Bacterial Cell Walls and Gram Staining

Flashcards covering bacterial cell wall structure, specifically focusing on gram-negative bacteria, and the detailed steps and principles of the Gram staining procedure.

Gram-negative organism

A type of bacteria whose infection can cause severe symptoms due to components in its cell wall; its outer membrane is affected by antibiotics.

Core polysaccharide

A component attached to Lipid A in gram-negative bacteria, containing sugars, providing stability, and being similar amongst different bacteria.

O polysaccharide (O antigen)

Made of sugar molecules, functions as an antigen, and is used for distinguishing species or strains of gram-negative bacteria (e.g., in E. Coli O157:H7).

LPS (Lipopolysaccharide)

A complex molecule found in the outer membrane of gram-negative bacteria, composed of Lipid A, core polysaccharide, and O polysaccharide.

Gram-negative cell wall

Characterized by a much thinner peptidoglycan layer located between an inner plasma membrane and an outer membrane that contains LPS and porins.

Porins

Channels embedded in the outer membrane of gram-negative bacteria, allowing passage of certain molecules.

Gram stain

A four-step differential staining procedure that classifies bacteria into gram-positive or gram-negative categories based on their cell wall composition, and also reveals their size, shape, and arrangement.

Differential stain

A staining technique that allows for the differentiation between two different types of organisms, or between different structures within an organism, on a single slide.

Crystal violet (Primary stain)

The first stain applied in the Gram stain procedure, which dyes all bacterial cells purple.

Gram's iodine (Mordant)

The second reagent added in the Gram stain, which forms a large crystal violet-iodine complex within the cell, making the crystal violet harder to wash out.

Alcohol (Decolorizer)

The third and critical step in Gram staining; it dehydrates the thick peptidoglycan of gram-positive cells, trapping the complex, while it punches holes in the outer membrane of gram-negative cells, allowing the complex to leak out.

Safranin (Counterstain)

The final stain applied in the Gram stain procedure; it stains decolorized gram-negative cells red, while gram-positive cells retain the purple primary stain.

Gram-positive cells (after Gram stain)

Appear purple because their thick peptidoglycan layer retains the crystal violet-iodine complex after decolorization.

Gram-negative cells (after Gram stain)

Appear red because the crystal violet-iodine complex leaks out after decolorization, allowing them to take up the red counterstain, safranin.

Crystal violet-iodine complex

A large insoluble complex formed inside bacterial cells during Gram staining by the interaction of crystal violet and Gram's iodine.