Mezlocillin: tentative interpretive standards for disk diffusion susceptibility testing

Abstract

The susceptibility of 447 clinical bacterial isolates to mezlocillin and carbenicillin was tested by standardized agar disk diffusion and reference broth microdilution methods. Tentative interpretive criteria for disk susceptibility testing by using 75-,ug mezlocillin disks are proposed: susceptible, 516 mm; indeterminate, 13 to 15 mm; and resistant, c12 mm. These would be applicable to both Pseudomonas species and the Enterobacteriaceae, but not to Staphylococcus aureus. For S. aureus, the breakpoints for susceptible, 529 mm, and resistant, c28 mm, hold for mezlocillin as well as for the other penicillinase-susceptible penicilhins.Mezlocillin is a new ureidopenicillin with a well-documented broad spectrum of in vitro antimicrobial activity (4,5,17,18 Organisms. The 447 selected recent clinical bacterial isolates tested are listed in Table 1. Of the isolates, 55% were tested by both methods at the Centers for Disease Control, Atlanta, Ga., and 55% were tested at the University of California, Davis, Medical Center, Sacramento, Calif. Ten percent of the isolates were tested at both institutions as a quality control of the methods, in addition to the strains recommended by the NCCLS, viz., Eacherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923, and Pseudomonas aeruginosa ATCC 25853. RESULTSThe distribution of MICs of mezlocillin and carbenicillin for the 447 selected clinical isolates is shown in Table 1. The differences in activity against the Enterobacteriaceae species were not great for most species. Exceptions to this were Citrobacter diversus and Klebsiella pneumoniae, which were eight times more susceptible to mezlocillin than to carbenicillin. E. coli was about twice as susceptible to mezlocillin. The 83 P. aeruginosa strains tested showed slightly less than one dilution step greater susceptibility to mezlocillin than to carbenicillin. This difference favoring mezlocillin was up to eightfold greater for other Pseudomonas species. Among the gram-positive isolates, mezlocillin had eight times greater activity than did carbenicillin against Streptococcus faecalis. Against penicillinase-producing S. aureus, carbenicillin was the more active drug, but non-beta-lactamase-producing strains were equally susceptible (MIC c 1 ug/ml) to both drugs. on May 7, 2018 by guest http://aac.asm.org/ Downloaded from

Setting interpretive breakpoints for antimicrobial susceptibility testing using disk diffusion

“…He also used population distribution studies of zone diameter values for isolates in order to confirm breakpoint setting [25]. His findings were later confirmed by Fuchs et al [26].…”