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Bacteria that are Gram-positive are stained dark blue or violet by Gram staining, in contrast to Gram-negative bacteria. The stain is caused by a higher amount of peptidoglycan in the cell wall, which typically lacks the secondary membrane and lipopolysaccharide layer found in other bacteria.

The largest group of Gram-positive bacteria are the Firmicutes; well-known genera include Bacillus, Listeria, Staphylococcus, Streptococcus, Enterococcus, and Clostridium. Other major groups include the Actinobacteria, Planctomycetes, Deinococci, and Thermotogae.