MUELLER-HINTON AGAR
Code: CM0337
An antimicrobial susceptibility testing medium which may be used in internationally recognised standard procedures.
Typical Formula: gm/litre
Beef, dehydrated infusion from:300.0
Casein hydrolysate:17.5
Starch:1.5
Agar:17.0
pH 7.3 ± 0.1 @ 25°C
Directions
Add 38g to 1 litre of distilled water. Bring to the boil to dissolve the medium completely. Sterilise by autoclaving at 121°C for 15 minutes.
Description
Mueller-Hinton Agar was designed to be a reproducible culture medium for the isolation of pathogenic Neisseria species (Mueller & 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, GC selective media (GC Agar Base CM0367 plus supplements SR0056/SR0091/SR0095/SR0101/SR0104/SR0105) have replaced Meuller-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 its performance is specified by the NCCLS5,6.
Oxoid Mueller-Hinton Agar meets the requirements of WHO7,8. Criticisms have been made about variation in performance of Mueller-Hinton Agar between and with manufacturers’ batches/lots of medium9. The causes of such variation are:
1. The effects of differences in concentration of divalent cations Mg++ and Ca++. These effects are shown as MIC variations with aminoglycosides against Pseudomonas aeruginosa and tetracycline against staphylococci10,11,12.
2. Variation in thymine and thymidine content, which affect sulphonamide and trimethoprim MIC values13,14.
3. Differences in the characteristics of the agar used in the medium, especially diffusion properties15.
In the light of such criticisms the NCCLS called interested manufacturers together to discuss the standardisation and stabilisation 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 standards6.
The result of this cooperative effort is that Mueller-Hinton Agar is now a standard medium and declares on the label that it conforms to the NCCLS standard M6-A.
`This lot meets the NCCLS standard M6-A for dehydrated Mueller-Hinton agar.’
For further details of antimicrobial susceptibility testing see relevant section.
Mueller-Hinton Agar supplemented with yeast, NAD and haematin is used specifically for the susceptibility testing of Haemophilus influenzae16. For further details see Haemophilus Test Medium (HTM), CM0898.
Mueller-Hinton Agar and Broth are used as the basis of solid and liquid media containing cefoperazone, trimethoprim, pipera