Mueller Hinton agar is a non-selective, non-differential medium commonly used for performing the Kirby Bauer antibiotic susceptibility test. This test involves placement of paper disks impregnated with a collection of antibiotics onto the agar to determine whether they can influence the normal growth of a bacterial lawn on the agar surface. Lawns are “seeded” with bacteria, antibiotic disks are added, the plate is incubated overnight to promote growth, and the sizes of zones of inhibition (where bacteria were prevented from growing) are determined and compared to standard tables to determine the microbe’s susceptibility to each of the antibiotics used.
Mueller Hinton agar supports the growth of most medically-interesting bacteria, although some fastidious bacteria may require the addition of 5% sheep blood or other specifically-required ingredients to support their growth. The content of the medium is formulated to avoid ingredients that reduce the impact of growth products of the microbe and chemicals that interfere with antibiotic activity. The medium must be made to specifications and adjusted to neutral pH prior to use.
Contents of Mueller Hinton Agar in 1 liter of distilled water are:
- 2.0 g beef extract
- 17.5 g casein hydrolysate
- 1.5 g starch
- 17.0 g agar
- pH adjusted to 25 C.