Mueller Hinton Agar

Mueller and Hinton developed Mueller Hinton Agar (MHA) in 1941 for the isolation of pathogenic Neisseria species. It is now commonly used for the routine susceptibility testing of non-fastidious microorganism by the Kirby-Bauer disk diffusion technique.

MHA is recommended for the diffusion of antimicrobial agents impregnated on paper disc through an agar gel as described in CLSI Approved Standard. Zone diameters are established for each antimicrobial agent determining resistant, intermediate, and sensitive results for pathogenic microorganisms

Mueller Hinton Agar with 5% sheep blood and Mueller Hinton Agar with Hemoglobin have been recommended for antimicrobial susceptibility testing of Streptococcus pneumoniae and Haemophilus influenza.

MHA

Mueller Hinton Media contains Beef Extract, Acid Hydrolysate of Casein, Starch and Agar. Beef Extract and Acid Hydrolysate of Casein provide nitrogen, vitamins, carbon, amino acids, sulphur and other essential nutrients. Starch is added to absorb any toxic metabolites produced. Starch hydrolysis yields dextrose, which serves as a source of energy. Agar is the solidifying agent. Final pH 7.3 ± 0.1 at 25ºC

Composition of Mueller Hinton Agar (MHA)

Ingredients	Gms/Litre
Beef extract	2.0
Acid hydrolysate of casein	17.5
Starch	1.5
Agar	17.0

 Uses of MHA

1. The major use of Mueller Hinton Agar is for antimicrobial susceptibility testing. It has become the standard medium for the  Kirby-Bauer method

2. It can be used to cultivate Neisseria

3. It is specified in FDA Bacteriological Analytical Manual for food testing, and procedures commonly performed on aerobic and facultative anaerobic bacteria.

Why MHA 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. It supports the growth of most non-fastidious bacterial pathogens andiv.

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. This medium is recommended for susceptibility testing of pure cultures only. Inoculum density may affect the zone size.

4. Heavy inoculum may result in smaller zones or too less inoculum may result in bigger zones. 

5. Fastidious organisms may not grow on this medium and may require supplementation of blood. 

6. As antimicrobial susceptibility is carried with antibiotic disc, proper storage of the disc is desired which may affect the potency of the disc. 

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References

l Atlas R.M and Snyder J.W. 2014. Handbook of Media for Clinical and Public Health Microbiology. CRC Press. Taylor & Francis Group. 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742. Page no.324-325

l http://himedialabs.com/TD/M173.pdf

l https://microbiologyinfo.com/mueller-hinton-agar-mha-composition-principle-uses-and-preparation/

l https://microbeonline.com/why-mueller-hinton-agar-is-used-in-routine-antibiotic-susceptibility-testing