Other Susceptibility Testscontinued Clinical Microbiology Review 230
TEST EXPLANATION
Tests for oxacillin- (methicillin-) resistant
S. aureus(MRSA)
Vancomycin screen
E test
D test
Oxacillin used as class representative for penicillinase-resistant penicillins. Organism resistant
to it is resistant to all. (Oxacillin-resistant S. aureusreferred to as methicillin-resistant for his-
torical reasons.) Oxacillin screen plate: Mueller-Hinton with 4% NaCl & oxacillin (6 μg /mL)
inoculated & incubated overnight. Any growth = resistant. Not suitable for testing coag-neg
staph (CNS). CLSI recommends cefoxitin disk diffusion method for both S. aureus& CNS. Many
labs use PCR. Can be done directly on specimen. Faster results.
For detection of vancomycin-resistant enterococci (VRE). Brain-heart infusion agar plus
6 μg vancomycin/mL inoculated & incubated overnight. Growth = resistance. Can also be
used for S. aureus.
Plastic strip containing antibiotic concentration gradient placed on inoculum lawn on Mueller-
Hinton plate & incubated overnight. MIC = point where border of growth inhibition intersects
strip. Combines convenience of disk diffusion with ability to generate MIC data. Useful for
testing fastidious organisms such as S. pneumoniae, other strep, H. influenzae, & anaerobes.
To detect inducible clindamycin resistance in MRSA isolates that are resistant to erythromycin
& susceptible to clindamycin on initial testing. Erythromycin & clindamycin disks placed
15–26 mm apart on Mueller-Hinton agar inoculated with organism. After overnight incuba-
tion, flattened zone between disks (D-shaped zone of inhibition around clindamycin disk)
means erythromycin induces clindamycin resistance. Clindamycin reported as resistant.
Similar procedure for beta-hemolytic strep except Mueller-Hinton with sheep blood used &
disks placed 12 mm apart.
