The effect of a combination of Iso-Sensitest agar and ampicillin on the detection of enterobacter beta-lactamase.

The ampicillin MICs for faecal strains of E. cloacae (n = 11) and Enterobacter species not identified to species level (n = 3) were noticed to be two-fold to four-fold lower on Iso-Sensitest than on Mueller-Hinton II agar. The levels of inducible beta-lactamase on Iso-Sensitest, Mueller-Hinton, and a selective MacConkey-based agar, with and without ampicillin, were determined. The level produced on Iso-Sensitest with ampicillin was comparable to that induced by Mueller-Hinton alone. We conclude that when routine procedures for the detection of beta-lactamase-based resistance are evaluated, not only should care be taken to choose the right antibiotics, but also different combinations of beta-lactams and agar should be tested.