Phenotypic detection of mec A-positive staphylococcal blood stream isolates: high accuracy of simple disk diffusion tests.
Detection of oxacillin-resistance in staphylococci by phenotypic methods remains problematic. Although standardized susceptibility test methods are adequate for Staphylococcus aureus, many are less satisfactory for the coagulase-negative staphylococci (CNS). We have studied 108 consecutive blood culture isolates of staphylococci. The mec A gene was detected by PCR in one S. aureus and 55 CNS isolates. Susceptibility testing was performed as follows: oxacillin (1-microgram), ceftizoxime (30-microgram), and cephalothin (30-microgram) by disk diffusion; oxacillin, ceftizoxime, cephalothin, methicillin, ampicillin, ampicillin/ sulbactam, penicillin, cefazolin, imipenem, and meropenem by the broth microdilution method. In addition, isolates were tested by the oxacillin agar screen plate method. The single oxacillin-resistant S. aureus strain was detected by all oxacillin susceptibility test methods and by the ceftizoxime disk and MIC methods. Two oxacillin-susceptible S. aureus were intermediate (minor error) by ceftizoxime broth microdilution (MIC, 16 micrograms/mL). The most sensitive, simple phenotypic methods for detection of oxacillin-resistant CNS (mec A positive) were as follows: oxacillin disk diffusion at 98%, oxacillin screen plate at 91%, oxacillin broth microdilution at 87%, ceftizoxime disk diffusion at 100%, ceftizoxime broth microdilution at 87%, and methicillin broth microdilution at 83%. These results indicate that oxacillin and ceftizoxime disk diffusion tests are the most accurate phenotypic methods in routine clinical use for detection of oxacillin-resistant CNS. Oxacillin broth microdilution MIC testing (2% NaCl supplement) would perform more satisfactorily (100% sensitivity) with an adjusted interpretive breakpoint at < or = 0.5 microgram/mL, in contrast to the lower accuracy of the "so-called" reference agar screen test.
Department of Pathology, University of Iowa College of Medicine, Iowa City 52242, USA.
M G
MG
Cormican
W W
WW
Wilke
M S
MS
Barrett
M A
MA
Pfaller
R N
RN
Jones
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000032:analysis*; D001431:Bacteriological Techniques; D004269:DNA, Bacterial / Q000032:analysis*; D004352:Drug Resistance, Microbial; D005798:Genes, Bacterial; D006801:Humans; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests; D016133:Polymerase Chain Reaction; D012680:Sensitivity and Specificity; D013203:Staphylococcal Infections / Q000097:blood*; D013210:Staphylococcus / Q000235:genetics* / Q000302:isolation & purification
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D004269:DNA, Bacterial