Provisional quality control parameters and interpretive criteria for testing susceptibility of Streptococcus pneumoniae and Haemophilus influenzae to quinupristin/dalfopristin (RP59500). Antimicrobial Testing Quality Control Group.
Studies were undertaken to select tentative criteria for susceptibility testing of quinupristin/dalfopristin against Streptococcus pneumoniae and Haemophilus influenzae. Against 612 isolates of Streptococcus pneumoniae, MICs of quinupristin/dalfopristin were < or = 1.0 microg/ml for all but one strain. With a tentative MIC breakpoint of either < or = 1.0 microg/ml or < or = 2.0 microg/ml for susceptible, a disk diffusion zone diameter breakpoint of > or = 19 mm embraced all but two of the susceptible pneumococci; > or = 16 mm included all strains. For Haemophilus influenzae, MICs of quinupristin/dalfopristin clustered near the tentative breakpoints; 91.5% of the MICs were 2.0 to 8.0 microg/ml. This precluded satisfactory performance of the disk diffusion test in discriminating between resistant and susceptible isolates unless MIC breakpoints are modified for this species: clinical experience will be needed before that can be justified. Based on data from a multilaboratory study, the following quality control limits are proposed for Streptococcus pneumoniae ATCC 49619 when testing quinupristin/dalfopristin: 0.25 to 1.0 microg/ml for broth microdilution tests and 19 to 24 mm for disk diffusion tests. For tests of Haemophilus influenzae ATCC 29247, MIC limits are 2.0 to 16 microg/ml; disk tests were very reproducible but are not yet recommended.
The Clinical Microbiology Institute, Wilsonville, Oregon 97070, USA.
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P
Fuchs
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A
Barry
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S
Brown
D000900:Anti-Bacterial Agents / Q000008:administration & dosage / Q000494:pharmacology*; D004338:Drug Combinations; D006193:Haemophilus influenzae / Q000187:drug effects*; D008826:Microbial Sensitivity Tests / Q000592:standards*; D011786:Quality Control; D013296:Streptococcus pneumoniae / Q000187:drug effects*; D014769:Virginiamycin / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000494:pharmacology
D000900:Anti-Bacterial Agents; D004338:Drug Combinations; D014769:Virginiamycin; C113825:quinupristin; C113826:dalfopristin