Mueller-Hinton agar

Mueller-Hinton agar is a specialized culture medium widely employed in clinical microbiology for antimicrobial susceptibility testing. It was originally formulated by John Howard Mueller and Jane Hinton to support the growth of Neisseria and other fastidious organisms. Its standardized composition and low levels of inhibitors make it the medium of choice for standardized disk diffusion and broth dilution methods recommended by international bodies like the Clinical and Laboratory Standards Institute and the European Committee on Antimicrobial Susceptibility Testing.

Composition and preparation

The fundamental components of this medium include an infusion from beef and casein, which provide essential amino acids and nitrogen sources. It is enriched with starch, which is believed to absorb any toxic fatty acids released during bacterial growth, and solidified with agar. The final pH is adjusted to 7.2–7.4 at room temperature. Preparation involves dissolving the dehydrated powder in distilled water, followed by autoclaving for sterilization. For specific tests, such as for Haemophilus influenzae or Neisseria gonorrhoeae, the medium is supplemented with lysed horse blood and nicotinamide adenine dinucleotide.

Historical development

The medium was developed in the 1940s by researchers John Howard Mueller, working at Harvard Medical School, and his colleague Jane Hinton, one of the first African-American women to publish in the field of bacteriology. Their initial work, published in the Journal of Bacteriology, aimed to create a transparent medium for studying bacterial growth and hemolysis. Its utility for susceptibility testing was later recognized and refined, notably by W. M. M. Kirby and A. W. Bauer, who developed the standardized Kirby-Bauer disk diffusion test. This adoption was a pivotal moment in the standardization of antibiotic testing globally.

Standardization and quality control

Rigorous standardization is maintained under guidelines from the Clinical and Laboratory Standards Institute in the United States and the European Committee on Antimicrobial Susceptibility Testing. These protocols specify exact parameters for depth, pH, and divalent cation content, particularly calcium and magnesium, which can affect aminoglycoside and tetracycline activity. Quality control involves testing each batch with reference strains from collections like the American Type Culture Collection, such as Staphylococcus aureus ATCC 25923 and Escherichia coli ATCC 25922. Performance is also monitored by programs like the College of American Pathologists proficiency surveys.

Clinical and microbiological applications

Its primary application is in the Kirby-Bauer disk diffusion test to determine bacterial susceptibility to antimicrobial agents. It is the recommended medium for testing common pathogens like Staphylococcus aureus, Pseudomonas aeruginosa, and members of the Enterobacteriaceae. The medium is also used in Etest and agar dilution methods. Furthermore, it serves as the base for specialized tests, including screening for methicillin-resistant Staphylococcus aureus and detecting extended-spectrum beta-lactamase production in organisms like Klebsiella pneumoniae.

Limitations and modifications

A key limitation is its insufficient nutrients to support the growth of certain fastidious pathogens, such as Streptococcus pneumoniae and the aforementioned Haemophilus influenzae. To address this, supplements like sheep blood or chocolate agar are used. Variations include Mueller-Hinton agar with 5% sheep blood and Mueller-Hinton chocolate agar. Another modification is Mueller-Hinton agar with 2% NaCl, used for detecting oxacillin resistance in Staphylococcus. Performance can also be affected by thymidine content, which may interfere with trimethoprim and sulfonamide testing, leading to the use of low-thymidine formulations.

Category:Microbiology techniques Category:Laboratory equipment