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 fuchsine) 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.
The following characteristics are generally present in a Gram-positive bacterium:
- 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.
Full article ▸