Agar dilution

Agar dilution
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Purpose	Determination of antimicrobial susceptibility
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Agar dilution is a method used in microbiology to determine the minimum inhibitory concentration (MIC) of antimicrobial agents against specific bacteria. This technique is a standard procedure in clinical laboratories for assessing the antibiotic resistance of bacterial isolates.

Principle

The principle of agar dilution involves incorporating different concentrations of an antimicrobial agent into a solid growth medium, typically agar, and then inoculating the surface with a standardized number of bacterial cells. The lowest concentration of the antimicrobial that prevents visible growth of the bacterium is considered the MIC.

Procedure

Preparation of Agar Plates

1. Selection of Medium: The choice of agar medium depends on the type of bacteria being tested. Commonly used media include Mueller-Hinton agar for non-fastidious organisms. 2. Incorporation of Antimicrobial Agent: The antimicrobial agent is diluted in a series of two-fold dilutions. Each concentration is mixed with molten agar and poured into petri dishes to solidify. 3. Control Plates: Plates without the antimicrobial agent are prepared as controls to ensure bacterial viability.

Inoculation

1. Preparation of Inoculum: A bacterial suspension is prepared from a fresh culture, adjusted to a specific turbidity, usually equivalent to a 0.5 McFarland standard. 2. Application to Agar: A multipoint inoculator or a manual method is used to apply a defined volume of the bacterial suspension onto the surface of each agar plate.

Incubation

- The inoculated plates are incubated at an appropriate temperature, typically 35-37°C, for 16-20 hours.

Interpretation

- After incubation, plates are examined for bacterial growth. The MIC is the lowest concentration of the antimicrobial agent that completely inhibits visible growth.

Advantages

- Quantitative Results: Provides precise MIC values, which are crucial for determining the appropriate dosage of antibiotics. - Standardization: Highly standardized, allowing for reproducibility and comparison across different laboratories.

Disadvantages

- Labor Intensive: Requires preparation of multiple agar plates and careful inoculation. - Time Consuming: Longer time to results compared to automated methods.

Applications

- Used in clinical microbiology laboratories to guide antibiotic therapy decisions. - Essential for antimicrobial resistance surveillance programs.

Comparison with Other Methods

- Broth Dilution: Similar in principle but uses liquid medium. Agar dilution is more labor-intensive but provides clearer endpoints. - Disk Diffusion: Simpler and faster but less precise than agar dilution.

See Also

• Antimicrobial susceptibility testing
• Broth microdilution
• Etest

External Links

• [CDC Guidelines on Antimicrobial Susceptibility Testing]