Bacterial capsule

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The outer red layer in this diagram is the capsule, which is distinct from the cell envelope. This bacterium is gram-positive, as its cell envelope comprises a single cell membrane (orange) and a thick peptidoglycan-containing cell wall (purple). Prokaryote cell.svg
The outer red layer in this diagram is the capsule, which is distinct from the cell envelope. This bacterium is gram-positive, as its cell envelope comprises a single cell membrane (orange) and a thick peptidoglycan-containing cell wall (purple).

The bacterial capsule is a large structure common to many bacteria. [1] It is a polysaccharide layer that lies outside the cell envelope, and is thus deemed part of the outer envelope of a bacterial cell. It is a well-organized layer, not easily washed off, and it can be the cause of various diseases. [2] [3]

Contents

The capsule—which can be found in both gram negative and gram-positive bacteria—is different from the second lipid membrane – bacterial outer membrane, which contains lipopolysaccharides and lipoproteins and is found only in gram-negative bacteria. When the amorphous viscid secretion (that makes up the capsule) diffuses into the surrounding medium and remains as a loose undemarcated secretion, it is known as a slime layer. Capsule and slime layer are sometimes summarized under the term glycocalyx.

A bacterial capsule has a semi-rigid border that follows the contour of the cell. The capsule excludes India Ink when dyed. A slime layer is a non-rigid matrix that is easily deformed and is not able to exclude India Ink. Biofilms are composed of many cells and their outer barriers. The primary functions of both capsules and slime layers are for protection and adhesion. Bacteria Capsules and Slime Layers.jpg
A bacterial capsule has a semi-rigid border that follows the contour of the cell. The capsule excludes India Ink when dyed. A slime layer is a non-rigid matrix that is easily deformed and is not able to exclude India Ink. Biofilms are composed of many cells and their outer barriers. The primary functions of both capsules and slime layers are for protection and adhesion.

Composition

Most bacterial capsules are composed of polysaccharide, [4] but some species use other materials, such as poly-D-glutamic acid in Bacillus anthracis . Because most capsules are so tightly packed, they are difficult to stain because most standard stains cannot penetrate the capsule. To visualize encapsulated bacteria using a microscope, a sample is treated with a dark stain, such as India ink. The structure of the capsule prevents the stain from penetrating the cell. When viewed, bacterial capsules appear as a bright halo around the cell on a dark background. [5]

Function

The bacterial capsule serves as a shield, giving protection from toxins, and from drying out. Capsules allow adhesion to surfaces and help enable the bacteria to evade the host immune system. [6] The water content in the capsule gives the protection against drying out. The capsule is considered a virulence factor because it enhances the ability of bacteria to cause disease (e.g. prevents phagocytosis). The capsule can protect cells from engulfment by eukaryotic cells, such as macrophages. [7] A capsule-specific antibody may be required for phagocytosis to occur. They also exclude bacterial viruses and most hydrophobic toxic materials such as detergents.[ citation needed ] Immunity to one capsule type does not result in immunity to the other types. Capsules also help cells adhere to surfaces. As a group where the capsule is present they are known as polysaccharide encapsulated bacteria or encapsulated bacteria. [8]

Diversity

The capsule is found most commonly among gram-negative bacteria:

However, some gram-positive bacteria may also have a capsule:

The yeast Cryptococcus neoformans , [23] though not a bacterium, has a similar capsule. [24] [25]

Capsules too small to be seen with an ordinary microscope, such as the M protein of Streptococcus pyogenes , are called microcapsules.

Demonstration of capsule

  1. India ink staining: the capsule appears as a clear halo around the bacterium as the ink can't penetrate the capsule. [26] : 87 
  2. Maneval's capsule stain: the capsule appears as a clear halo between the pink-stained bacterium and the bluish-grey stained background. The background stain is the acidic stain Congo red (which changes color to bluish-grey due to the pH), and the pink stain is fuchsine.
  3. Serological methods: Capsular material is antigenic and can be demonstrated by mixing it with a specific anticapsular serum. When examined under the microscope, the capsule appears 'swollen' due to an increase in its refractivity. This phenomenon is the basis of quellung reaction.

Use in vaccination

Vaccination using capsular material is effective against some organisms (e.g., H. influenzae type b, [27] [28] S. pneumoniae , and N. meningitidis [29] ). However, polysaccharides are not highly antigenic, especially in children, so many capsular vaccines contain polysaccharides conjugated with protein carriers, such as the tetanus toxoid or diphtheria toxoid. This stimulates a much more robust immune response. [30]

See also

Related Research Articles

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Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall sandwiched between an inner membrane (cytoplasmic), and an outer membrane.

<i>Neisseria</i> Genus of bacteria

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<i>Streptococcus pneumoniae</i> Species of bacterium

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<i>Cryptococcus neoformans</i> Species of yeast

Cryptococcus neoformans is an encapsulated yeast belonging to the class Tremellomycetes and an obligate aerobe that can live in both plants and animals. Its teleomorph is a filamentous fungus, formerly referred to Filobasidiella neoformans. In its yeast state, it is often found in bird excrement. Cryptococcus neoformans can cause disease in apparently immunocompetent, as well as immunocompromised, hosts.

<i>Haemophilus influenzae</i> Species of bacterium

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<i>Klebsiella</i> Genus of gram-negative bacteria

Klebsiella is a genus of Gram-negative, oxidase-negative, rod-shaped bacteria with a prominent polysaccharide-based capsule.

An overwhelming post-splenectomy infection (OPSI) is a rare but rapidly fatal infection occurring in individuals following removal of the spleen. The infections are typically characterized by either meningitis or sepsis, and are caused by encapsulated organisms including Streptococcus pneumoniae. It is a medical emergency and requires immediate treatment. Death has been reported to occur within 12 hours.

<span class="mw-page-title-main">Bacterial pneumonia</span> Disease of the lungs

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<span class="mw-page-title-main">Conjugate vaccine</span> Type of vaccine

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<i>Neisseria meningitidis</i> Species of bacterium that can cause meningitis

Neisseria meningitidis, often referred to as the meningococcus, is a Gram-negative bacterium that can cause meningitis and other forms of meningococcal disease such as meningococcemia, a life-threatening sepsis. The bacterium is referred to as a coccus because it is round, and more specifically a diplococcus because of its tendency to form pairs.

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<span class="mw-page-title-main">Quellung reaction</span> Reaction in which antibodies bind to bacterial capsule

The quellung reaction, also called the Neufeld reaction, is a biochemical reaction in which antibodies bind to the bacterial capsule of Streptococcus pneumoniae, Klebsiella pneumoniae, Neisseria meningitidis, Bacillus anthracis, Haemophilus influenzae, Escherichia coli, and Salmonella. The antibody reaction allows these species to be visualized under a microscope. If the reaction is positive, the capsule becomes opaque and appears to enlarge.

<span class="mw-page-title-main">Cefodizime</span> Chemical compound

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IgA protease is an enzyme. This enzyme catalyses the following chemical reaction[reaction equation needed]

References

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  2. ↑ Gao S, Lewis GD, Ashokkumar M, Hemar Y (January 2014). "Inactivation of microorganisms by low-frequency high-power ultrasound: 1. Effect of growth phase and capsule properties of the bacteria". Ultrasonics Sonochemistry. 21 (1): 446–53. doi:10.1016/j.ultsonch.2013.06.006. PMID   23835398. S2CID   24149924.
  3. ↑ Hathaway LJ, Grandgirard D, Valente LG, Täuber MG, Leib SL (March 2016). "Streptococcus pneumoniae capsule determines disease severity in experimental pneumococcal meningitis". Open Biology. 6 (3): 150269. doi:10.1098/rsob.150269. PMC   4821241 . PMID   27009189.
  4. ↑ " bacterial capsule " at Dorland's Medical Dictionary
  5. ↑ "Basteria: Capsules and Slime Layers". Encyclopædia Britannica. Archived from the original on 2013-03-08.
  6. ↑ Santos-López A, Rodríguez-Beltrán J, San Millán Á (July 2021). "The bacterial capsule is a gatekeeper for mobile DNA". PLOS Biol. 19 (7): e3001308. doi: 10.1371/journal.pbio.3001308 . PMC   8260180 . PMID   34228713.
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  13. ↑ Dorman MJ, Feltwell T, Goulding DA, Parkhill J, Short FL (November 2018). "Klebsiella pneumoniae Defined by density-TraDISort". mBio. 9 (6). doi:10.1128/mBio.01863-18. PMC   6247091 . PMID   30459193.
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  15. ↑ Schouls L, van der Heide H, Witteveen S, Zomer B, van der Ende A, Burger M, Schot C (February 2008). "Two variants among Haemophilus influenzae serotype b strains with distinct bcs4, hcsA and hcsB genes display differences in expression of the polysaccharide capsule". BMC Microbiology. 8 (1): 35. doi: 10.1186/1471-2180-8-35 . PMC   2267795 . PMID   18298818.
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  17. ↑ Gibson DL, White AP, Snyder SD, Martin S, Heiss C, Azadi P, et al. (November 2006). "Salmonella produces an O-antigen capsule regulated by AgfD and important for environmental persistence". Journal of Bacteriology. 188 (22): 7722–30. doi:10.1128/JB.00809-06. PMC   1636306 . PMID   17079680.
  18. ↑ Kenyon, Johanna J.; Hall, Ruth M. (2013-04-16). de Crécy-Lagard, Valerie (ed.). "Variation in the Complex Carbohydrate Biosynthesis Loci of Acinetobacter baumannii Genomes". PLOS ONE. 8 (4): e62160. Bibcode:2013PLoSO...862160K. doi: 10.1371/journal.pone.0062160 . ISSN   1932-6203. PMC   3628348 . PMID   23614028.
  19. ↑ Singh, Jennifer K.; Adams, Felise G.; Brown, Melissa H. (2019-01-09). "Diversity and Function of Capsular Polysaccharide in Acinetobacter baumannii". Frontiers in Microbiology. 9: 3301. doi: 10.3389/fmicb.2018.03301 . ISSN   1664-302X. PMC   6333632 . PMID   30687280.
  20. ↑ Hamaguchi S, Zafar MA, Cammer M, Weiser JN (March 2018). "Capsule Prolongs Survival of Streptococcus pneumoniae during Starvation". Infection and Immunity. 86 (3). doi:10.1128/IAI.00802-17. PMC   5820961 . PMID   29311231.
  21. ↑ Hyams C, Camberlein E, Cohen JM, Bax K, Brown JS (February 2010). "The Streptococcus pneumoniae capsule inhibits complement activity and neutrophil phagocytosis by multiple mechanisms". Infection and Immunity. 78 (2): 704–15. doi:10.1128/IAI.00881-09. PMC   2812187 . PMID   19948837.
  22. ↑ Rao, Shreesha; Chen, Mei‐Yun; Sudpraseart, Chiranan; Lin, Peiry; Yoshida, Terutoyo; Wang, Pei‐Chi; Chen, Shih‐Chu (2022). "Genotyping and phenotyping of Lactococcus garvieae isolates from fish by pulse‐field gel electrophoresis (PFGE) and electron microscopy indicate geographical and capsular variations". Journal of Fish Diseases. 45 (6): 771–781. doi:10.1111/jfd.13601. PMID   35235703. S2CID   247220475.
  23. ↑ O'Meara TR, Alspaugh JA (July 2012). "The Cryptococcus neoformans capsule: a sword and a shield". Clinical Microbiology Reviews. 25 (3): 387–408. doi:10.1128/CMR.00001-12. PMC   3416491 . PMID   22763631.
  24. ↑ Gates MA, Thorkildson P, Kozel TR (April 2004). "Molecular architecture of the Cryptococcus neoformans capsule". Molecular Microbiology. 52 (1): 13–24. doi: 10.1111/j.1365-2958.2003.03957.x . PMID   15049807.
  25. ↑ Casadevall A, Coelho C, Cordero RJ, Dragotakes Q, Jung E, Vij R, Wear MP (December 2019). "Cryptococcus neoformans". Virulence. 10 (1): 822–831. doi:10.1080/21505594.2018.1431087. PMC   6779390 . PMID   29436899.
  26. ↑ Rudolph K (1996). "Chapter 3: Pseudomonas synringae pathovars". In Singh RP, Kohmoto K, Singh US (eds.). Pathogenesis & Host Specificity in Plant Diseases. Vol. 1: Prokaryotes (1st ed.). Amsterdam: Elsevier Science. ISBN   978-0-08-098473-5.
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  30. ↑ Goldblatt D (January 2000). "Conjugate vaccines". Clinical and Experimental Immunology. 119 (1): 1–3. doi:10.1046/j.1365-2249.2000.01109.x. PMC   1905528 . PMID   10671089.
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