Gram Stain Vocabulary

Vocabulary flashcards covering Gram staining concepts mentioned in the notes.

Gram stain

A differential stain that classifies bacteria by cell wall structure; Gram-positive retain crystal violet and appear purple, while Gram-negative lose it after decolorization and are counterstained pink.

Decolorizer

Alcohol (or acetone) used in Gram staining to dissolve the outer lipid layer of Gram-negative bacteria, causing loss of the crystal violet dye from the thinner peptidoglycan layer.

Crystal violet (purple)

The primary stain in Gram staining; colors all cells purple; Gram-positive retain it after processing, while Gram-negative typically do not after decolorization.

Safranin (counterstain)

The counterstain used after decolorization; stains cells that have lost the primary stain pink/red; can stain cells that didn’t retain the purple dye.

Gram-positive

Bacteria with a thicker peptidoglycan layer that retain the crystal violet dye and appear purple after Gram staining.

Gram-negative

Bacteria with a thinner peptidoglycan layer and an outer lipid-containing membrane; decolorized by alcohol and appear colorless until counterstained.

Peptidoglycan

A polymer forming the bacterial cell wall; thicker in Gram-positive bacteria and thinner in Gram-negative bacteria.

Lipid outer layer

A protective lipid-rich outer layer found in some bacteria (e.g., mycobacteria) that makes staining more difficult.

Mycobacterium

Genus of bacteria with a lipid-rich outer barrier and a relatively thick peptidoglycan; includes M. tuberculosis; presents staining challenges.

Mycobacterium tuberculosis

Species within the genus Mycobacterium that causes tuberculosis.