Antimicrobial Testing with Mueller Hinton Agar

BioLabTests offers a wide range of testing, from environmental, product and antimicrobial testing to hygiene audit support and microbiological research. Some of our services for antimicrobial testing include the Kirby Bauer Test and the Minimum Inhibitory Concentration (MIC) Testing. Here we share further information on the tests, their purpose for real-life applications and of course some trivia about the origins of the agar used for this type of testing (…you know how geeky we are).

The test method

The Kirby Bauer Test is widely used to determine the sensitivity or resistance of bacteria to various antimicrobial compounds, and it uses so called Mueller Hinton agar.

Mueller Hinton agar is inoculated with bacteria via the spread method, and a range of discs impregnated with an antimicrobial are placed on the surface for incubation. After incubation, the plates are observed for a ‘zone of inhibition’; a circular area relating to the level of antimicrobial activity upon the bacteria. Any zone of inhibition present indicates a degree of sensitivity to the antimicrobial whereby no zone indicates total bacterial resistance. Generally, the larger the zone, the more potent the antimicrobial!

The susceptibility of the bacteria can then be measured by taking the diameter of the inhibition zone from the centre of the disc and matching the number to a standard of values, depending on the antimicrobial and concentration.

Real-life applications for this particular testing

Whilst this method is primarily used for antibiotic susceptibility, it can be used to test other antimicrobials, such as the effectiveness of detergents and hand sanitiser.

Testing using the Kirby Bauer method can for example determine whether a microorganism, such as MRSA, is resistant to a specific antimicrobial. This could be useful to support research in finding a suitable detergent for laboratories to effectively ensure that there is no MRSA contamination.

In recent years there has been a lot of interest in antimicrobials, for example research into the antimicrobial activity of silver nanoparticles with and without the combination of antibiotics which has shown to reduce bacterial growth. This is something the Mueller Hinton agar is highly useful for.

The origins of Mueller Hinton Agar

The lady who we owe the discovery of the agar used for the Kirby Bauer test method is Jane Hinton, born in Canton Massachusetts on May 1, 1919. She grew up surrounded by science. Her father, a successful bacteriologist and pathologist, was the first African-American professor at Harvard University and the first African-American author of a textbook. Her parents ensured she had the best education available for black students at the time across Europe.

After completing her undergraduate degree, she worked at her father’s laboratories in Harvard. It was there she assisted John Howard Mueller, with whom she developed the Mueller-Hinton agar medium at the age of just 20 years old.

The Mueller-Hinton agar is a non-selective, non-differential medium capable of growing a wide range of non-fastidious organisms. It is considered a ‘loose’ agar, which helps to mediate the rate of diffusion of the antimicrobial more effectively than other types of media. They also discovered that the addition of starch acts to absorb any toxic factors found during growth. This discovery, alongside other ingredients, makes this medium ideal for standardised antimicrobial disc susceptibility testing.