Gram-negative bacteria

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Microscopic image of gram-negative Pseudomonas aeruginosa bacteria (pink-red rods) Pseudomonas aeruginosa Gram.jpg
Microscopic image of gram-negative Pseudomonas aeruginosa bacteria (pink-red rods)

Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. [1] Their defining characteristic is that their cell envelope consists of a thin peptidoglycan cell wall sandwiched between an inner (cytoplasmic) membrane and an outer membrane. [2] These bacteria are found in all environments that support life on Earth.

Contents

Within this category, notable species include the model organism Escherichia coli , along with various pathogenic bacteria, such as Pseudomonas aeruginosa , Chlamydia trachomatis , and Yersinia pestis . They pose significant challenges in the medical field due to their outer membrane, which acts as a protective barrier against numerous antibiotics (including penicillin), detergents that would normally damage the inner cell membrane, and the antimicrobial enzyme lysozyme produced by animals as part of their innate immune system. Furthermore, the outer leaflet of this membrane contains a complex lipopolysaccharide (LPS) whose lipid A component can trigger a toxic reaction when the bacteria are lysed by immune cells. This reaction may lead to septic shock, resulting in low blood pressure, respiratory failure, reduced oxygen delivery, and lactic acidosis. [3]

Several classes of antibiotics have been developed to target gram-negative bacteria, including aminopenicillins, ureidopenicillins, cephalosporins, beta-lactam-betalactamase inhibitor combinations (such as piperacillin-tazobactam), folate antagonists, quinolones, and carbapenems. Many of these antibiotics also cover gram-positive bacteria. The antibiotics that specifically target gram-negative organisms include aminoglycosides, monobactams (such as aztreonam), and ciprofloxacin.

Characteristics

Gram-negative (LPS-diderm) cell wall structure Gram negative cell wall.svg
Gram-negative (LPS-diderm) cell wall structure
Gram-positive and -negative bacteria are differentiated chiefly by their cell wall structure Gram-Cell-wall.svg
Gram-positive and -negative bacteria are differentiated chiefly by their cell wall structure

Conventional gram-negative (LPS-diderm) bacteria display the following characteristics:[ citation needed ]

However, the LPS-diderm group (corresponding to kingdom Pseudomonadati, formerly 'Hydrobacteria') is not the only type of bacteria that stain negative. [4] Mycobacterium (or rather most of Mycobacteriales), which does not belong in the group, have independently evolved an outer cell membrane, with a cell wall made of mycolic acid. [5] This gives it very different structure and features. [6] [7] [8] [9]

Classification

Along with cell shape, Gram staining is a rapid diagnostic tool and once was used to group species at the subdivision of Bacteria. Historically, the kingdom Monera was divided into four divisions based on Gram staining: Firmicutes (+), Gracillicutes (−), Mollicutes (0) and Mendocutes (var.). [10] Since 1987, the monophyly of the gram-negative bacteria has been disproven with molecular studies. [11] However some authors, such as Cavalier-Smith still treat them as a monophyletic taxon (though not a clade; his definition of monophyly requires a single common ancestor but does not require holophyly, the property that all descendants be encompassed by the taxon) and refer to the group as a subkingdom "Negibacteria". [12]

Current knowledge divides the gram-negatives into two large groups and some straddlers. The more "conventional" Gram-negatives with an LPS outer membrane do share a common ancestor and are grouped in kingdom Pseudomonadati. [4] The less conventional ones are, as mentioned above, the order Mycobacteriales, have a mycolic acid cell wall and an outer membrane. [5] The kingdom and the order are each monophyletic (or rather, not holyphyletic), but the "LPS-diderm" and "mycolic-diderm" groups are not, because some bacteria in the kingdom and the order do not, in fact, stain gram negative. They will be discussed in the next section.

Taxonomy

Bacteria are traditionally classified based on their Gram-staining response into the gram-positive and gram-negative bacteria. Having just one membrane, the gram-positive bacteria are also known as monoderm bacteria, while gram-negative bacteria, having two membranes, are also known as diderm bacteria. It was traditionally thought that the groups represent lineages, i.e., the extra membrane only evolved once, such that gram-negative bacteria are more closely related to one another than to any gram-positive bacteria. While this is often true, the classification system breaks down in some cases, with lineage groupings not matching the staining result. [13] [14] [6] [7] Thus, Gram staining cannot be reliably used to assess familial relationships of bacteria. Nevertheless, staining often gives reliable information about the composition of the cell membrane, distinguishing between the presence or absence of an outer lipid membrane. [13] [15]

Of these two structurally distinct groups of prokaryotic organisms, monoderm prokaryotes are thought to be ancestral. Based upon a number of different observations, including that the gram-positive bacteria are the most sensitive to antibiotics and that the gram-negative bacteria are, in general, resistant to antibiotics, it has been proposed that the outer cell membrane in gram-negative bacteria (diderms) evolved as a protective mechanism against antibiotic selection pressure. [13] [14] [15] [8] Some bacteria such as Deinococcus , which stain gram-positive due to the presence of a thick peptidoglycan layer, but also possess an outer cell membrane are suggested as intermediates in the transition between monoderm (gram-positive) and diderm (gram-negative) bacteria. [13] [8]

The conventional LPS-diderm group of gram-negative bacteria (e.g., Pseudomonadota, Aquificota, Chlamydiota, Bacteroidota, Chlorobiota, "Cyanobacteria", Fibrobacterota, Verrucomicrobiota, Planctomycetota, Spirochaetota, Acidobacteriota) are uniquely identified by a few conserved signature indel (CSI) in the HSP60 (GroEL) protein. The presence of this CSI in all sequenced species of conventional 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. [8] They have accordingly been assigned a kingdom Pseudomonadati (formerly "Hydrobacteria"). [4]

The difficulty lies in the other taxa that also have a diderm structure.

Example species

The proteobacteria are a major superphylum of gram-negative bacteria, including E. coli , Salmonella , Shigella , and other Enterobacteriaceae, Pseudomonas , Moraxella , Helicobacter , Stenotrophomonas , Bdellovibrio , acetic acid bacteria, Legionella etc. Other notable groups of gram-negative bacteria include the cyanobacteria, spirochaetes, and green sulfur bacteria. [17] [18] [19]

Medically-relevant gram-negative diplococci include the four types that cause a sexually transmitted disease ( Neisseria gonorrhoeae [20] ), a meningitis ( Neisseria meningitidis [21] ), and respiratory symptoms ( Moraxella catarrhalis , [22] A coccobacillus Haemophilus influenzae is another medically relevant coccal type. [23]

Medically relevant gram-negative bacilli include a multitude of species. Some of them cause primarily respiratory problems ( Klebsiella pneumoniae , Legionella pneumophila , Pseudomonas aeruginosa ), primarily urinary problems ( Escherichia coli , Proteus mirabilis , Enterobacter cloacae , Serratia marcescens ), and primarily gastrointestinal problems ( Helicobacter pylori , Salmonella enteritidis , Salmonella typhi ).[ citation needed ]

Gram-negative bacteria associated with hospital-acquired infections include Acinetobacter baumannii , which cause bacteremia, secondary meningitis, and ventilator-associated pneumonia in hospital intensive-care units.

Bacterial transformation

Transformation is one of three processes for horizontal gene transfer, in which exogenous genetic material passes from one bacterium to another, the other two being conjugation (transfer of genetic material between two bacterial cells in direct contact) and transduction (injection of foreign DNA by a bacteriophage virus into the host bacterium). [24] [25] In transformation, the genetic material passes through the intervening medium, and uptake is completely dependent on the recipient bacterium. [24]

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. [24] Transformation has been studied in medically important gram-negative bacteria species such as Helicobacter pylori , Legionella pneumophila , Neisseria meningitidis , Neisseria gonorrhoeae , Haemophilus influenzae and Vibrio cholerae . [26] It has also been studied in gram-negative species found in soil such as Pseudomonas stutzeri , Acinetobacter baylyi , and gram-negative plant pathogens such as Ralstonia solanacearum and Xylella fastidiosa . [26]

Role in disease

Example of a workup algorithm of possible bacterial infection in cases with no specifically requested targets (non-bacteria, mycobacteria etc.), with most common situations and agents seen in a New England setting. Clinically significant Gram-negative bacteria are usually rods, as shown near bottom right. Although some gram-negative bacteria can be recognized by "bench tests", diagnosis in the modern microbiology lab usually involves MALDI-TOF and/or multitarget assay. Diagnostic algorithm of possible bacterial infection.png
Example of a workup algorithm of possible bacterial infection in cases with no specifically requested targets (non-bacteria, mycobacteria etc.), with most common situations and agents seen in a New England setting. Clinically significant Gram-negative bacteria are usually rods, as shown near bottom right. Although some gram-negative bacteria can be recognized by "bench tests", diagnosis in the modern microbiology lab usually involves MALDI-TOF and/or multitarget assay.

One of the several unique characteristics of gram-negative bacteria is the structure of the bacterial outer membrane. The outer leaflet of this membrane contains lipopolysaccharide (LPS), whose lipid A portion acts as an endotoxin. [1] If gram-negative bacteria enter the circulatory system, LPS can trigger an innate immune response, activating the immune system and producing cytokines (hormonal regulators). This leads to inflammation and can cause a toxic reaction, resulting in fever, an increased respiratory rate, and low blood pressure. That is why some infections with gram-negative bacteria can lead to life-threatening septic shock. [3]

The outer membrane protects the bacteria from several antibiotics, dyes, and detergents that would normally damage either the inner membrane or the cell wall (made of peptidoglycan). The outer membrane provides these bacteria with resistance to lysozyme and penicillin. The periplasmic space (space between the two cell membranes) also contains enzymes which break down or modify antibiotics. Drugs commonly used to treat gram negative infections include amino, carboxy and ureido penicillins (ampicillin, amoxicillin, pipercillin, ticarcillin). These drugs may be combined with beta-lactamase inhibitors to combat the presence of enzymes that can digest these drugs (known as beta-lactamases) in the peri-plasmic space. Other classes of drugs that have gram negative spectrum include cephalosporins, monobactams (aztreonam), aminoglycosides, quinolones, macrolides, chloramphenicol, folate antagonists, and carbapenems. [27]

Orthographic note

The adjectives gram-positive and gram-negative derive from the surname of Hans Christian Gram, a Danish bacteriologist; as eponymous adjectives, their initial letter can be either capital G or lower-case g, depending on which style guide (e.g., that of the CDC), if any, governs the document being written. [28] This is further explained at Gram staining § Orthographic note .

See also

References

Notes

  1. 1 2 Baron S, Salton MR, Kim KS (1996). "Structure". In Baron S (ed.). Medical Microbiology (4th ed.). University of Texas Medical Branch at Galveston. ISBN   978-0-9631172-1-2. PMID   21413343. Archived from the original on 2021-07-06. Retrieved 2021-08-18.
  2. ↑ Silhavy, T. J.; Kahne, D.; Walker, S. (2010-05-01). "The Bacterial Cell Envelope". Cold Spring Harbor Perspectives in Biology. 2 (5): a000414. doi:10.1101/cshperspect.a000414. ISSN   1943-0264. PMC   2857177 . PMID   20452953.
  3. 1 2 Pelletier, Lawrence L. (1996). "Microbiology of the Circulatory System". In Baron S (ed.). Medical Microbiology (4th ed.). University of Texas Medical Branch at Galveston. ISBN   978-0-9631172-1-2. PMID   21413321. Archived from the original on 2022-04-13. Retrieved 2021-05-12.
  4. 1 2 3 Göker, Markus; Oren, Aharon (22 January 2024). "Valid publication of names of two domains and seven kingdoms of prokaryotes". International Journal of Systematic and Evolutionary Microbiology. 74 (1). doi:10.1099/ijsem.0.006242. PMID   38252124.
  5. 1 2 3 Gupta, Radhey S. (22 February 2019). "Commentary: Genome-Based Taxonomic Classification of the Phylum Actinobacteria". Frontiers in Microbiology. 10: 206. doi: 10.3389/fmicb.2019.00206 . PMC   6395429 . PMID   30853945. Mycolic acids are important constituents of the cell envelopes of most members.
  6. 1 2 Desvaux M, Hébraud M, Talon R, Henderson IR (April 2009). "Secretion and subcellular localizations of bacterial proteins: a semantic awareness issue". Trends Microbiol. 17 (4): 139–45. doi:10.1016/j.tim.2009.01.004. PMID   19299134.
  7. 1 2 3 Sutcliffe IC (October 2010). "A phylum level perspective on bacterial cell envelope architecture". Trends Microbiol. 18 (10): 464–70. doi:10.1016/j.tim.2010.06.005. PMID   20637628.
  8. 1 2 3 4 5 6 Gupta RS (August 2011). "Origin of diderm (gram-negative) bacteria: antibiotic selection pressure rather than endosymbiosis likely led to the evolution of bacterial cells with two membranes". Antonie van Leeuwenhoek. 100 (2): 171–82. doi:10.1007/s10482-011-9616-8. PMC   3133647 . PMID   21717204.
  9. 1 2 Marchandin H, Teyssier C, Campos J, Jean-Pierre H, Roger F, Gay B, Carlier JP, Jumas-Bilak E (June 2010). "Negativicoccus succinicivorans gen. nov., sp. nov., isolated from human clinical samples, emended description of the family Veillonellaceae and description of Negativicutes classis nov., Selenomonadales ord. nov. and Acidaminococcaceae fam. nov. in the bacterial phylum Firmicutes". Int. J. Syst. Evol. Microbiol. 60 (Pt 6): 1271–9. doi: 10.1099/ijs.0.013102-0 . PMID   19667386.
  10. ↑ Gibbons, N. E.; Murray, R. G. E. (1978). "Proposals Concerning the Higher Taxa of Bacteria". International Journal of Systematic Bacteriology. 28 (1): 1–6. doi: 10.1099/00207713-28-1-1 .
  11. ↑ Woese CR (June 1987). "Bacterial evolution". Microbiol. Rev. 51 (2): 221–71. doi:10.1128/MMBR.51.2.221-271.1987. PMC   373105 . PMID   2439888.
  12. ↑ Cavalier-Smith, T. (2006). "Rooting the tree of life by transition analyses". Biol. Direct. 1: 19. doi: 10.1186/1745-6150-1-19 . PMC   1586193 . PMID   16834776.
  13. 1 2 3 4 Gupta, RS (December 1998). "Protein phylogenies and signature sequences: A reappraisal of evolutionary relationships among archaebacteria, eubacteria, and eukaryotes". Microbiol. Mol. Biol. Rev. 62 (4): 1435–91. doi:10.1128/MMBR.62.4.1435-1491.1998. PMC   98952 . PMID   9841678.
  14. 1 2 Gupta RS (2000). "The natural evolutionary relationships among prokaryotes" (PDF). Crit. Rev. Microbiol. 26 (2): 111–31. CiteSeerX   10.1.1.496.1356 . doi:10.1080/10408410091154219. PMID   10890353. S2CID   30541897. Archived (PDF) from the original on 2018-07-20. Retrieved 2017-10-24.
  15. 1 2 Gupta RS (August 1998). "What are archaebacteria: life's third domain or monoderm prokaryotes related to gram-positive bacteria? A new proposal for the classification of prokaryotic organisms". Mol. Microbiol. 29 (3): 695–707. doi: 10.1046/j.1365-2958.1998.00978.x . PMID   9723910.
  16. ↑ Choi, JK; Poudel, S; Yee, N; Goff, JL (3 October 2024). "Deeply branching Bacillota species exhibit atypical Gram-negative staining". Microbiology Spectrum. 12 (10): e0073224. doi:10.1128/spectrum.00732-24. PMC   11448272 . PMID   39162559.
  17. ↑ Castenholz, Richard W. (2015), "General Characteristics of the Cyanobacteria", Bergey's Manual of Systematics of Archaea and Bacteria, John Wiley & Sons, Ltd, pp. 1–23, doi:10.1002/9781118960608.cbm00019, ISBN   978-1-118-96060-8 , retrieved 2025-01-25
  18. ↑ Cole, John R. (1990-01-01), Carter, G. R.; Cole, John R. (eds.), "5 - Spirochetes", Diagnostic Procedure in Veterinary Bacteriology and Mycology (Fifth Edition), San Diego: Academic Press, pp. 41–60, doi:10.1016/b978-0-12-161775-2.50009-8, ISBN   978-0-12-161775-2 , retrieved 2025-01-25
  19. ↑ Schmidt, Thomas M. (2019). Encyclopedia of Microbiology (4th ed.). San Diego: Elsevier Science & Technology. pp. 527–537. ISBN   978-0-12-811737-8.
  20. ↑ Yeshanew, Addisu Gize; Geremew, Rozina Ambachew (2018-07-17). "MNeisseria Gonorrhoae and their antimicrobial susceptibility patterns among symptomatic patients from Gondar town, north West Ethiopia". Antimicrobial Resistance and Infection Control. 7 (85): 85. doi: 10.1186/s13756-018-0376-3 . PMC   6050735 . PMID   30026943.
  21. ↑ "Vaccine Preventable Diseases Surveillance Manual". CDC. April 3, 2018. Retrieved January 26, 2024.
  22. ↑ Verduin, Cees M.; Hol, Cees; Fleer, Andre; van Dijk, Hans; van Belkum, Alex (January 2002). "Moraxella catarrhalis: from Emerging to Established Pathogen". Clinical Microbiology Reviews. 15 (1): 125–144. doi:10.1128/CMR.15.1.125-144.2002. PMC   118065 . PMID   11781271.
  23. ↑ "For Clinicians: Haemophilus influenzae". CDC. February 13, 2018. Retrieved January 26, 2024.
  24. 1 2 3 Johnston C, Martin B, Fichant G, Polard P, Claverys JP (2014). "Bacterial transformation: distribution, shared mechanisms and divergent control". Nat. Rev. Microbiol. 12 (3): 181–96. doi:10.1038/nrmicro3199. PMID   24509783. S2CID   23559881.
  25. ↑ Korotetskiy I, Shilov S, Kuznetsova T, Kerimzhanova B, Korotetskaya N, Ivanova L, Zubenko N, Parenova R, Reva O (2023). "Analysis of Whole-Genome Sequences of Pathogenic Gram-Positive and Gram-Negative Isolates from the Same Hospital Environment to Investigate Common Evolutionary Trends Associated with Horizontal Gene Exchange, Mutations and DNA Methylation Patterning". Microorganisms. 11 (2): 323. doi: 10.3390/microorganisms11020323 . PMC   9961978 . PMID   36838287.
  26. 1 2 Seitz P, Blokesch M (2013). "Cues and regulatory pathways involved in natural competence and transformation in pathogenic and environmental Gram-negative bacteria". FEMS Microbiol. Rev. 37 (3): 336–63. doi: 10.1111/j.1574-6976.2012.00353.x . PMID   22928673.
  27. ↑ Glück, Thomas (12 December 2003). "Gram-Negative Bacteria and Broad-Spectrum Antibiotics: Good News Except for Fluoroquinolones". www.jwatch.org. NEJM Journal Watch. Archived from the original on 10 March 2018. Retrieved 10 March 2018.
  28. ↑ "Preferred Usage - Emerging Infectious Disease journal - CDC". CDC.gov. Centers for Disease Control and Prevention. Archived from the original on 2018-01-29. Retrieved 2018-03-04.
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