Ex. 4-3 Flashcards
What does MSA stand for?
Mannitol Salt Agar
What type of medium is Mannitol Salt Agar?
Undefined, selective, and differential
Selects for Gram-positive microbes (selective for members of the genus Staphylococcus due to their salt tolerance)
Differentiates between Staphylococcus species based on their ability to ferment mannitol to acid end products and those that do not.
What does MSA contain?
Carbohydrate mannitol
7.5% sodium chloride
pH indicator phenol red
What ingredient in MSA makes it selective?
Sodium chloride makes the medium selective because it’s concentration is high enough to dehydrate and kill most bacteria.
What ingredient makes MSA differential?
Mannitol provides the substrate for fermentation and makes the medium differential.
What does the phenol red do in the MSA medium?
Indicates whether fermentation with an acid end-product has taken place by changing color as the pH changes
What would you observe on the Mannitol Salt Agar using Staphylococci that do not ferment mannitol?
Growth that appears pink or red and the medium remains unchanged
What would you observe on the Mannitol Salt Agar using Staphylococci that does ferment mannitol such as Staphylococcus aureus?
Bright yellow colonies usually surrounded by a yellow halo
What causes the yellow growth of Staphylococcus aureus on the red Mannitol Salt Agar?
It ferments Mannitol, which produces acids and lowers the pH of the medium.
What is the main reason for using Mannitol Salt Agar?
For isolation and differentiation of Staphylococcus aureus from other staphylococcus species. (not used to determine the ability of an isolate to ferment mannitol-other mediums better for that)
What microbes did we use in our mannitol salt agar lab and what were the results?
Escherichia coli, Staphylococcus epidermidis, and Enterococcus faecalis
- it only grew on Staphylococcus epidermidis
(Ef is Gram-positive but didn’t grow because it doesn’t like Salt)
