Gram-positive bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
are those that are stained dark blue or violet by
Gram stainingGram staining is a method of differentiating bacterial species into two large groups ....
. This is in contrast to Gram-negative bacteria, which cannot retain the
crystal violetCrystal violet or Gentian violet is a triarylmethane dye. The dye is used as a histological stain and in Gram’s method of classifying bacteria. Crystal violet has antibacterial, antifungal, and anthelmintic properties and was formerly important as a topical antiseptic...
stain, instead taking up the
counterstainA counterstain is a stain with color contrasting to the principal stain, making the stained structure more easily visible.An example is the malachite green counterstain to the fuchsine stain in the Gimenez staining technique....
(
safraninSafranin is a biological stain used in histology and cytology. Safranin is used as a counterstain in some staining protocols, colouring all cell nuclei red. This is the classic counterstain in a Gram stain...
or
fuchsineFuchsine or rosaniline hydrochloride is a magenta dye with chemical formula C20H19N3·HCl. There are other similar chemical formulations of products sold as fuchsine, and several dozen other synonyms of this molecule....
) and appearing red or pink. Gram-positive organisms are able to retain the crystal violet stain because of the high amount of
peptidoglycanPeptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
in the
cell wallThe cell envelope is the cell membrane and cell wall plus an outer membrane, if one is present.Most bacterial cell envelopes fall into two major categories: Gram positive and Gram negative. These are differentiated by their Gram staining characteristics....
. Gram-positive cell walls typically lack the outer membrane found in Gram-negative bacteria.
Characteristics
The following characteristics are generally present in a Gram-positive bacterium:
- cytoplasmic lipid membrane
- thick peptidoglycan
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
layer
- teichoic acids and lipoids are present, forming lipoteichoic acid
Lipoteichoic acid is a major constituent of the cell wall of Gram-positive bacteria. These organisms have an inner membrane and, external to it, a thick peptidoglycan layer. It consists of teichoic acids, long chains of ribitol phosphate and is anchored to the lipid bilayer via a glyceride...
s, which serve to act as chelatingChelation is the formation or presence of two or more separate coordinate bonds between apolydentate ligand and a single central atom....
agents, and also for certain types of adherence.
- capsule polysaccharides (only in some species)
- flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
(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.
- The individual peptidoglycan molecules are cross-linked by pentaglycine chains by a DD-transpeptidase enzyme. In gram-negative bacteria, the transpeptidase creates a covalent bond directly between peptidoglycan molecules, with no intervening bridge.
Both Gram-positive and Gram-negative bacteria may have a membrane called an
S-layerAn S-layer is a part of the cell envelope commonly found in bacteria, as well as among archaea.It consists of a monomolecular layer composed of identical proteins or glycoproteins. This two-dimensional structure is built via self-assembly and encloses the whole cell surface. Thus, the S-layer...
. In Gram-negative bacteria, the S-layer is attached directly 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 acidTeichoic acids are bacterial polysaccharides of glycerol phosphate or ribitol phosphate linked via phosphodiester bonds.-Location and structure:...
s 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.
Classification
Along with cell shape, Gram staining is a rapid diagnostic tool of use to group species of Bacteria. In traditional and even some areas of contemporary microbiological practice, such staining, alongside growth requirement and antibiotic susceptibility testing, and other macroscopic and physiologic tests, forms the full basis for classification and subdivision of the Bacteria (e.g., see Figure, and pre-1990 versions of Bergey's Manual).
As such, historically, the kindgom
MoneraMonera is a superseded kingdom that contains unicellular organisms without a nucleus , such as bacteria....
was divided into four
divisionsIn biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...
based primarily on Gram staining: Firmicutes (positive in staining), Gracillicutes (negative in staining), Mollicutes (neutral in staining) and Mendocutes (variable in staining).
Since 1987 and the seminal 16S ribosomal RNA phylogenetic studies of
Carl WoeseCarl Richard Woese is an American microbiologist and physicist. Woese is famous for defining the Archaea in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique pioneered by Woese and which is now standard practice. He was also the originator of the RNA world hypothesis in 1977,...
(Department of Microbiology, University of Illinois) and collaborators and colleagues, the
monophylyIn common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
of the Gram-positive bacteria has been challenged, with striking productive implications for the therapeutic and general study of these organisms. Based on molecular studies of 16S sequences, Woese recognised 12
bacterial phylaThe bacterial phyla are the major lineages of the domain Bacteria.In the scientific classification established by Carl von Linné, each bacterial strain has to be assigned to a species , which is a lower level of a hierarchy of ranks...
, two being Gram-positive: high-GC Gram-positives and low-GC Gram-positives (where G and C refer to the
guanineGuanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...
and
cytosineCytosine is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine . It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached . The nucleoside of cytosine is cytidine...
content in their genomes), which are now referred to by these names, or as
ActinobacteriaActinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content. They can be terrestrial or aquatic. Actinobacteria is one of the dominant phyla of the bacteria....
and
FirmicutesThe Firmicutes are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer-membrane that causes them to stain Gram-negative...
. The former, the
ActinobacteriaActinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content. They can be terrestrial or aquatic. Actinobacteria is one of the dominant phyla of the bacteria....
, are the high GC content Gram-positive bacteria. The latter, the Firmicutes are the "low-GC" Gram-positive bacteria, which actually have 45%–60% GC content but lower than that of the Actinobacteria,. The Firmicutes contain the well-known genera that are majority of Gram-positives of medical interest:
StaphylococcusStaphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....
,
StreptococcusStreptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning...
,
EnterococcusEnterococcus is a genus of lactic acid bacteria of the phylum Firmicutes. Enterococci are Gram-positive cocci that often occur in pairs or short chains, and are difficult to distinguish from streptococci on physical characteristics alone. Two species are common commensal organisms in the...
(cocci),
BacillusBacillus is a genus of Gram-positive, rod-shaped bacteria and a member of the division Firmicutes. Bacillus species can be obligate aerobes or facultative anaerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species...
,
CorynebacteriumCorynebacterium is a genus of Gram-positive rod-shaped bacteria. They are widely distributed in nature and are mostly innocuous. Some are useful in industrial settings such as C. glutamicum. Others can cause human disease. C...
,
NocardiaNocardia is a genus of weakly staining Gram-positive, catalase-positive, rod-shaped bacteria. It forms partially acid-fast beaded branching filaments . It has a total of 85 species. Some species are non-pathogenic while others are responsible for nocardiosis. Nocardia are found worldwide in soil...
,
ClostridiumClostridium is a genus of Gram-positive bacteria, belonging to the Firmicutes. They are obligate anaerobes capable of producing endospores. Individual cells are rod-shaped, which gives them their name, from the Greek kloster or spindle...
,
ActinobacteriaActinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content. They can be terrestrial or aquatic. Actinobacteria is one of the dominant phyla of the bacteria....
and
Listeria (bacilli/rods). This group also been expanded to include the
MycoplasmaMycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
, or Mollicutes, bacteria-like organisms that lack cell walls and cannot be Gram-stained, but appear to have derived evolutionarily from such forms.
Despite the wide acceptance and practical record of utility of the new molecular phylogeny, a small group, including Cavalier-Smith, still treat the Monera as a monophyletic
cladeA clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
and refer to the group as division "Posibacteria".
Exceptions
In general, Gram-positive bacteria have a single lipid bilayer (monoderms), whereas Gram-negative have two (diderms). Some taxa lack peptidoglycan (such as the domain
Archaea, the class
Mollicutes, some members of the
Rhickettsiales, and the insect-endosymbionts of the
Enterobacteriales) and are Gram-variable. This, however, does not always hold true.
The
Deinococcus-ThermusThe Deinococcus-Thermus are a small group of bacteria composed of cocci highly resistant to environmental hazards.There are two main groups.* The Deinococcales include two families, with three genera, Deinococcus and Truepera, the former with several species that are resistant to radiation; they...
bacteria have Gram-positive stains, although they are structurally similar to Gram-negative bacteria with two layers (diderms).
The
ChloroflexiChloroflexi is one of four classes of bacteria in the phylum Chloroflexi, known as filamentous green non-sulfur bacteria. They produce energy from light and are named for their green pigment, usually found in photosynthetic bodies called chlorosomes....
have a single layer, yet (with some exceptions) stain negative. Two related phyla to the
Chloroflexi, the TM7 clade and the
Ktedonobacteria, are also monoderms.
Some Firmicute species are not Gram-positive; these belong to the class
MollicutesThe Mollicutes are a class of bacteria distinguished by the absence of a cell wall. The word "Mollicutes" is derived from the Latin mollis , and cutis . They are parasites of various animals and plants, living on or in the host's cells. Individuals are very small, typically only 0.2–0.3 μm in size...
(alternatively considered a class of the phylum
TenericutesTenericutes is a phylum of bacteria that contains the Class Mollicutes. The name was validated in 1984 as a new division ....
), which lack peptidoglycan (Gram-indeterminate), and the class
Negativicutes, which includes
Selenomonas and which stain Gram-negative.
Pathogenesis
Most pathogens in humans are Gram-positive organisms. In the classical sense, six Gram-positive genera are typically pathogenic in humans. Two of these,
StreptococcusStreptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning...
and
StaphylococcusStaphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....
, are cocci (sphere-shaped bacteria). The remaining organisms are
bacilliBacilli refers to a taxonomic class of bacteria. It includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens like Bacillus anthracis .-Ambiguity:...
(rod-shaped bacteria) and can be subdivided based on their ability to form
sporeIn biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s. The non-spore formers are
CorynebacteriumCorynebacterium is a genus of Gram-positive rod-shaped bacteria. They are widely distributed in nature and are mostly innocuous. Some are useful in industrial settings such as C. glutamicum. Others can cause human disease. C...
and
Listeria (a coccobacillus), whereas
BacillusBacillus is a genus of Gram-positive, rod-shaped bacteria and a member of the division Firmicutes. Bacillus species can be obligate aerobes or facultative anaerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species...
and
ClostridiumClostridium is a genus of Gram-positive bacteria, belonging to the Firmicutes. They are obligate anaerobes capable of producing endospores. Individual cells are rod-shaped, which gives them their name, from the Greek kloster or spindle...
produce spores. The spore-forming bacteria can again be divided based on their
respirationCellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
:
Bacillus is a
facultative anaerobeA facultative anaerobic organism is an organism, usually a bacterium, that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation...
, while
Clostridium is an
obligate anaerobeObligate anaerobes are microorganisms that live and grow in the absence of molecular oxygen; some of these are killed by oxygen. -Metabolism:...
.
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