Gram staining

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A Gram stain of mixed Staphylococcus aureus (Staphylococcus aureus ATCC 25923, gram-positive cocci, in purple) and Escherichia coli (Escherichia coli ATCC 11775, gram-negative bacilli, in red), the most common Gram stain reference bacteria

Gram staining (or Gram's method) is a way of classifying bacteria into two large groups: gram-positive and gram-negative. The name comes from its inventor, Hans Christian Gram.

Gram staining stains bacteria according to the chemical and physical properties of their cell walls. It stains peptidoglycan, a thick layer in gram-positive bacteria.[1] So, gram-positive bacteria are coloured by the crystal violet dye. A counterstain (commonly safranin or fuchsin) added after the crystal violet gives all gram-negative bacteria a red or pink colour.

The Gram stain is almost always the first step in the identification of a bacterial organism. However, not all bacteria can be classified by this technique. They are called 'gram-variable' or 'gram-indeterminate'.

References[change | change source]

  1. Bergey, David H. et al (1994). Bergey's Manual of determinative bacteriology (9th ed.). Lippincott Williams & Wilkins. ISBN 0-683-00603-7.