What is gram-negstive?

Updated: 4/28/2022
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There are two categories of bacteria, gram-positive and gram-negative. They are categorized based on the structural differences of their cell wall.

While gram-(+) only have one layer of membrane, gram-(-) has an outer and inner membrane. The thick lipid bilayer making up the outer membrane is harder to penetrate with antibiotics.


  • Gram-(+) retains crystal violet dye after being washed with solution and will appear purple. This happens because it's membrane is 50-90% peptidolglycan, a sugar and peptide.
  • Gram-(-) does not retain the crystal violet dye after being washed with solution and will appear pink because of the counter-stain in the solution, usually Safranin. This happens because the outer membrane is only ~10% peptidolglycan.

Pathogenic Properties

  • Gram-(-) have lipopolysaccharides (LPS) made up of toxic fatty acids (lipid A) and a polysaccharide (which includes antigen O) in their outer membrane. LPS at high levels in the body causes septic shock.

Antibiotic resistance:

Both gram-(+) and gram-(-) can show antibiotic resistance.

Gram-(-) is more likely to show antibiotic resistance because:

  1. Their outer membrane (see above)
  2. They generally have greater facility in exchanging DNA among other cells of the same species.


  • Some gram-(+) bacteria: streptococcal, staphylococcal
  • gram-(+) types can cause: step throat, toxic shock, blood poisoning
  • Some gram-(-) bacteria: shigella, salmonella, E. coli
  • gram-(-) types can cause: pneumonia, gonorrhea,
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