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) 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.
The following characteristics are generally present in a Gram-positive bacterium:
capsule polysaccharides (only in some species)
flagellum (only in some species)
- cytoplasmic lipid membrane
- thick peptidoglycan layer
- 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 Bacillus
, and Clostridium
. 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 guanosine
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.
bacteria have Gram-positive stains, although they are structurally similar to Gram-negative bacteria.
bacteria in humans are gram-negative organisms. Classically, six gram-positive organisms are typically pathogenic in humans. Two of these, Streptococcus
, are cocci
(round 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
, while Bacillus
produce spores. The spore-forming bacteria can again be divided based on their respiration
is a facultative anaerobe
, while Clostridium
is an obligate anaerobe