See also medically important gram + bacteria organisms and Medically Important Gram Negative Bacteria

Differences between Gram Negative and Gram Positive Bacteria

Gram-negative bacteria organisms have thinner cell walls which differ in composiiton from their Gram-positive counterparts. As a result, they lose the crystal violet stain (and take the color of the red counterstain) in Gram’s method of staining. Gram negative bacteria can be distinguished from gram-positive according to the following:

• Cell Envelope: Outer membrane (gram – only): Gram negative bacteria are covered by an outer membrane which contains lipopolysaccharide (LPS) and porins. The LPS is a major antigenic component and functions as a mitogen. Mitogens are agents that are able to induce cell division in a large percentage of T or B cells. Unlike immunogens, which activate only lymphocytes bearing specific receptors, mitogens activate many clones of T or B cells irrespective of their antigen specificity. Porins form pores through the outer membrane and allow diffusion of hydrophilic molecules. Endotoxins, which are part of the lipopolysaccharide complex that forms the outer envelope of gram-ngegative bacteria, are released during the lysis of a microorganism or cell division. An essential part of quality assurance and quality control invovles testing end-product injectable drugs, medical devices, and raw materials for endotoxins. The importance of testing for endotoxins in humans cannot be emphasized enough because of the pyrogenic or immunorespeonsive reactions that can occur after exposure to bacterial pyrogens. Humans are particularly sensitive to minute amounts of endotoxins, and mild gram-negative bacterial infections can often casue a pyrogenic response. The presence of endotoxin in the bloostream can cause fever, inflammation and often irreversible shock. (Joiner, “comparison of endotoxin testing methods for pharmaceutical products” J Pharm Compd 6; 408-409 (2002)
• Higher Lipid content: Gram – bacteria have a higher lipid content due to their outer membrane
• Less peptidoglycan (sometimes called “cell wall”): Gram – have a thinner peptidoyglycan layer than Gram-positive cell walls in what is called the periplasmic space (the area between the internal face of the outer membrane and the external surface of the cytoplasmic membrane). The linker between the N-acetylglucosamine-N-acetylmuramic acid disaccharide units include DAP rather than a pentaglycine chain as with gram + bacteria. This thin layer fails to retain the crystal violet dye but does retain a pinkish red safranin dye.

 Gram-positive bacteria can be distinguished from gram-negative according to the following:

• thick cells wall composed largely of peptidoglycan, which consists of repeating N-acetylglucosamine-N-acetylmuramic acid disaccharide units that are linked via short peptide bonds. In gram + bacteria, this linkage has a pentaglycine chain which is lacking in gram – bacteria.

This thick cell wall retains crystal violet dye. Thus gram-positive bacteria will stain purple in comparison with gram-negative bacterial which stain red by the counterstain safranin. 

Since the peptidoglycan layer is a target for penicillin, this drug works better for gram-positive bacteria.

• Scaffolding proteins like teichoic and lipoteichoic acids are also associated with the peptidoglycan layer in gram positive bacteria.
• No lipopolysaccharide (LPS) as with gram-negative bacteria.
• No outer membrane