Penicillin zone edge or "beach and cliff" test

A standard inoculum of the organism of interest is inoculated onto a Mueller-Hinton agar plate and incubated in the presence of a penicillin disk. After incubation, the zone edge of inhibition of growth around the penicillin disk is examined.
(A) A fuzzy zone edge, or "beach," indicates the absence of a beta-lactamase.
(B) A sharp zone edge, or "cliff," indicates the presence of a beta-lactamase.