Gram-positive bacterium, any of various types of bacteria that are characterized by having a thick peptidoglycan cell wall and by the absence of an outer membrane composed of lipopolysaccharide. Gram-positive bacteria are named because of their reaction to the Gram stain, a microbiological staining technique that is used to identify and characterize bacteria. Gram-positive bacteria characteristically stain purple following the Gram reaction, owing to their thick cell walls; this is in contrast to Gram-negative bacteria, which stain pink or red, owing to their thin cell walls.

Gram staining is carried out by first heat-fixing bacterial cells to a slide. The slide is then treated with crystal-violet stain, followed by washing with an iodine solution. In the final steps the slide is washed with an organic solvent, such as alcohol or acetone, and a counterstain, such as safranin, is added. Both Gram-positive and Gram-negative bacteria initially turn purple, owing to the crystal-violet stain. In the subsequent wash steps, Gram-positive bacteria retain their purple colour; by comparison, the purple stain is removed from Gram-negative bacteria by the solvent, causing them to take up the colour of the counterstain.

Examples of Gram-positive bacteria include Bacillus, Clostridia, Listeria, Staphylococcus, and Streptococcus.