Gram Stain Status
Country: United States
Date: February 2005
What does "What is the gram stain status of E-coli?" mean?
Gram positive bacteria are stained violet in color when they are exposed to
Gram stain (name after the bacteriologist that discovered it). Gram
negative bacteria, like E.coli for example, are not stained by this stain.
What distinguishes Gram+ from Gram- bacteria is the amount of
lipopolysaccharide (endotoxin) and peptidoglycan in the cell wall of the
bacterium. E. coli cell walls are loaded with endotoxin while Gram+ cell
walls contain a lot of peptidoglycan
Ron Baker, Ph.D..
A gram stain is a differential stain-i.e. uses two dyes. One of the dyes is
purple and the other is red. Bacterial cell walls come in two different
forms based on the amount of a substance called peptidoglycan, PG for short,
that it contains. One type of bacterial cell wall contains almost all PG and
the other only about 20% PG. The purple dye sticks to PG and cell walls that
have more will resist being decolorized by alcohol. So in the staining
procedure, the purple dye is put on first. If the cell wall is the type that
resists decolorizing, it will hang on to the purple color. If it is the second
type, because it is only 20% PG, the purple color can be washed out using
This would leave the bacteria colorless, so a second dye is added to
re-colorize these bacteria. The first type is called gram positive (I always
remember purple=positive) and the second gram negative which stain red. So
the question is asking whether E. coli are gram positive (purple) or gram
negative (pink). This is the first step in identifying bacteria because it
divides them into two basic groups.
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Update: June 2012