Evaluation of the E test by using selected gram-positive bacteria

The E test (AB Biodisk NA Inc.) was compared with standard reference methods using the National Committee for Clinical Laboratory Standards's recommendations for determining the MICs of four selected antibiotics against 208 clinical isolates of gram-positive bacteria. These bacteria included 32 strains of Streptococcus pneumoniae, 25 strains of Enterococcus faecium, 20 strains of oxacillin-sensitive Staphylococcus aureus (OSSA), 96 strains of oxacillin-resistant S. aureus (ORSA), and 35 strains of coagulase-negative staphylococci. Evaluation included MIC accuracy within 1 dilution, reproducibility testing, and cost analysis. There was 94% agreement between the E test and the reference method in testing S. pneumoniae and penicillin G. There was 92% agreement with ampicillin and 100% agreement with vancomycin in testing E. faecium isolates. Accuracy of the oxacillin E test with staphylococci was significantly improved by the use of salt-supplemented Mueller-Hinton agar, for an agreement of 100% with coagulase-negative staphylococci and oxacillin-sensitive S. aureus and that of 85% with oxacillin-resistant S. aureus, with no major discrepancies. The E test with American Type Culture Collection isolates and clinical strains gave excellent reproducibility and was less costly than microdilution panels when used to test fewer than three antibiotics. The E test is a simple, reliable, reproducible, and cost-effective method for MIC determination for gram-positive organisms.