Other Susceptibility Tests Clinical Microbiology Review 229
TEST EXPLANATION
Minimal inhibitory concentration (MIC)
Minimal bactericidal concentration (MBC)
Serum bactericidal test (Schlicter test)
b-Lactamase
Screen for penicillin susceptibility in
S. pneumoniae
Standard inoculum added to serial dilutions of antibiotics & incubated. MIC = least amount
of antibiotic (highest dilution) that prevents visible growth. Can be done in tubes, microtiter
trays, or automated analyzers.Clear tubes from MIC are subcultured to agar. MBC = concentration of antibiotic that results
in 99.9% ↓in CFU per mL.Serial dilutions of patient’s peak & trough specimens inoculated with standardized amount
of patient’s pathogen & incubated overnight. Serumstatic titer = highest dilution that in-
hibits growth. Clear tubes subcultured. Serumcidal titer = highest dilution with 99.9% ↓in
CFU/mL. Rarely performed.Detects enzyme that cleaves b-lactam ring, rendering penicillin & cephalosporins ineffective.
Test Haemophilus, N. gonorrhoeae, Moraxella. Bacteria applied to moistened disk impreg-
nated with cephalosporin nitrocefin (cefinase disk). Red color if b-lactam ring is broken,
i.e., organism produces b-lactamase.Test with oxacillin disk instead of penicillin disk. More sensitive in detecting resistance.
Mueller-Hinton with 5% sheep blood inoculated with organism. Oxacillin disk (1 μg) placed.
Incubated overnight in 5%–7% CO 2. Zone of inhibition ≥20 mm = susceptible to penicillin.
<20 mm, perform MIC.continued...