PRINCIPLES OF STAINING 

1. Simple stains – increase contrast

2. Differential stains – differentiate   between organisms or structures

3. Gram stain – differentiates between two kinds of cell wall structures

Preparing smears for staining

1. Bacteria on slide

2. Air Dry

3. Bacteria are HEAT FIXED to the slide

4. Stain is applied

Bacteria are slightly negative, so are attracted to the positive chromophore of the BASIC DYE

Common Basic Dyes

•crystal violet

•methylene blue

•safranin

•basic fuchsin

Gram Staining

1884 Hans Christian Gram

most important stain used in Bacteriology

Divides all Bacteria into 2 groups:

•Gram (+)

•Gram (-)

Identification of a Bacteria 

Result

•Gram (+)  PURPLE

•Gram (-)   RED

•Difference - due to structure of cell wall

–Gram (+)      Thick cell wall

–Gram (-)       Thin cell wall

This is the pictures of result :

GRAM (-)          and        GRAM (+)