Comparatively Speaking: Gram-positive vs. Gram-negative Bacteria

September 11, 2012 | Contact Author | By: Anthony J. O'Lenick Jr., Siltech LLC, and Kelly Dobos, Kao Corp.
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Bacteria are subdivided into two broad groups based on their ability to take up crystal violet dye during  Gram staining. This process was named after Hans Christian Gram, a Danish scientist who developed it in the 1880s. Gram-positive cells will appear purple or blue, while Gram-negative cells will appear pink after application of a counterstain.

Susceptibility to staining is determined by the structure of the bacterial cell wall. Gram-positive bacteria have cell walls made up of high amounts of peptidoglycan, a polysaccharide with peptide cross-links, which stains dark purple in the presence of crystal violet. A Gram-negative bacterial cell wall contains far less peptidoglycan and is covered by a second lipopolysaccharide membrane that is removed before counterstaining.

Gram staining is still an important part of the diagnostic process in medical microbiology. The antibiotic penicillin interferes with the generation of peptidoglycan, making it effective in treating infections caused by Gram-positive bacteria. But because of the differences in the types of bacterial cell wall, it is less effective in treatment of Gram-negative infections.

The differences in bacterial cell walls also highlights the importance of selecting a broad-spectrum preservative package to protect cosmetic and personal care products, as a single preservative is rarely effective against all types of organisms.