3.2 Sample Detection Staining

A collection of vocabulary terms related to microbial staining techniques, including Gram staining, acid-fast staining, and complex dye mixtures like Giemsa.

Cation

A positively charged ion, such as the sodium ion in table salt.

Anion

A negatively charged ion, such as the chloride ion in table salt.

Chromophore

The colored ion in a dye salt that is bound to a color component.

Acidic dye

A dye with a negatively charged (anionic) chromophore that stains positively charged structures in acidic conditions.

Basic dye

A dye with a positively charged (cationic) chromophore that stains negatively charged structures, such as membranes and proteins, in basic conditions.

Simple stains

A single basic dye used to determine cell size, shape, and arrangement, involving a soak in the dye for $30$ to $60\,\text{seconds}$ before rinsing.

Differential stains

Staining techniques that use at least two dyes in a multistep process to distinguish different cells, structures, or chemicals in a sample.

Gram stain

A differential stain that divides bacteria into gram positive and gram negative groups based on structural and chemical differences in their cell walls.

Peptidoglycan

A structural layer in the bacterial cell wall that can be either thick and attached to the plasma membrane or very thin and covered by an outer membrane.

Gram positive

Bacteria with a thick peptidoglycan layer that stain purple during the Gram stain procedure.

Gram negative

Bacteria with a thin peptidoglycan layer and an outer membrane of lipopolysaccharides, which appear pink at the end of the Gram stain procedure.

Crystal violet

The primary stain in the Gram stain process that colors all cells violet or purple by sticking to the thick peptidoglycan wall.

Mordant

A substance, such as iodine, that binds to a dye and makes it less soluble, forming a stable complex trapped in the cell.

Decolorizing step

The third step of Gram staining where ethanol and acetone, or $95\%$ ethanol, is used to break down thin peptidoglycan walls, making gram negative cells colorless.

Safranin

A red counterstain used in Gram staining to provide color and contrast to gram negative cells that were previously decolorized.

Capsule

A negatively charged structure made of polysaccharides, also known as a glycocalyx, that contributes to a microbe's virulent factor.

Negative staining

A simple stain using an acidic, anionic dye that is repelled by a cell's negative surface charge to highlight structures like bacterial capsules.

Acid fast staining

A differential stain used to identify Mycobacterium or Nocardia which have a thick lipid or waxy layer in their cell wall.

Carbol Fuchsin red

The primary stain used in acid fast staining that is permeated into the waxy cell wall layer using heat.

Giemsa stain

A neutral mixture of methylene blue, neocin, and azure used to visualize blood samples and diagnose malaria or other protozoal infections.