noun, singular: gram-negative bacterium
Gram staining is a useful and quick laboratory procedure to know whether a bacterial cell is gram-positive or gram-negative. Gram-positive bacterial cells will appear violet (from the crystal violet dye) whereas gram-negative bacterial cells will appear pink (usually from the safranin dye) when viewed under the microscope after gram staining. Initially, all bacterial cells are violet in colour from the initial dye treatment however some cells will lose the stain following decolorization and counterstaining with safranin. Other cells though will retain the violet stain. Those that lose the violet color and take the counterstain are called gram-negative bacteria. Those that retain the violet color after the procedure are called gram-positive bacteria.
Gram-negative bacteria are able to take the color of the counterstain because their thinner cell walls allow the acetone-alcohol to wash out the initial stain. Gram-positive bacteria, on the other hand, have thick cell walls that resist decolorization and counterstaining in Gram's method.
Examples of bacteria that are gram-negative are Escherichia coli, Salmonella spp., Neisseria spp., Shigella spp., Hemophilus influenzae, Klebsiella pneumoniae, Legionella pneumophila, Pseudomonas aeruginosa, etc.