Swabs taken from
patients, staff and the environment are cultured on agar plates.
They are incubated at 35° Celsius.
The bacterial colonies are visible within one to two days.
As there are thousands of different organisms on the skin
it is difficult to differentiate the S.aureus without using
“selective” media to take advantage of the specific
characteristics of the bacteria.
One such selective agar is mannitol salt agar (MSA) containing methicillin.
S.aureus, unlike many bacteria, is salt tolerant. Using media that contains
a high salt concentration will therefore allow growth of S.aureus
while inhibiting many of the other organisms.
Further
selection can be achieved by adding methicillin or similar antibiotics
to the agar.
Mannitol (a carbohydrate) is a useful agar additive because S.aureus
utilises it to produce an acidic byproduct, which changes the colour of
the agar around colonies from pink to yellow.
The scientist can then readily identify probable MRSA colonies by
their yellow colour. Further tests are performed to confirm
the culture results.
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