An improved selective medium for the isolation of Escherichia coli O157.

Sorbitol-MacConkey medium has become widely used for the isolation of verotoxigenic (VT+) Escherichia coli O157. However, many organisms other than VT+ E. coli O157, especially other serogroups of E. coli and Proteus spp., may not ferment sorbitol, and thus may be confused initially with VT+ E. coli O157. Rhamnose is not fermented by VT+ E. coli O157, but is by most sorbitol non-fermenting E. coli of other serogroups. Cefixime is a cephalosporin antibiotic that is more active against Proteus spp. than against E. coli. Inclusion of rhamnose and cefixime in sorbitol-MacConkey agar improves its selectivity for the isolation of VT+ E. coli O157.