IDENTIFICATION AND USE: Mueller Hinton Agar is based on the formula recommended by Mueller and Hinton2 for the primary isolation of Neisseria species. Mueller and Hinton selected pea meal extract agar as a simple transparent medium containing heat stable ingredients.3 During their modification, starch replaced the growth-promoting properties of pea extract, acting as a “protective colloid” against toxic substances.

Bauer, Kirby, Sherris and Tuck4 recommended Mueller Hinton Agar for performing antibiotic susceptibility tests using a single disk of high concentration. This unsupplemented medium has been selected by the Clinical and Laboratory Standard Institute (CLSI)1 for several reasons. This medium is low in sulfonamide, trimethoprim and tetracycline inhibitors, and provides satisfactory growth of most non-fastidious pathogens along with demonstrating batch-to-batch reproducibility.

Mueller Hinton Agar is often abbreviated as M-H Agar, and complies with requirements of the World Health Organization. Mueller Hinton Agar is specified in FDA Bacteriological Analytical Manual for food testing, and procedures commonly performed on aerobic and facultatively anaerobic bacteria.7 A variety of supplementscan be added to Mueller Hinton Agar, including 5% defibrinated sheep or horse blood, 1% growth supplement and 2% sodium chloride.

PRINCIPLES OF THE PROCEDURE: Beef Extract and Acid Hydrolysate of Casein provide nitrogen, vitamins, carbon, and amino acids in Mueller Hinton Agar. Starch is added to absorb any toxic metabolites produced. Agar is the solidifying agent.

A suitable medium is essential for testing the susceptibility of microorganisms to sulfonamides and trimethoprim. Antagonism to sulfonamide activity is demonstrated by para-aminobenzoic acid (PABA) and its analogs. Reduced activity of trimethoprim, resulting in smaller growth inhibition zones and inner zonal growth, is demonstrated on medium possessing high levels of thymide. The PABA and thymine/thymidine content of Mueller Hinton Agar are reduced to a minimum, reducing the inactivation of sulfonamides and trimethoprim.