Mueller-Hinton agar (MHA) is the best medium to use for routine antimicrobial susceptibility testing using the Kirby-Bauer disc diffusion method for nonfastidious bacteria (both aerobe and facultative anaerobe). The use of media other than Mueller-Hinton agar may result in erroneous results.
Mueller-Hinton agar is also the standard medium used for most broth dilution testing as the conditions of this medium (i.e. pH, cation concentration, and thymidine contents) are well maintained.
Mueller Hinton agar (MHA) can be purchased from commercial suppliers or can also be prepared from the dehydrated medium. Be sure to prepare the media according to the manufacturer’s directions.
Formula for Mueller-Hinton agar per liter of purified water
- Beef extract: 2.0 g
- Acid Hydrolysate of casein: 17.5 g
- Starch: 1.5 g
- Agar: 17.0 g
Preparation of Mueller-Hinton agar
- Suspend 38 g of medium (or the components listed above) in 1 liter of purified water.
- Mix thoroughly.
- Heat with frequent agitation and boil for 1 minute to completely dissolve the components.
- Autoclave at 121°C for 15 minutes.
- Cool to 45°C
- Pour cooled Mueller Hinton Agar into sterile Petri dishes on a level, horizontal surface to give uniform depth.
Note: The plates must be poured to a depth of 4 mm (approximately 25 ml of liquid agar for 100-mm plates and 60 ml of liquid agar for 150-mm plates, but in any case to a measured depth of 4 mm). Plates that are too shallow will produce false susceptible results as the antimicrobial compound will diffuse further than it should, creating larger zones of inhibition. Conversely, plates poured to a depth >4 mm will result in false resistant results.
- Allow solidifying at room temperature.
- Check prepared MHA to ensure the final pH is 7.3 ±1 at 25°C.
Note: If the pH is <7.2 certain drugs will appear to lose potency (aminoglycosides, quinolones, macrolides), while other agents may appear to have excessive activity (tetracycline). If the pH is >7.4, the opposite results may occur.
- Prepared media can be stored at 4 to 8°C. MHA is stable for approximately 70 days from the date of preparation.
Note: Mueller Hinton agar should be tested with known strains of organism at least weekly in order to verify that the media and disks are working as expected.
Why Mueller-Hinton agar is used in routine antibiotic susceptibility testing (AST)?
Mueller-Hinton agar is the best medium for routine antibiotic susceptibility testing (AST) because of the following reasons:
- It shows acceptable batch-to-batch reproducibility for susceptibility testing
- It supports satisfactory growth of most nonfastidious pathogens
- It is low in sulfonamide, trimethoprim, and tetracycline inhibitors (i.e. concentration of inhibitors thymidine and thymine is low in MHA)
- A large body of data and experience has been collected concerning susceptibility tests performed with this medium.
Modifications of Muller Hinton agar
- Mueller Hinton agar medium supplemented with 5% sheep blood is recommended for determining the antimicrobial susceptibility of
- Streptococcus pneumoniae
- Neisseria meningitidis
- Haemophilus test medium (HTM) is the preferred medium for the antimicrobial susceptibility testing of H. influenzae using modified Kirby Bauer disk diffusion method. HTM medium consists of following ingredients: thymidine free MHA supplemented with 15 μg/ml NAD, 15 μg/ml bovine hemin, and 5 mg/ml yeast extract.