Disk diffusion susceptibility testing for LY163892 (KT-3777), a new orally administered 1-carbacephem.

LY163892, a new orally administered 1-carbacephem, was found to have a spectrum of antimicrobial activity very similar to that of cefaclor. Diffusion tests with 30-micrograms LY163892 disks produced acceptable interpretive error rates with greater than or equal to 22 mm as the susceptibility zone diameter. This was required to reduce potential false-susceptibility results, particularly among Enterobacter spp. and Providencia stuartii: both species included strains that produce beta-lactamases capable of hydrolyzing Ly163892. Preliminary recommendations for LY163892 disk tests are presented, but the continued use of the 30-micrograms cephalothin "class representative" disk might be the best procedure to minimize LY163892 interpretive errors for clinical trials. A separate LY163892 disk for testing Haemophilus influenzae strains should be seriously considered.