Gram-positive Bacillus anthracis bacteria (purple rods) in cerebrospinal fluid sample. The other cells are white blood cells.

Diagram of bacterial cell walls

Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to gram-negative bacteria, which cannot hold the crystal violet stain. Instead they take up the counterstain (safranin or fuchsine) and appear red or pink.