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Bacteria staining method has long been misexplained

Gram technique works, but not the way scientists thought

3:20pm, May 6, 2015
gram staining bacteria

COLOR CODED  An old lab technique for differentiating bacteria as either gram-positive (left) or gram-negative (right) has been incorrectly explained for decades.

With delicate hues of purple and pink, a lab technique called gram staining has reliably characterized bacteria for more than a century. Yet many scientists are mistaken about why the vivid method works, new research finds.

Contrary to standard scientific texts, the purple dye called crystal violet, a main ingredient in gram staining, does not actually enter bacterial cells, researchers report April 27 in ACS Chemical Biology. Instead, the dye gets trapped in a tight package of sugar-filled polymers, called peptidoglycan, which envelops bacterial cells. The thickness and integrity of the sweet bacterial armor determines whether crystal violet leaves a cell purple or not.  That royal shade, or lack of it, reveals a cell’s type of outer structure.

Published by Hans Christian Gram in 1884, gram staining distinguishes gram-positive bacteria (purple) from gram-negative bacteria (pink

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