Chromatic™ MH – Mueller Hinton chromogenic for microbial differentiation and antibiogram

Description
Chromatic™ MH is a Mueller Hinton chromogenic medium used for microbial differentiation simultaneously with the antibiogram for various microorganisms directly from clinical samples. The antibiogram is optimized for Gram-negative bacteria.

Typical formula (g/l)

Beef extract 2.0
Acid hydrolyzed casein 17.5
Starch 1.5
Chromogenic mixture 0.5
Agar 17.0
Final pH 7.3 ± 0.2 at 25°C

Principle of the method

Acid hydrolyzed casein and beef extract provide amino acids, nitrogen, carbon, minerals, vitamins, and other essential nutrients for the growth of microorganisms. Starch acts as a protective colloid against toxic molecules and, with its hydrolysis in the autoclave, provides a small amount of glucose as an energy source.

The chromogenic mixture allows for the identification of microorganisms based on the color and morphology of the colonies. Agar acts as a solidifying agent.

The Kirby-Bauer method is based on the diffusion of the antimicrobial through the agar. Each paper disc contains a specific concentration of antimicrobial agent, which inhibits the growth of the microorganism by forming a halo around the disc. The diameter of the inhibition halo is correlated with the minimum inhibitory concentration (MIC).

The Gradient Strip method is a quantitative analysis for determining the MIC of an antimicrobial agent and for identifying resistance mechanisms. The strip contains an exponential gradient of the antimicrobial agent and is placed on the surface of the inoculated plate. The MIC value is read where the edge of the inhibition ellipse intersects the strip.

Chromatic™ MH Agar is formulated with low levels of thymine and thymidine and controlled levels of calcium and magnesium ions.

Preparation

For the dehydrated medium, suspend 38.5 g of powder in 1 liter of sterile distilled or deionized water, mix well, and heat until completely dissolved. Sterilize in an autoclave at 121°C for 15 minutes, cool to 45-50°C, and dispense into Petri dishes under aseptic conditions.

Test procedure

Inoculate the plate by directly streaking 10 µl of sample on the surface of the agar. If necessary, dilute the sample in saline solution and mix with a vortex before seeding.
Apply the antimicrobial discs or MIC Test Strips on the surface of the inoculated plate.
Incubate at 37°C for 18-24 hours in an aerobic atmosphere.

Interpretation of results

After incubation, observe the color of the colonies and interpret the results according to the ID table.

Microorganisms and typical colony colors:
E. coli pink-reddish-mauve
Klebsiella spp, Enterobacter spp, Serratia spp green-blue
Proteus spp brown
Pseudomonas spp yellowish-green
S. aureus cream
E. faecalis green-turquoise

For the antibiogram, measure the diameter of the inhibition zone around the disc or read the MIC value. Interpret the diameters of the inhibition zones or the MIC values according to current standards and classify the microorganism as sensitive, intermediate, or resistant to the tested agents.

Appearance and storage

Dehydrated medium: fine, dry, homogeneous, beige
Prepared medium: amber, slightly opalescent

The powder is highly hygroscopic and should be stored at 10-30°C in a dry environment, in the original container tightly closed. Ready plates should be stored at 2-8°C away from light. Do not use the product after the expiration date or if it shows signs of contamination or deterioration.

Shelf life

Dehydrated medium: 4 years
Ready-to-use plates: 4 months

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