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Name: Phyllis
Status: Other
Grade:  Other
Location: GA
Country: United States
Date: February 2005


Question:
What does "What is the gram stain status of E-coli?" mean?



Replies:
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 alcohol.

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.

vanhoeck



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