Lab 7 and 8: Gram Staining and Bacterial Smears

Gram Staining

Differential staining technique for bacteria identification.

Hans Christian Gram

Introduced Gram staining in 1884.

Gram-positive bacteria

Have thick peptidoglycan layer in cell wall.

Gram-negative bacteria

Have thin peptidoglycan layer and lipopolysaccharide.

Primary stain

First stain applied, usually crystal violet.

Crystal violet

Primary stain that colors all bacteria purple.

Gram's iodine

Mordant that forms complex with crystal violet.

Mordant

Substance that fixes dye in cells.

Decolorizer

95% ethanol used to remove stain from G- bacteria.

Counterstain

Secondary stain, usually safranin, colors G- bacteria pink.

Peptidoglycan

Polymer forming the cell wall in bacteria.

Lipopolysaccharide

Component of the outer membrane in G- bacteria.

Heat fixing

Process of killing and adhering bacteria to slides.

Bacti-Cinerator

Device used for heat fixing slides.

Aseptic technique

Method to prevent contamination during bacterial handling.

Smear preparation

Spreading bacteria on a slide for staining.

Blotting

Gently drying the slide to avoid dilution.

Rinsing

Washing excess stain from the slide.

Positive stain

Stain that colors the surface of cells.

Chromophore

Color-bearing part of a stain solution.

Bacterial morphology

Shape and arrangement of bacterial cells.

Simple staining

Technique to enhance visibility of bacteria.

Distilled water

Used for rinsing and preparing smears.

Bibulous paper

Absorbent paper used for blotting slides.

Inoculating loop

Tool for transferring bacteria to slides.

Staining procedure

Steps followed to apply stains to bacteria.

Observation under microscope

Final step to view stained bacteria.