Caption:
Gram-positive bacteria, light micrograph. These are bacilli (rod-shaped) bacteria. Gram staining divides bacteria into two groups (Gram-positive or Gram-negative) based on the chemical and physiological properties of their cell walls. Gram-positive bacteria retain the crystal violet dye during the Gram stain process and so appear blue or violet under a microscope. Gram-negative bacteria appear red or pink. Magnification: x2800 when printed at 10 centimetres wide.