Microscopic Morphology

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Medstudents thanks Dr. Martin Nachbar, M.D. from New York University, because this section can only be here with permission of the Hippocrates Project of the New York University School of Medicine

GRAM STAIN

The Gram stain provides information about two important features of microorganisms - (1) their morphology, that is their size, shape, and arrangement in space, and (2) their ability to retain gentian violet (a blue-violet stain), which establishes them as Gram positive organisms or their failure to retain gentian violet and to be counterstained with safronin red (a pink stain) which establishes them as Gram negative organisms. The differential staining of the two types of organisms is a reflection of differences in the basic composition of their cell wall or cell envelope.

Here are some typical examples

Gram positive organisms

• cocci clusters

• cocci chain

• bacilli

• pleomorphic rods

• branched rods

Gram negative organisms

• cocci

• coccobacilli

• bacilli

• pleomorphic rods

SPECIAL STAINS

Some organisms do not stain well (or at all) with the Gram stain. In addition, visualization of certain features of some bacteria or fungi require other methods. Below are some examples of such procedures.

• acidfast stain

• capsule stain

• flagella stain

• spore stain

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