Fractional Inhibitory Concentration

FIC, or Fractional Inhibitory Concentration, is a microbiological concept used to determine the interaction between two antimicrobial agents when used in combination. The FIC index is calculated to assess whether the combined effect of the antimicrobial agents is synergistic, additive, or antagonistic.

Here's a basic protocol for conducting an FIC assay:

Materials Needed:

Microbial cultures (bacteria)
Two antimicrobial agents (e.g., antibiotics)
Broth medium (e.g., Mueller-Hinton broth)
Sterile test tubes or microplates
Pipettes and tips
Incubator
Agar plates for subculture (Mueller-Hinton agar or appropriate medium)
Colony counter or other means of bacterial enumeration

Procedure:

Preparation of Microbial Inoculum:
a. Grow the bacterial culture on appropriate agar plates or in broth culture until it reaches the desired growth phase (usually log phase).
b. Adjust the microbial density to match a specific turbidity standard (e.g., 0.5 McFarland standard) for bacterial cultures.

Preparation of Antimicrobial Dilutions:
a. Prepare a series of dilutions of each antimicrobial agent individually in broth medium to achieve a range of concentrations.
b. Each dilution should be added to separate test tubes or wells of a microplate.

Preparation of Combination Dilutions:
a. Prepare a series of dilutions of the two antimicrobial agents in combination in broth medium. These dilutions should include a constant ratio of the concentrations of the two agents.
b. Each dilution should be added to separate test tubes or wells of a microplate.

Inoculation and Incubation:
a. Inoculate each test tube or well with a standardized amount of the microbial inoculum.
b. Incubate the tubes or plates at the appropriate temperature and conditions for the microorganism you are testing.

Subculture on Agar Plates:
a. After the incubation period, take samples from the tubes or wells that showed no visible growth (indicating inhibition) and streak them onto agar plates.
b. Incubate the agar plates at the appropriate temperature and conditions for the microorganism.

Enumeration and FIC Calculation:
a. After incubation, count the number of bacterial colonies on the agar plates for each antimicrobial concentration.
b. Calculate the Fractional Inhibitory Concentration (FIC) index for each combination by dividing the MIC of the combination by the MIC of each antimicrobial agent alone.
c. Calculate the sum of the FIC values for the two agents in each combination to determine whether the interaction is synergistic, additive, or antagonistic.

Data Interpretation:
a. A synergistic effect is indicated by an FIC index of ≤0.5.
b. An additive effect is indicated by an FIC index between >0.5 and ≤1.
c. An antagonistic effect is indicated by an FIC index >1.

Always follow established laboratory protocols, guidelines, and safety precautions when working with microorganisms and antimicrobial agents. The interpretation of FIC results should be done in consultation with experts in the field, as various factors can influence the outcomes of the assay.