2. HANS CHRISTIAN GRAM
The Gram stain was devised
by the Danish physician,
Hans Christian Gram,
while working in Berlin in
1883. He later published
this procedure in 1884.
3. GRAM’S STAIN
Gram staining (or Gram's method) is a method of
differentiating bacterial species into two large groups
Gram Positive Gram Negative
Gram staining differentiates bacteria by the chemical and
physical properties of their cell walls.
4. GRAM POSITIVE BACTERIA
• Gram positive bacteria have a thick
cell wall of peptidoglycan.
• Peptidoglycan is a polymer
consisting of sugar amino acids
that form a mesh like outside the
plasma membrane of bacteria
forming cell wall.
• In Gram positive bacteria,
between the cell wall and cell
membrane, there is a "membrane
teichoic acid".
5. GRAM NEGATIVE BACTERIA
Gram negative bacteria have
an outer membrane of
phospholipids and bacterial
Lipopolysaccharides outside of
their thin peptidoglycan layer.
The space between the outer
membrane and the
peptidoglycan layer is called
the periplasmic space.
6.
7. PRINCIPLE OF GRAM’S STAINING
The structure of the organism ‘s cell wall
determines whether the organism is gram positive
or negative.
When stained with a primary stain and fixed by a
mordant, some bacteria are able to retain the
primary stain by resisting declorization while other
get decolorized by decolorizer.
Those bacteria which retain the primary stain are
called Gram positive.
Those bacteria which get decolorized and then get
counterstained are called Gram negative.
8. 1. Crystal violet - all bacteria take crystal violet- so all
appears violet.
2. Iodine – Crystal Violet-iodine(CV-I) complex is
formed.
3. Acetone- bacteria with high lipid content loose CV-I
complex(appear colourless) but bacteria with less
lipid content retains CV-I complex ( appear violet).
4. Safranine/ basic fuchsin – only colourless bacteria
takes – appear pink.
9.
10. PROCEDURE
1. Make a smear & dry thoroughly in cool air. Fix the dried
film by passing it briefly through a bunsen flame.
2. Flood the slide with crystal violate sol. for upto 1 min.
Wash off briefly with tap water & drain.
3.Flood the slide with gram’s iodine sol. & allow to act as
a mordant for about 1 min. Wash off with tap water &
drain.
11. 4.Decolourise the smear with acetone for 10-30 sec. taking
care not to overdecolourise & immediately wash off with
water.
5.Flood the slide with safranin sol. & counterstain for about
30 sec, wash off with tap water, drain & blot dry with filter
paper & examine under oil immersion objective.
