Expert: Walter Hintz - 12/15/2007

Question
hi!could u pls help me with this question thrown by my instructor?
a. why do bacteria remain unstained in the negative staining procedure?
b. what makes a gram positive bacteria stained gram negative? thnks so much for your help!!!

Answer
 a)Go over again the gram staining procedure;
 1. The bacteria are stained with a dark blue stain and iodine is used to fix the stain
 2. Alcohol is added to remove the stain
 3. A red counterstain is added
 4. The slide is washed

 The secret is in the cell wall of the bacteria.  Some contain  large amounts of a  protein called peptidoglycan. The blue stain is held by this compound and cannot be taken out.  The bacteria is called Gram positive.

When there is less compound the blue is washed out and there is no color until the red is added.  Red bacteria are Gram negative.

b)Gram positive bacteria are always stained blue (positive) not red (negative).  I do not know why your instructor asked this question