LB6-4-3 Mannitol Salt Agar

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Microbio

3 Broth Cultures

Escherichia Coli
Staphylococcus Epidermidis
Staphylococcus Aureus

Mannitol salt is both:

Selective media & Differential media

Selective Media

Favors growth of Staphylococcus species
Inhibits growth of Gram (-) bacteria

Differential Media

Isolate Staphylococcus aureus from other staph species

MSA Contains:

1.NaCl-7.5%-inhibits Gram (-) bacteria (E. Coli)

2.Mannitol-carbohydrate
S. aurerus produces enzyme-mannitol dehydrogenase-breaks mannitol into pyruvate which ferments to produce an acid

3. Phenol Red-pH indicator
Yellow (acid) < 6.8
Red (neutral) 7.4-8.4
Pink (base) > 8.4

1. What purpose does the Nutrient Agar plate serve? In what way does it increase validity of the test result?

Serves as a control
Increases validity-shows that Gram (-) bacteria (e. coli) are viable

What would be the likely consequence of omitting the NaCl in Mannitol Salt Agar? Why?

Salt is the selective ingredient. With no NaCl, all organisms, Gram (+) & Gram (-) would grow equally

With the diversity of microorganisms in the world, how can a single test such as MSA be used to confidently identify staphylococcus aures?

Salt levels in MSA inhibit all Gram (-) leaving gram (+) cells.
Only staph species can tolerate high salt, eliminating all other gram (+) microbes

Only S. Aures can ferminent mannitol, eliminating all other staph species.

RESULTS:

E. Coli-no growth! Why? 7.5% salt inhibition

S. Aureus-Yellow!-Enzyme breakdown Mannitol-produce acid

S. Epidermidis

S. Aures fermits mannitol acid (yellow)
S. Epidermidis grows but cant fermit mannitol-no acid
E. Coli-No growth-high salt

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