Broth Microdilution
One-Sentence Definition
Broth microdilution determines the minimum inhibitory concentration by growing a standardized inoculum in a series of two-fold antibiotic dilutions in liquid medium, and is the reference method for susceptibility testing.
Simple Explanation
Serial halving dilutions of the drug in a 96-well plate. The first well with no visible growth gives the MIC.
Method
- Two-fold dilution series of each antibiotic in cation-adjusted Mueller-Hinton broth across a microtitre plate
- Inoculate to ~5 × 10⁵ CFU/mL — inoculum accuracy is the single biggest error source
- Incubate 16–20 h
- Read the lowest concentration with no visible growth = MIC
- Interpret against EUCAST/CLSI breakpoints
Why it is the reference method
- Quantitative and reproducible
- The method against which all other susceptibility methods, including genomic prediction, are validated — see Model Evaluation in Clinical Microbiology
- Required for agents where disk diffusion performs poorly (colistin must be tested by broth microdilution; it diffuses badly in agar)
Practical forms
- Commercial automated panels (Vitek, Phoenix, MicroScan) are miniaturized, partially automated broth microdilution with algorithmic reading
- Manual reference panels for confirmation, research, and new-agent evaluation
Limits
- Labour-intensive in manual form; fixed drug panels in commercial systems
- Truncated dilution ranges can hide how resistant an organism really is
- Reading errors at trailing endpoints (especially for antifungals and bacteriostatic agents)
Related MOCs
Active Recall Questions
- What defines the MIC well?
- Why must colistin be tested by broth microdilution?
- Why does inoculum density matter more than almost any other variable?