exercise 24 KTTK: Mannitol salt agar (micro. lab)

Mannitol salt agar (MSA) is

selective and differential medium

selective

due to high concentration of NaCl, or salt, (7.5%) which dehydrates and kills most bacteria

differential

• Organisms able to ferment mannitol produce acidic end products
• Phenol red is pH indicator indicating presence or absence of acidic end products
• Yellow below pH of 6.8
• Red from pH of 7.4 to 8.4
• Pink at or above pH of 8.4

phenol red

is pH indicator indicating presence or absence of acidic end products

yellow

below pH of 6.8

red 

from pH of 7.4 to 8.4

pink

at or above pH of 8.4

why phenol red is added to the agar

to indicate a change in pH, usually to detect the production of acid from carbohydrate fermentation by bacteria

Know what kind of organisms thrive on the agar and why

Staphylococci thrive on mannitol salt agar due to high salt tolerance (live on
salty skin)

which organism is able to ferment the mannitol and what happens if it does

Staphylococcus aureus is able to ferment mannitol to acid end products, so pH is lowered below 6.8 leading to appearance of yellow colonies

Know why (application) mannitol salt agar is used

Mannitol salt agar (MSA) is used for isolation and differentiation of Staphylococcus aureus from other Staphylococcus species

MSA is not used

o determine whether organism can ferment mannitol or not

know staphylococcus aureus results

Staphylococcus aureus is able to ferment mannitol to acid end products, so pH is lowered below 6.8 leading to appearance of yellow colonies

know staphylococcus epidermidis results

don’t ferment mannitol, pH remains above appear red or pink

know Escherichia coli results

no growth not fermented the mannitol

mannitol salt agar (MSA) medium recipe

beef extract, peptone, sodium chloride, D-mannitol, phenol red, agar, distilled/deionized water