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Gram-positive bacteria differ from gram-negative bacteria in the structure of their cell walls. The cell walls of gram-positive bacteria are made up of twenty times as much murein or peptidoglycan than gram-negative bacteria. These complex polymers of sugars and amino acids cross-link and layer the cell wall.

The thick outer matrix of peptidoglycan, teichoic acid, polysaccharides, and other proteins serve a number of purposes, including membrane transport regulation, cell expansion, and shape formation.

Almost all bacteria can be classified as gram-positive or gram-negative. The classification relies on the positive or negative results from Gram’s staining method, which uses complex purple dye and iodine. Because gram-positive bacteria have more layers of peptidoglycan in their cell walls than gram-negative, they can retain the dye.

Six common gram-positive bacteria that infect humans and their shapes follow:
Streptococcus (cocci)
Staphylococcus (cocci)
Bacillus (bacilli, protective spore) - causes anthrax and gastroenteritis
Clostridium (bacilli, protective spore) - causes botulism, tetanus, gas gangrene, and pseudomembranous colitis
Corynebacterium (bacilli, no protective spore) - causes diphtheria
Listeria (bacilli, no protective spore) - causes meningitis

Note: see information on bacterial morphology.