Correlation of moxalactam (LY127935) susceptibility tests by disk diffusion and agar plate dilution methods

Susceptibility of pathogenic aerobic bacteria to moxalactam (LY127935) was compared by two methods, diffusion from 30-microgram disks and agar plate dilution. The two methods gave a satisfactory degree of correlation when compared by linear regression, but the slope of the linear regression was significantly steeper for gram-negative bacilli (Enterobacteriaceae and Pseudomonas aeruginosa) than for coccal organisms (streptococci and staphylococci). The largest zone diameters by disk diffusion were found with Citrobacter, Proteus, Escherichia coli, Klebsiella, and Enterobacter strains. The error rate-bounded method of Metzler and DeHaan (J. Infect. Dis. 130:588-594, 1974) gave useful breakpoints of disk zone sizes for moxalactam resistance and susceptibility.