MIC Test Strip | Etest | Principle | Reading

MIC strips allow the quantitative determination of minimum inhibitory concentrations (MIC) of antibiotics / antifungals against bacteria / fungi as well as the detection of resistance mechanisms.

MIC strips are specific paper strips impregnated with predefined gradients of antibiotic concentration (covering 15 double dilutions as for conventional MIC methods).

Principle

It is a thin, inert, non-porous plastic strip with a g / mL MIC reading scale on one side, and a preset antibiotic gradient on the other side.

When applied to a seeded agar surface, the preformed antibiotic gradient immediately transfers to the agar to form a stable, continuous and exponential gradient of antibiotic concentrations below the strip.

Example:

Etest® :developed by AB Biodisk, Solna, Sweden, and currently commercialized by bioMérieux,
MIC Test Strip® : developed by LIOFILCHEM, Italy

Procedure

❶ Suspend well-isolated colonies from an agar plate to obtain a turbidity of 0.5 McFarland (con ﬂ uent or almost con ﬂ uent growth should be obtained after incubation).

❷ Dip a sterile swab into the inoculum suspension and squeeze it against the wall of the test tube to remove excess fluid.

❸ Allow excess moisture to be absorbed so that the surface is completely dry (15 - 20 minutes) before applying the CMI strips.

❹ Apply the strip to the agar surface with the scale facing up and the strip code facing the outside of the plate, pressing it with sterile forceps onto the agar surface and ensuring that all of the length of the antibiotic gradient is in full contact with the agar surface. Once applied, do not move the tape.

❺ If air bubbles are trapped under the strip, gently move them to the edge using sterile forceps, taking care not to move the strip over the agar. When removing air bubbles, start at the lowest concentration and work your way up.

❻ Incubate the agar plates in the inverted position at 35 ± 2 ° C.

❼ After 18 hours of incubation or more, a symmetrical inhibition ellipse centered along the strip is formed. The MIC is read directly from the g / mL scale at the point where the edge of the inhibition ellipse intersects the MIC test strip.

Reading CMI strips

After 18 hours of incubation or more, a symmetrical inhibition ellipse centered along the strip is formed. The MIC is read directly from the g / mL scale at the point where the edge of the inhibition ellipse intersects the MIC test strip.