Rapid screening of colonies from Listeria selective agar

A rapid method ('Liststrip', Lab M) of screening aesculin-positive colonies from Oxford selective agar was evaluated. This method relies on the ability of all Listeria spp. to hydrolyse aesculin and to have a rapid phosphatase but neither a pyroglutamic acid beta-naphthylamide amidase nor produce indole. Of 198 Listeria spp., all gave identical results (as above) in 10 min (except four isolates of L. murrayi), whereas of 112 aesculin-positive colonies from Oxford agar that were not Listeria only one gave a similar result. The method was compared with Gram stain and oxidase as a screening procedure. This strip test method is simple enough to be used by relatively unskilled staff and provides a rapid and reliable method to screen colonies from Oxford selective agar.