Gram Stain Flashcards
Examples of slightly curved rod morphologies
Helicobacter pylori & Vibrio cholerae
Examples of flexible spiral morphologies
Treponema pallidum & Borrelia burgdoferi
Gram stain
most commonly used differential stain in bacteriology
Gram stain identifies
Gram (+) bacteria & gram (-) bacteria by differentiating thick (+) & thin (-) peptidoglycan layers
Gram (+) bacteria
have teichoic acid (bacterial polysaccharides that make cell wall more rigid)
cell wall composed of a thick peptidoglycan layer that helps retain the crystal dye to make them gram (+)
Gram (-) bacteria
high lipid content due to the outer membrane
cell wall composed of thin peptidoglycan layer
Some bacteria are…
closely surrounded by a capsule composed of polysaccharide that make them resistant to phagocytosis, increase adherence to surfaces & nutrients, and protect them against dessication
General differential stains
detect differences between organisms or differences between parts of the same organism
Gram stain steps
primary stain - crystal violet
mordant - iodine
decolorizer - ethanol
counterstain - safranin
Gram stain steps
primary stain - crystal violet
mordant - iodine
decolorizer - ethanol
counterstain - safranin
Gram positive organisms
purple
rods - Bacillus subtilus
cocci - Micrococcus luteus & Staphylococcus aureus
Gram negative organisms
pink
rods - Escherichia coli & Proteus mirabilis
cocci - Neisseria
False results for gram stain
Variable results are a consequence of poor technique
Decolorization is the most crucial step:
-don’t leave alcohol for too long bc it will cause stain to over decolorize (this is why we wash with water)
- don’t leave alcohol for too short, it will under colorize
Identify morphologies
Spirillum (spiral)
Identify morphologies
Top: Bacilli (rods)
Bottom: Cocci (spheres)
Identify species
gram (+) - Micrococcus luteus
gram (-) - Escherichia coli
