Inbacteriology,Gram-positive bacteria arebacteria that give a positive result in theGram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type ofcell wall.
The Gram stain is used by microbiologists to place bacteria into two main categories, Gram-positive (+) andGram-negative (−). Gram-positive bacteria have a thick layer ofpeptidoglycan within the cell wall, and Gram-negative bacteria have a thin layer of peptidoglycan.
Gram-positive bacteria retain thecrystal violet stain used in the test, resulting in a purple color when observed through anoptical microscope. The thick layer of peptidoglycan in the bacterial cell wall retains thestain after it has been fixed in place by iodine. During the decolorization step, the decolorizer removes crystal violet from all other cells.
Conversely,Gram-negative bacteria cannot retain the violet stain after the decolorization step;alcohol used in this stage degrades the outer membrane of Gram-negative cells, making the cell wall more porous and incapable of retaining the crystal violet stain. Their peptidoglycan layer is much thinner and sandwiched between aninner cell membrane and abacterial outer membrane, causing them to take up thecounterstain (safranin orfuchsine) and appear red or pink.
Despite their thicker peptidoglycan layer, Gram-positive bacteria are more receptive to certaincell wall–targetingantibiotics than Gram-negative bacteria, due to the absence of the outer membrane.[1]
In general, the following characteristics are present in Gram-positive bacteria:[2]
Only some species have acapsule, usually consisting ofpolysaccharides. Only some species areflagellates, and those withflagella have just twobasal body rings for support, in contrast to the four found in Gram-negative bacteria. Both Gram-positive and Gram-negative bacteria commonly have a surface layer called anS-layer. In Gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Gram-negative bacteria's S-layer is attached directly to theouter membrane. Specific to Gram-positive bacteria is the presence ofteichoic acids in the cell wall. Some of these are lipoteichoic acids, which have a lipid component in the cell membrane that can assist in anchoring the peptidoglycan.[3]
Along withcell shape,Gram staining is a rapid method used to differentiate bacterial species. Such staining, together with growth requirement and antibiotic susceptibility testing, and other macroscopic and physiologic tests, forms a basis for practical classification and subdivision of the bacteria (e.g., see figure and pre-1990 versions ofBergey's Manual of Systematic Bacteriology).[citation needed]
Historically, the kingdomMonera was divided into fourdivisions based primarily on Gram staining:Bacillota (positive in staining),Gracilicutes (negative in staining),Mollicutes (neutral in staining) and Mendocutes (variable in staining).[4] Based on16S ribosomal RNA phylogenetic studies of the late microbiologistCarl Woese and collaborators and colleagues at theUniversity of Illinois, themonophyly of the Gram-positive bacteria was challenged,[5] with major implications for the therapeutic and general study of these organisms. Based onmolecular studies of the 16S sequences, Woese recognised twelvebacterial phyla. Two of these were Gram-positive and were divided on the proportion of theguanine andcytosine content in theirDNA. The high G + C phylum was made up of theActinobacteria, and the low G + C phylum contained theFirmicutes.[5] The Actinomycetota include theCorynebacterium,Mycobacterium,Nocardia andStreptomyces genera. The (low G + C) Bacillota, have a 45–60% GC content, but this is lower than that of the Actinomycetota.[2]
 | It has been suggested that this section besplit out into another article titledGram stain. (Discuss)(November 2023) |
Although bacteria are traditionally divided into two main groups, Gram-positive and Gram-negative, based on theirGram 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 bound 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 bound by a single membrane, but stain Gram-negative due to either lack of the peptidoglycan layer, as in themycoplasmas, 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 bound by a single cell membrane, the termmonoderm bacteria has been proposed.[6][10]
In contrast to Gram-positive bacteria, all typical Gram-negative bacteria are bound 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: theperiplasmic space or the periplasmic compartment. These bacteria have been designated asdiderm bacteria.[6][10] The distinction between the monoderm and diderm bacteria is supported by conserved signature indels in a number of important proteins (viz. DnaK, GroEL).[6][7][10][11] Of these two structurally distinct groups of bacteria, monoderms are indicated to be ancestral. Based upon a number of observations including that the Gram-positive bacteria are the major producers of antibiotics and that, in general, Gram-negative bacteria are resistant to them, it has been proposed that the outer cell membrane in Gram-negative bacteria (diderms) has evolved as a protective mechanism againstantibiotic selection pressure.[6][7][10][11] Some bacteria, such asDeinococcus, which stain Gram-positive due to the presence of a thick peptidoglycan layer and also possess an outer cell membrane are suggested as intermediates in the transition between monoderm (Gram-positive) and diderm (Gram-negative) bacteria.[6][11] The diderm bacteria can also be further differentiated between simple diderms lacking lipopolysaccharide, the archetypical diderm bacteria where the outer cell membrane contains lipopolysaccharide, and the diderm bacteria where outer cell membrane is made up ofmycolic acid.[8][11][12]
In general, Gram-positive bacteria are monoderms and have a singlelipid bilayer whereas Gram-negative bacteria are diderms and have two bilayers. Exceptions include:
Some Bacillota species are not Gram-positive. The class Negativicutes, which includesSelenomonas, are diderm and stain Gram-negative.[12] Additionally, a number of bacterial taxa (viz.Negativicutes,Fusobacteriota,Synergistota, andElusimicrobiota) that are either part of the phylum Bacillota or branch in its proximity are found to possess a diderm cell structure.[9][11][12] However, a conserved signature indel (CSI) in theHSP60 (GroEL) protein distinguishes all traditional phyla of Gram-negative bacteria (e.g.,Pseudomonadota,Aquificota,Chlamydiota,Bacteroidota,Chlorobiota, "Cyanobacteria",Fibrobacterota,Verrucomicrobiota,Planctomycetota,Spirochaetota,Acidobacteriota) from these other atypical diderm bacteria, as well as other phyla of monoderm bacteria (e.g.,Actinomycetota,Bacillota,Thermotogota,Chloroflexota).[11] The presence of this CSI in all sequenced species of conventional LPS (lipopolysaccharide)-containing Gram-negative bacterial phyla provides evidence that these phyla of bacteria form a monophyletic clade and that no loss of the outer membrane from any species from this group has occurred.[11]
In the classical sense, six Gram-positive genera are typically pathogenic in humans. Two of these,Streptococcus andStaphylococcus, arecocci (sphere-shaped). The remaining organisms arebacilli (rod-shaped) and can be subdivided based on their ability to formspores. The non-spore formers areCorynebacterium andListeria (a coccobacillus), whereasBacillus andClostridium produce spores.[17] The spore-forming bacteria can again be divided based on theirrespiration:Bacillus is afacultative anaerobe, whileClostridium is anobligate anaerobe.[18] Also,Rathybacter,Leifsonia, andClavibacter are three Gram-positive genera that cause plant disease. Gram-positive bacteria are capable of causing serious and sometimes fatalinfections in newborn infants.[19] Novel species of clinically relevant Gram-positive bacteria also includeCatabacter hongkongensis, which is an emerging pathogen belonging toBacillota.[20]
Transformation is one of three processes forhorizontal gene transfer, in which exogenous genetic material passes from a donor bacterium to a recipient bacterium, the other two processes beingconjugation (transfer ofgenetic material between two bacterial cells in direct contact) andtransduction (injection of donor bacterial DNA by abacteriophage 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 andGram-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 asStreptococcus pneumoniae,Streptococcus mutans,Staphylococcus aureus andStreptococcus sanguinis and in Gram-positive soil bacteriaBacillus subtilis andBacillus cereus.[23]
The adjectivesGram-positive andGram-negative derive from the surname ofHans Christian Gram; aseponymous adjectives, their initial letterG can be either a capital or lower-case, depending on whichstyle guide, if any, governs the document being written. For instance, the style guide of the AmericanCDC recommends writing:Gram stain,this species is gram negative, anda gram-negative species.[24]
This article incorporatespublic domain material fromScience Primer.NCBI. Archived fromthe original on 2009-12-08.| Medical microbiology | |||||||
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