Gram-positive bacteria

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Rod-shaped gram-positive Bacillus anthracis bacteria in a cerebrospinal fluid sample stand out from round white blood cells, which also accept the crystal violet stain.
gram-negative bacilli

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

stain
after it is washed away from the rest of the sample, in the decolorization stage of the test.

Conversely,

inner cell membrane and a bacterial outer membrane, causing them to take up the counterstain (safranin or fuchsine
) and appear red or pink.

Despite their thicker peptidoglycan layer, gram-positive bacteria are more receptive to certain

antibiotics than gram-negative bacteria, due to the absence of the outer membrane.[1]

Characteristics

Gram-positive and gram-negative cell wall structure
Structure of gram-positive cell wall

In general, the following characteristics are present in gram-positive bacteria:[2]

  1. Cytoplasmic lipid membrane
  2. Thick peptidoglycan layer
  3. chelating
    agents, and also for certain types of adherence.
  4. Peptidoglycan chains are cross-linked to form rigid cell walls by a bacterial enzyme DD-transpeptidase.
  5. A much smaller volume of periplasm than that in gram-negative bacteria.

Only some species have a

flagella, have only two basal body rings to support them, whereas gram-negative have four. Both gram-positive and gram-negative bacteria commonly have a surface layer called an S-layer. In gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Gram-negative bacteria's S-layer is attached directly to the outer membrane. Specific to gram-positive bacteria is the presence of teichoic 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]

Classification

Along with

Bergey's Manual of Systematic Bacteriology
).

Species identification hierarchy in clinical settings

University of Illinois, the monophyly of the gram-positive bacteria was challenged,[5] with major implications for the therapeutic and general study of these organisms. Based on molecular studies of the 16S sequences, Woese recognised twelve bacterial phyla. Two of these were gram-positive and were divided on the proportion of the guanine and cytosine content in their DNA. The high G + C phylum was made up of the Actinobacteria, and the low G + C phylum contained the Firmicutes.[5] The Actinomycetota include the Corynebacterium, Mycobacterium, Nocardia and Streptomyces genera. The (low G + C) Bacillota, have a 45–60% GC content, but this is lower than that of the Actinomycetota.[2]

Importance of the outer cell membrane in bacterial classification

N-acetylmuramic acid

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

diderm 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 against antibiotic selection pressure.[6][7][10][11] Some bacteria, such as Deinococcus, 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 of mycolic acid.[8][11][12]

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
    TM7 clade and the Ktedonobacteria, are also monoderms.[15][16]

Some Bacillota species are not gram-positive. The class Negativicutes, which includes

Thermotogota, Chloroflexota, etc.).[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]

Pathogenicity

Colonies of a gram-positive pathogen of the oral cavity, Actinomyces sp.

In the classical sense, six gram-positive genera are typically pathogenic in humans. Two of these,

Bacillota.[20]

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

Gram staining § Orthographic note
.

References

  1. ^ Basic Biology (18 March 2016). "Bacteria".
  2. ^
    ISBN 978-0131443297
    .
  3. PMID 24024634
    .
  4. doi:10.1099/00207713-28-1-1
    .
  5. ^
    PMID 2439888
    .
  6. ^
    PMID 9841678
    .
  7. ^
    S2CID 30541897. Archived
    (PDF) from the original on 2013-06-25.
  8. ^
    PMID 19299134
    .
  9. ^
    PMID 20637628
    .
  10. ^
    S2CID 41206658
    .
  11. ^
    PMID 21717204
    .
  12. ^
    PMID 19667386
    .
  13. PMID 20495028
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  14. PMID 20860732
    .
  15. PMID 11133473
    .
  16. PMID 16751552
    .
  17. ISBN 978-0-940780-81-1
    .
  18. ^ 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.
  19. ISBN 9781451192681
    . Access provided by the University of Pittsburgh.
  20. PMID 17122022
    .
  21. ^
    S2CID 23559881
    .
  22. PMID 36838287
    .
  23. PMID 18295550
    .
  24. ^ "Emerging Infectious Diseases Journal Style Guide". CDC.gov. Centers for Disease Control and Prevention.

External links

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