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Mueller Hinton Agar

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Mueller Hinton Agar (MHA) 

   The major use of Mueller Hinton Agar is for antimicrobial susceptibility testing.

Composition of MHA

Ingredients  
Beef Extract                                  2.00 gm
Acid Hydrolysate of Casein         17.50 gm
Starch                                           1.50 gm
Agar                                             17.00 gm
Distilled Water                             1000 ml
Final pH 7.3 ± 0.1 at 25ºC


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Why Mueller Hinton agar is used for antibiotic susceptibility testing?
1.    It is a non-selective, non-differential medium. This means that almost all organisms plated on here will grow.
2.    It contains starch. Starch is known to absorb toxins released from bacteria, so that they cannot interfere with the antibiotics. It also mediates the rate of diffusion of the antibiotics through the agar.
3.    It is a loose agar. This allows for better diffusion of the antibiotics than most other plates. A better diffusion leads to a truer zone of inhibition.
4.    MHA shows acceptable batch-to-batch reproducibility for susceptibility testing.
5.    MHA is low in sulfonamide, trimethoprim, and tetracycline inhibitors (i.e. concentration of inhibitors thymidine and thymine is low in MHA).
6.    Both the para-aminobenzoic acid (PABA) and thymine/thymidine content in Mueller Hinton Agar are reduced to a minimum, thus markedly reducing the inactivation of sulfonamides and trimethoprim when the media is used for testing the susceptibility of bacterial isolates to these antimicrobics.

Preparation of MHA
1.    Suspend 38 gm of the medium in one liter of distilled water.
2.    Heat with frequent agitation and boil for one minute to completely dissolve the medium.
3.    Autoclave at 121°C for 15 minutes. Cool to room temperature.
4.    Pour cooled Mueller Hinton Agar into sterile petri dishes on a level, horizontal surface to give uniform depth.
5.    Allow to cool to room temperature.
6.    Check for the final pH 7.3 ± 0.1 at 25ºC.
7.    Store the plates at 2-8 ºC.

Quality control:

 Incubate  at 35ºC. for 18-24 hours.

If microbial growth on medium,it couldn't use 

If don't microbial growth on medium,it could use 

Image result for mueller hinton agar
Limitations of MHA
1.    Numerous factors can affect results: inoculum size, rate of growth, medium formulation and pH. Strict adherence to protocol is required to ensure reliable results.
2.    Drug inactivation may result from the prolonged incubation times required by slow growers.
3.    Variation in the concentration of divalent cations, primarily calcium and magnesium affects result of aminoglycoside, tetracycline, and colistin test with P. aeruginosa isolates.
Referance:
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