Gram Stains:

The Gram stain is one of the most important first steps in identifying bacteria. While it provides valuable information quickly, interpreting a Gram stain requires practice. Staining quality, decolorization, and the characteristics of the organism all influence the final result.

Some organisms, such as Clostridium species, overdecolorize easily and may appear Gram negative despite being Gram positive. For this reason, don’t rely on color alone. Always evaluate the organism’s:

• Gram reaction (purple or pink)
• Size
• Shape
• Cell arrangement (chains, clusters, pairs, etc.)

Additional clues, including colony morphology and growth on different culture media, can help confirm an organism’s identity.

Interpreting the Results

• Gram-positive bacteria stain purple because they retain the crystal violet-iodine complex.
• Gram-negative bacteria stain pink after taking up the counterstain.

A poor-quality Gram stain often has numerous purple artifacts scattered throughout the background, making the slide difficult to interpret.

A well-prepared Gram stain typically has a clean, lightly pink background. Polymorphonuclear leukocytes (PMNs)—also called neutrophils, segs, or polys—should have nuclei that stain a light purple, serving as a good indicator that the staining process was performed correctly.

Gram Stain Procedure: