Gram-positive bacteria
In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.
The Gram stain is used by microbiologists to place bacteria into two main categories, Gram-positive (+) and Gram-negative (-). Gram-positive bacteria have a thick layer of peptidoglycan within the cell wall, and Gram-negative bacteria have a thin layer of peptidoglycan.
Gram-positive bacteria take up the
Conversely,
Despite their thicker peptidoglycan layer, gram-positive bacteria are more receptive to certain
Characteristics
In general, the following characteristics are present in gram-positive bacteria:[2]
- Cytoplasmic lipid membrane
- Thick peptidoglycan layer
- chelatingagents, and also for certain types of adherence.
- Peptidoglycan chains are cross-linked to form rigid cell walls by a bacterial enzyme DD-transpeptidase.
- A much smaller volume of periplasm than that in gram-negative bacteria.
Only some species have a
Classification
Along with
Importance of the outer cell membrane in bacterial classification
It has been suggested that this section be split out into another article titled Gram stain. (Discuss) (November 2023) |
Although bacteria are traditionally divided into two main groups, gram-positive and gram-negative, based on their Gram stain retention property, this classification system is ambiguous as it refers to three distinct aspects (staining result, envelope organization, taxonomic group), which do not necessarily coalesce for some bacterial species.[6][7][8][9] The gram-positive and gram-negative staining response is also not a reliable characteristic as these two kinds of bacteria do not form phylogenetic coherent groups.[6] However, although Gram staining response is an empirical criterion, its basis lies in the marked differences in the ultrastructure and chemical composition of the bacterial cell wall, marked by the absence or presence of an outer lipid membrane.[6][10]
All gram-positive bacteria are bounded by a single-unit lipid membrane, and, in general, they contain a thick layer (20–80 nm) of peptidoglycan responsible for retaining the Gram stain. A number of other bacteria—that are bounded by a single membrane, but stain gram-negative due to either lack of the peptidoglycan layer, as in the mycoplasmas, or their inability to retain the Gram stain because of their cell wall composition—also show close relationship to the gram-positive bacteria. For the bacterial cells bounded by a single cell membrane, the term monoderm bacteria has been proposed.[6][10]
In contrast to gram-positive bacteria, all typical gram-negative bacteria are bounded by a cytoplasmic membrane and an outer cell membrane; they contain only a thin layer of peptidoglycan (2–3 nm) between these membranes. The presence of inner and outer cell membranes defines a new compartment in these cells: the
Exceptions
In general, gram-positive bacteria are monoderms and have a single lipid bilayer whereas gram-negative bacteria are diderms and have two bilayers. Exceptions include:
- Some taxa lack peptidoglycan (such as the class Enterobacteriales) and are gram-indeterminate.
- The Deinococcota have gram-positive stains, although they are structurally similar to gram-negative bacteria with two layers.
- The
Some Bacillota species are not gram-positive. The class Negativicutes, which includes
Pathogenicity
In the classical sense, six gram-positive genera are typically pathogenic in humans. Two of these,
Bacterial transformation
Transformation is one of three processes for horizontal gene transfer, in which exogenous genetic material passes from a donor bacterium to a recipient bacterium, the other two processes being conjugation (transfer of genetic material between two bacterial cells in direct contact) and transduction (injection of donor bacterial DNA by a bacteriophage virus into a recipient host bacterium).[21][22] In transformation, the genetic material passes through the intervening medium, and uptake is completely dependent on the recipient bacterium.[21]
As of 2014 about 80 species of bacteria were known to be capable of transformation, about evenly divided between gram-positive and gram-negative bacteria; the number might be an overestimate since several of the reports are supported by single papers.[21] Transformation among gram-positive bacteria has been studied in medically important species such as Streptococcus pneumoniae, Streptococcus mutans, Staphylococcus aureus and Streptococcus sanguinis and in gram-positive soil bacterium Bacillus subtilis, Bacillus cereus.[23]
Orthographic note
The adjectives gram-positive and gram-negative derive from the surname of
References
- ^ Basic Biology (18 March 2016). "Bacteria".
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- ^ Sahebnasagh, R.; Saderi, H.; Owlia, P. (4–7 September 2011). Detection of methicillin-resistant Staphylococcus aureus strains from clinical samples in Tehran by detection of the mecA and nuc genes. The First Iranian International Congress of Medical Bacteriology. Tabriz, Iran.
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- ^ S2CID 23559881.
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- ^ "Emerging Infectious Diseases Journal Style Guide". CDC.gov. Centers for Disease Control and Prevention.
External links
- This article incorporates NCBI. Archived from the originalon 2009-12-08.
- 3D structures of proteins associated with plasma membrane of gram-positive bacteria
- 3D structures of proteins associated with outer membrane of gram-positive bacteria