The Gram Stain

1. The Gram stain is named for the Danish bacteriologist who first devised it in 1844 (History, 2004). It is usually the first test used to identify bacteria. The Gram stain is based on the ability of bacterial cell walls to retain crystal violet dye during solvent treatment. Bacterial cells with walls containing small amounts of peptidoglycan and lipopolysaccharide are Gram-negative; those containing relatively high amounts of peptidoglycan and lipopolysaccharide are Gram-positive (The Gram, 2004). Microorganisms which retain the crystal violet-iodine complex appear purple-brown under the microscope, and are considered Gram-positive. Those which do not retain the stain appear red and are considered Gram-negative. An example of a Gram-positive bacterium is Staphylococcus aureus. An example of a Gram-negative organism is Salmonella typhii.

The Gram stain is carried out on a smear of the sample containing the bacteria (History, 2004; The Gram, 2004). The steps are:

1. Prepare a smear by placing one drop of the bacterial culture on a glass slide and smear it onto the slide using a wooden stick or by

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