Gram Stain and Bacterial Morphology

This flashcard set covers the principles of Gram staining, the reagents involved, bacterial morphologies like bacilli and cocci, and the outcomes of staining errors.

Gram staining

A differential stain that classifies bacteria as Gram-positive or Gram-negative.

Gram-positive bacteria color

The color purple, which is the result of the Gram stain on these organisms.

Gram-negative bacteria color

The color pink, which is the result of the Gram stain on these organisms.

Gram-positive cell wall

A bacterial structure characterized by thick peptidoglycan.

Gram-negative cell wall

A bacterial structure characterized by thin peptidoglycan and an outer membrane.

Order of Gram stain reagents

Crystal Violet $\rightarrow$ Iodine $\rightarrow$ Decolorizer $\rightarrow$ Safranin.

Crystal Violet

The reagent used as the primary stain in Gram staining.

Iodine

The reagent used as the mordant in Gram staining.

Safranin

The reagent used as the counterstain in Gram staining.

Decolorizer

The reagent that removes crystal violet from Gram-negative cells.

E. coli

A bacteria that is Gram-negative and has bacilli (rods) morphology.

S. aureus

A bacteria that is Gram-positive and has cocci (spheres) morphology.

Cocci

Spherical-shaped bacteria.

Bacilli

Rod-shaped bacteria.

Spirilla

Spiral-shaped bacteria.

Thin smear

A preparation technique required for proper decolorization and easier morphology observation.

Over-decolorization

A staining error that causes Gram-positive cells to appear Gram-negative (pink).

Under-decolorization

A staining error that causes Gram-negative cells to appear Gram-positive (purple).

Thick smear error

An issue where Gram-negative cells may appear Gram-positive due to the density of the sample.

Skipping iodine

An error in the staining procedure that may cause Gram-positive cells to appear pink.

Gram-negative bacilli

Bacteria that appear as pink rods after Gram staining.

Gram-positive cocci

Bacteria that appear as purple spheres after Gram staining.