Mueller Hinton Agar (MHA)
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
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
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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