The page below is a sample from the LabCE course Reading Gram Stained Smears From Cultures. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Gram Stain

The Gram stain is probably the most important and universally used staining technique in microbiology. It provides a fundamental differentiation between types of bacteria. It should be noted that the morphology of most organisms stained with Gram's stain are MOST characteristic in smears prepared from an 18- to 24-hour broth subculture. Although preparation from colonies growing on an agar surface can be less characteristic, it is an extremely important part of the identification process.
The Gram stain is used to distinguish between gram-positive and gram-negative organisms, which is based on distinct and consistent differences in the bacterial cell wall. Gram-positive bacteria retain the primary stain and are blue to deep purple in color. Gram-negative organisms are those that decolorize during the staining process and, after counterstaining, are pink to red in color.
Bacterial organisms occur in three basic shapes:
  • Cocci, which are spherical
  • Bacilli, which are elongated or cylindrical (also known as rods)
  • Spirochetes, which are spirals
In this course, we will examine cocci and rods. Both cocci and rods may be either gram-positive or gram-negative. Examples of gram-positive cocci (GPC) are shown in the upper image and gram-negative cocci (GNC) are shown in the lower image. Both slides were made using organisms recovered from culture media.