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An MSA plate with Micrococcus sp. (1), Staphylococcus epidermidis (2) and S. aureus colonies (3). 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. [1]
Second, the isolate is cultured on mannitol salt agar, which is a selective medium with 7.5% NaCl that allows S. aureus to grow, producing yellow-colored colonies as a result of mannitol fermentation and subsequent drop in the medium's pH. [98] [99]
It is used to isolate Staphylococcus aureus from clinical specimens and food. It was first described by Vogel and Johnson, who modified the Tellurite Glycine Agar recipe by Zebovitz et al. by doubling the mannitol concentration to 1% (w/v) and adding Phenol red as a pH indicator. [1] It is widely available commercially.
Importantly, S. aureus is generally coagulase-positive, meaning that a positive coagulase test would indicate the presence of S. aureus or any of the other 11 coagulase-positive Staphylococci. [1] A negative coagulase test would instead show the presence of coagulase-negative organisms such as S. epidermidis or S. saprophyticus.
Microorganisms growing on an agar plate. Sterilization (British English: sterilisation) refers to any process that removes, kills, or deactivates all forms of life (particularly microorganisms such as fungi, bacteria, spores, and unicellular eukaryotic organisms) and other biological agents (such as prions or viruses) present in fluid or on a specific surface or object. [1]
Staphylococcus growth in Baird-Parker (BP) agar.. Baird-Parker agar is a type of agar used for the selective isolation of gram-positive Staphylococci species. [1] It contains lithium chloride and tellurite to inhibit the growth of alternative microbial flora, while the included pyruvate and glycine promote the growth of Staphylococci. [1]
Illustration of streak plate procedure to achieve isolated colonies using aseptic technique. The three-phase streaking pattern, also known as the T-Streak, is recommended for beginners. [5] However it is also limited in applications using greater than a single culture. [5] The plate is split by drawing a "T" to create three separate sections.
Streak a beta-lysin–producing strain of aureus down the center of a sheep blood agar plate. The test organism streak should be 3 to 4 cm long. Streak test organisms across the plate perpendicular to the S. aureus streak within 2 mm. (Multiple organisms can be tested on a single plate). Incubate at 35°-37°C in ambient air for 18-24 hours.