mikrodilution

Mikrodilution, or broth microdilution, is a laboratory method used to determine the antimicrobial susceptibility of microorganisms by exposing them to a range of antimicrobial concentrations in small-volume wells. The method yields the minimal inhibitory concentration (MIC), the lowest concentration that prevents visible growth after standard incubation.

Procedure: A standardized inoculum, typically about 5 x 10^5 CFU/mL for bacteria, is added to each well of a microtiter plate containing twofold serial dilutions of the test antimicrobial agent in broth (often Mueller-Hinton broth). Plates are incubated at 35-37°C for 16-20 hours (or longer for some organisms). The MIC is read as the lowest concentration with no visible growth, sometimes aided by a colorimetric or spectrophotometric endpoint.

Quality control and interpretation: Results are interpreted using clinical breakpoints published by guidelines bodies such as CLSI or EUCAST, which classify isolates as susceptible, intermediate (or I), or resistant at specified MIC values. Control strains with known MICs are tested to verify assay performance.

Applications and limitations: Microdilution is widely used in clinical microbiology, pharmacology, and surveillance to generate MIC data for many antibiotics against bacteria and some yeasts. It offers high throughput and standardization, and can be automated. Limitations include the need for accurate inoculum preparation and timing, potential variability with certain organisms or antibiotics, and the requirement for appropriate laboratory infrastructure.

Related methods: Other dilution-based approaches include broth macrodilution and agar dilution; gradient diffusion tests like Etest provide an MIC on an agar surface.