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

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 retain the crystal violet stain, instead taking up the counterstain (safranin or fuchsin) and appearing red or pink. Gram-positive organisms are able to retain the crystal violet stain because of the high amount of peptidoglycan in the cell wall. Gram-positive cell walls typically lack the outer membrane found in Gram-negative bacteria.

When treated as a clade, the term "Posibacteria" is sometimes used.[1]



Gram-positive and -negative cell wall structure
Structure of Gram-positive cell wall

The following characteristics are generally present in a Gram-positive bacterium:[2]

  1. cytoplasmic lipid membrane
  2. thick peptidoglycan layer
  3. capsule polysaccharides (only in some species)
  4. flagellum (only in some species)
    • if present, it contains two rings for support as opposed to four in Gram-negative bacteria because Gram-positive bacteria have only one membrane layer.


In the original bacterial phyla, the Gram-positive organisms made up the phylum Firmicutes, a name now used for the largest group. It includes many well-known genera such as Staphylococcus, Streptococcus, Enterococcus, (which are cocci) and Bacillus, Corynebacterium, Nocardia, Clostridium, Actinobacteria, and Listeria (which are rods and can be remembered by the mnemonic obconical). It has also been expanded to include the Mollicutes, bacteria-like Mycoplasma that lack cell walls and cannot be Gram stained, but are derived from such forms. Actinobacteria are the other major group of Gram-positive bacteria, which have a high guanine and cytosine content in their genomes (high G+C group). This contrasts with the Firmicutes, which have a low G+C content.

Both Gram-positive and Gram-negative bacteria may have a membrane called an S-layer. In Gram-negative bacteria, the S-layer is directly attached to the outer membrane. In Gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Unique to Gram-positive bacteria is the presence of teichoic acids in the cell wall. Some particular teichoic acids, lipoteichoic acids, have a lipid component and can assist in anchoring peptidoglycan, as the lipid component is embedded in the membrane.

Gram Positive Classification.png


The Deinococcus-Thermus bacteria have Gram-positive stains, although they are structurally similar to Gram-negative bacteria.


Most pathogenic bacteria in humans are gram-positive organisms. Classically, six gram-positive genera are typically pathogenic in humans. Two of these, Streptococcus and Staphylococcus, are cocci (sphere-shaped bacteria). The remaining organisms are bacilli (rod-shaped bacteria) and can be subdivided based on their ability to form spores. The non-spore formers are Corynebacterium and Listeria (a coccobacillus), while Bacillus and Clostridium produce spores.[3] The spore-forming bacteria can again be divided based on their respiration: Bacillus is a facultative anaerobe, while Clostridium is an obligate anaerobe.

See also


  1. ^ Cavalier-Smith T (2006). "Rooting the tree of life by transition analyses". Biol. Direct 1: 19. doi:10.1186/1745-6150-1-19. PMID 16834776.& PMC 1586193. 
  2. ^ Madigan M; Martinko J (editors). (2005). Brock Biology of Microorganisms (11th ed.). Prentice Hall. ISBN 0131443291. 
  3. ^ Gladwin, Mark; Bill Trattler (2007). Clinical Microbiology made ridiculously simple. Miami, FL: MedMaster, Inc. pp. 4–5. ISBN 978-0-940780-81-1. 

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