An antimicrobial susceptibility testing medium which may be used in internationally recognised standard procedures.
|Beef, dehydrated infusion from||
|pH 7.3 ± 0.1 @ 25°C|
Mueller-Hinton Agar was designed to be a reproducible culture medium for the isolation of pathogenic Neisseria species (Mueller and Hinton1). The inclusion of starch ensures that toxic factors found during growth will be absorbed and its presence is often essential to establish growth from very small inocula2.
However, specific GC media have replaced Mueller-Hinton Agar for this purpose.
The major use of Mueller-Hinton Agar is for Antimicrobial Susceptibility Testing (AST). It has become the standard medium for the Bauer-Kirby method3,4, and is specified by the Clinical & Laboratory Standards Institute (CLSI) formerly the National Committee for Clinical Laboratory Standards (NCCLS)5,6,7 and The European Committee on Antimicrobial Susceptibility Testing (EUCAST)8.
Oxoid Mueller-Hinton Agar meets the requirements of WHO9,10. Criticisms have been made about variation in performance of Mueller-Hinton Agar between and with manufacturers’ batches/lots of medium11. The causes of such variation are:
1. Differences in concentration of divalent cations Mg++ and Ca++. These effects are shown as MIC variations with aminoglycosides against Pseudomonas aeruginosa and tetracycline against staphylococci12,13,14.
2. Variation in thymine and thymidine content, which affect sulphonamide and trimethoprim MIC values15,16.
3. The concentration of manganese, which affects resistance interpretations with glycylcylines17.
4. The concentration of zine, which affects resistance interpretations with imipenem and potentially other carbapenems.18
5. Differences in the characteristics of the agar used in the medium, especially diffusion properties19.
In the light of such criticisms the CLSI called interested manufacturers together to discuss the standardization and stabilization of Mueller-Hinton Agar. Control methods were established whereby critical antimicrobial/organism combinations had to yield consistent zones of inhibition within 2mm of the specified diameters in the standards7.
The result of this cooperative effort is that Mueller-Hinton Agar became a standard medium and as well as being the medium of choice for CLSI methodology it is also specified by EUCAST8. The medium meets the criteria described in the Technical Specification ISO/TS 16782:201618.
Antibiotic susceptibility tests are performed in accordance with and meet the acceptance limits of the current ISO/TS 16782 using the following microorganisms:
Staphylococcus aureus ATCC®25923 WDCM00034
Staphylococcus aureus ATCC®29213 WDCM00131
Staphylococcus aureus ATCC®43300 WDCM00211
Staphylococcus aureus NCTC 12493 WDCM00212
Escherichia coli ATCC®25922 WDCM00013
Escherichia coli ATCC®35218
Pseudomonas aeruginosa ATCC®27853 WDCM00025
Enterococcus faecalis ATCC®33186 WDCM00210
Enterococcus faecalis ATCC®29212 WDCM00087
Streptococcus pneumoniae ATCC®49619
Haemophilus influenzae ATCC®49247
Haemophilus influenzae ATCC®49766
Mueller-Hinton Agar supplemented with yeast, NAD and haematin is used specifically for the susceptibility testing of Haemophilus influenzae20. For further details see Haemophilus Test Medium (HTM), CM0898.
Mueller-Hinton Agar supplemented with 5% defibrinated horse blood (SR0050) and 20 mg/L β-NAD is specified by EUCAST for testing fastidious organisms.
Mueller-Hinton Agar and Broth are used as the basis of solid and liquid media containing cefoperazone, trimethoprim, piperacillin and cycloheximide for selective isolation of Arcobacter spp. from meats21.
Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared plates at 2-8°C.