Microbiological Media

Mannitol salt agar or MSA is a commonly used selective and differential growth medium in microbiology. It encourages the growth of a group of certain bacteria while inhibiting the growth of others. This medium is important in medical laboratories as one method of distinguishing pathogenic microbes in a short period of time.^[1] It contains a high concentration (about 7.5%-10%) of salt (NaCl), making it selective for Gram-positive bacteria (Staphylococcusand Micrococcaceae) since this level of salt is inhibitory to most other bacteria.^[2] It is also a differential medium for mannitol-fermenting staphylococci, containing carbohydrate mannitol and the indicator phenol red, a pH indicator for detecting acid produced by mannitol-fermenting staphylococci. ^[3] Staphylococcus aureus produces yellow colonies with yellow zones, whereas other coagulase-negative staphylococci produce small pink or red colonies with no colour change to the medium.^[4]

Müller-Hinton agar is a microbiological growth medium that is commonly used for antibiotic susceptibility testing. It is also used to isolate and maintain Neisseria and Moraxella species.

It typically contains:

• 2.0g beef extract
• 17.5g casein hydrolysate
• 1.5g starch
• 17.0g agar
• dissolved in 1 liter of distilled water.
• pH adjusted to neutral at 25 °C.^[1]

Five percent sheep blood and nicotinamide adenine dinucleotide may also be added when susceptibility testing is done on Streptococcus species. This type is also commonly used for susceptibility testing of Campylobacter.

It has a few properties that make it excellent for antibiotic use. First of all, it is a nonselective, nondifferential medium. This means that almost all organisms plated on it will grow. Additionally, it contains starch. Starch is known to absorb toxins released from bacteria, so that they cannot interfere with the antibiotics. Second, it is a loose agar. This allows for better diffusion of the antibiotics than most other plates. A better diffusion leads to a truer zone of inhibition.