They are referring to how the germs react to one of the basic tests in bacteriology, the Gram stain or Gram’s stain, which distinguishes between two major classes of bacteria by how they take up certain dyes.
On a microscope slide, heat-treated gram-positive bacteria take up a purple dye (originally gentian violet), which is then fixed with an iodine solution, and they hold on to the color even after the slide is washed with a solvent.
Gram-negative bacteria, which have thinner but more complex and less permeable cell walls, do not retain the purple dye, but take up a counterstain, safranine, and end up a dark pink.
Gram-negative bacteria are of concern because they tend to be more resistant to antibiotics. Many cause serious diseases, including the plague, Legionnaire’s disease, and many bowel-related diseases, including cholera and salmonella.
Opportunistic infections from gram-negative bacteria are also a major danger in hospitals. When some of them escape from the gut and enter a patient’s bloodstream through medical procedures, they can cause septic shock, which is often fatal.
The test was named for Dr. Hans Christian Joachim Gram, a Danish doctor who developed it while working at the University of Berlin in 1884. He was trying to distinguish between two kinds of organisms that cause pneumonia: pneumococci and Klebsiella pneunzoniae.