Mueller-Hinton agar

Mueller-Hinton agar is a microbiological growth medium that is commonly used for antibiotic susceptibility testing, particularly for the Kirby-Bauer disk diffusion method. It is named after John Howard Mueller and Jane Hinton, who developed the medium in the 1940s. Mueller-Hinton agar is preferred for the Kirby-Bauer method due to its ability to produce consistent results, which are essential for accurately determining the susceptibility of bacterial strains to various antibiotics.

Composition

Mueller-Hinton agar is composed of:

• Beef infusion: 300.0 g
• Casein hydrolysate: 17.5 g
• Starch: 1.5 g
• Agar: 17.0 g
• Distilled water: 1000 mL

The beef infusion and casein hydrolysate provide the necessary nutrients for bacterial growth, including amino acids, peptides, and vitamins. Starch is added to absorb any toxic metabolites produced during bacterial growth. The agar serves as the solidifying agent.

Preparation

To prepare Mueller-Hinton agar, the ingredients are dissolved in distilled water and sterilized by autoclaving at 121°C for 15 minutes. Once sterilized, the medium can be poured into Petri dishes to solidify.

Usage

Mueller-Hinton agar is primarily used for the Kirby-Bauer disk diffusion method to test the susceptibility of bacterial strains to antibiotics. In this method, antibiotic-impregnated disks are placed on the agar surface inoculated with the test organism. As the bacteria grow, they encounter the antibiotic, and their growth is inhibited if they are susceptible. The area around the disk where bacteria cannot grow, known as the zone of inhibition, is measured. The size of the zone of inhibition is compared to standard values to determine if the bacteria are susceptible, intermediate, or resistant to the antibiotic.

Modifications

Several modifications of Mueller-Hinton agar exist, including:

• Mueller-Hinton agar with 5% sheep blood, used for testing fastidious organisms such as Streptococcus pneumoniae and Haemophilus influenzae.
• Mueller-Hinton agar with increased agar concentration (Mueller-Hinton II agar), used for testing the susceptibility of Neisseria gonorrhoeae and other fastidious organisms.

Limitations

While Mueller-Hinton agar is widely used for antibiotic susceptibility testing, it has some limitations. It is not suitable for testing the susceptibility of anaerobic bacteria or certain fastidious organisms that require specific nutrients or growth conditions not provided by Mueller-Hinton agar.

See Also

• Antibiotic susceptibility testing
• Kirby-Bauer disk diffusion method
• Agar plate
• Microbiological culture

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