Methods
Broth dilution
- Typically the gold standard
- Can be macro- or micro-broth dilution
- Increasing concentration of antimicrobial in the broth until growth is inhibited (MIC)
Agar dilution
- Same principle as broth dilution, but with agar plates
Kirby-Bauer disc diffusion
- Antimicrobial disc is placed on a Mueller-Hinton plate with controlled concentration of organisms
- Antimicrobial diffuses through the medium
- Standardized cutoffs for the diameter of the zone of inhibition
Non-phenotypic methods
- Examples include PBP2a rapid detection kits, PCR for known resistance genes (e.g. mecA), etc.
Specific Examples
Double-disc testing
- A variant of the Kirby-Bauer method above, but with two antimicrobial-eluting discs placed in close proximity
- Tests one antimicrobial inducing susceptibility or resistance to another antimicrobial
- Example includes the clindamycin D-test, where erythromycin may induce clindamycin resistance in S. aureus
