Josamycin: Interpretation of Inhibition Zones with the Bauer-Kirby Agar Disk Diffusion Test as Compared with Erythromycin

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Abstract
A total of 432 clinical isolates of Staphylococcus aureus (128), coagulase-negative staphylococci (123), group A and B .beta.-hemolytic streptococci (61), group D streptococci (30), Streptococcus pneumoniae (29), Haemophilus influenzae (19), Haemophilus parainfluenzae (12), and Legionella pneumophila (30) were examined with the agar dilution and Bauer-Kirby agar disk diffusion tests for susceptibility to josamycin as compared with erythromycin. On a weight-for-weight basis, erythromycin was more active than josamycin against all bacterial species, including L. pneumophila. Josamycin inhibited 18 of 23 S. aureus and 11 of 16 coagulase-negative staphylococcal strains resistant to erythromycin. Utilizing minimal inhibitory concentrations (MIC) breakpoints of .ltoreq. 2 .mu.g/ml (sensitive), 4 .mu.g/ml (intermediate) and of .gtoreq. 8 .mu.g/ml (resistant), and inhibition zone criteria of .gtoreq. 18 mm diameter (sensitive), 14-17 mm (intermediate), and .ltoreq. 13 mm (resistant), and excluding L. pneumophila, there was good correlation between erythromycin MIC and corresponding disk diffusion data for staphylococci and streptococci, but not for Haemophilus species. In comparison, josamycin yielded a significant number of minor discrepant data for group D streptococci and Haemophilus species. It is suggested that erythromycin and josamycin should not be tested against Haemophilus species, and that josamycin should be excluded from test batteries against enterococci. Erythromycin-resistant staphylococci require separate testing with josamycin.