Use of tellurite for the selection of verocytotoxigenic Escherichia coli O157

Summary. Potassium tellurite was assessed for the selection of verocytotoxigenic (VT+) Escherichia coli 0 1 57. MICs were higher for VT+ E. coli 0 1 57 than for other strains of E. coli and for Aeromonas spp. MacConkey medium containing sorbitol, tellurite and cefixime (TC-SMAC) permitted the growth of VT+ E. coli 0157 and Shigella sonnei but partially or completely inhibited the growth of 67 O/O of other strains of E. coli and all or most strains of other sorbitol-non-fermenting species tested. Of 39 1 rectal swabs from cattle screened on TC-SMAC medium, 26 yielded isolates of VT+ E. coli 0157 whereas sorbitol-MacConkey medium with cefixime and rhamnose yielded only nine isolates. Inclusion of potassium tellurite in sorbitol-MacConkey agar markedly increased the rate of isolation of VT+ E. coli 0157 from cattle rectal swabs and may do so for other types of specimen