BISC 303 Chapter Notes - Chapter 3: Gram Staining, Crystal Violet, Gram-Negative Bacteria
Mechanism of Gram
Staining
•The difference between typical Gram-positive bacteria and
typical Gram-negative bacteria is due to the physical
nature of their cell walls. If the cell wall is removed, typical
Gram positive bacteria stain Gram negative. Furthermore,
bacteria that never make cell walls, such as the
mycoplasmas, also stain Gram negative. 1... Differential
staining (section 2.3)
•During the Gram -staining procedure, bacteria are first
stained with crystal violet, a dye with a positive charge
that is attracted to the bacterial cell's net negative charge,
and next treated with iodine.
•Iodine is a mordant that interacts with the crystal violet,
forming an insoluble complex and thus promoting dye
retention.
•When bacteria are treated with ethanol in the
decolorization step, the alcohol is thought to shrink the
pores of the thick peptidoglycan found in the cell walls of
typical Gram-positive bacteria, causing the peptidoglycan
to act as a permeability barrier that prevents loss of crystal
violet.
•Thus the dyeiodine complex is retained during the
decolorization step and the bacteria remain purple, even
after the addition of a second dye, required by the
procedure.
find more resources at oneclass.com
find more resources at oneclass.com