Gram-positive vs. Gram-negative

Named for the scientist who created the test, Gram-positive and Gram-negative are two indications of the cellular structure of bacteria.

Gram-positive means retaining purple dye when stained with Gram's stain. Named for Danish scientist Gram, the stain is a test of cellular structure of bacteria. It differentiates between two types of bacteria, gram-positive being the one whose cell wall contains a thick layer of peptidoglycan, which retains the purple dye. Gram-positive bacteria are more susceptible to antibodies.

Gram-negative means not retaining purple dye when stained with Gram's stain. It refers to the type of bacteria that has a thinner but impenetrable cell wall that does not retain the purple dye. Gram-negative bacteria are more resistant to antibodies.

Detection of Gram-positive or Gram-negative bacteria is useful in medical testing as well as cheese making.