galactosidase induction.