Acid-Fast Stain Results

Purpose:  To differentiate between acid-fast and non acid-fast bacteria.

Principle:  Some bacteria contain a waxy lipid, mycolic acid, in there cell wall.  This lipid makes the cells more durable and is commonly associated with pathogens.  Acid fast cell walls are so durable that the stain (carbol fuschin) must be driven into the cells with heat.  The cells are then decolorized with acid-alcohol, all other cells will decolorize with this strong solvent, but acid fast bacteria will not.  Other cells are then counterstained with methylene blue.

To the right, a mixed culture has been acid-fast stained.
A.  Non Acid-fast bacteria
B.  Acid-fast bacteria

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