Detecting antimicrobial resistance
From IDWiki
Phenotypic methods
Broth dilution tests
- Gold standard
- Uses serial two-fold dilutions of the antimicrobial agent
- Macrobroth has a final volume of 1-2 mL
- Microbroth has a final volume of 50 µL
- Determines an MIC and MBC (min. bactericidal concentration)
- Read at lowest concentration that inhibits growth (i.e. no obvious growth)
- Trailing can occur; the MIC is read at the first spot where trailing begins
Agar dilution
- Concentration of antibiotic is incorporated into the agar
- Each agar plate has a different concentration
- Multiple inocula are set up on each plate
- Final inoculum of 10^4^ CFU per spot
Disk diffusion
- Drug released from disk with an antimicrobial agent diffuses through the agar
- Zone diameter is inversely proportional to the log of the MIC
- Interpretation usually uses reflected light reading above a black, nonreflective background; measured using a ruler on the back of an inverted dish
- If on blood agar, must be read from the front
Other
- Potentiation using standard methods
- Colorimetric methods
- Lateral flow assay
Genotypic methods
- Detected resistance may not predict antibiotic failure if genes are non-functional or not expressed, or may be expressed in insignificant levels
- Similarly, a negative results may not indicate susceptibility given genetic variation, novel mechanisms of resistance, or inhibition of amplification
- Genotypic methods
- PCR
- Sequencing
