Chemistry:Colanic acid

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Short description: Chemical compound
Colanic acid
Identifiers
3D model (JSmol)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Colanic acid is an exopolysaccharide synthesized by bacteria in the Enterobacteriaceae family. It is excreted by the cell to form a protective bacterial capsule, and it assists in the formation of biofilms.

Structure

Colanic acid is composed of polyanionic heteropolysaccharides with hexasaccharide repeating units, consisting of glucose, fucose, galactose, and glucuronic acid.[1][2] It also consists of O-acetyl groups and pyruvate side chains attached to these sugar molecules.[3] It forms a protective capsule around cells, primarily Enterobacteriaceae.[4] Colanic acid's high molecular weight and branching structure contribute to its high viscosity, while the carboxylic acid groups in its structure are the primary contributors to its acidity. It is considered mildly toxic when injected intraperitoneally in mice, and its effect on mammals can be compared to the effects of low doses of endotoxin,[2] which can cause diarrhea and malaise.

E. coli colonies that produce colanic acid are said to be colicinogenic, and appear larger, smoother, and more opaque than those that do not. The colanic acid itself is observed as amorphous, white, and fibrous and is water-soluble as well as soluble in dilute salt solutions.[2]

Function

The main function of colanic acid is to form a protective slimy capsule around the cell surface under stressful conditions to increase its chances of survival.[5] The stressful environment can come in the forms of desiccation, oxidative stress, and a low pH. Expression of colanic acid in E. coli has been shown to be required for the creation of normal E. coli biofilm architecture.

Colanic acid synthesis is up-regulated in biofilms, where acetylation plays a crucial role in modulating its structural conformation and physical and chemical properties. In E. coli, colanic acid plays an essential role in biofilm formation. However, it does not enhance bacterial adhesion, but instead blocks the establishment of specific binding between bacteria and the underlying substrate.[5]

Environmental factors

Temperature and pH

Colanic acid begins to be synthesized and accumulate at 19 °C. Nutrients modulate the production of colanic acid with maximal production occurring when glucose and proline are used as carbon and nitrogen sources. E. coli, a member of the Enterobacteriaceae family, is commonly used to study the conditions and effects of colanic acid production. A study showed that E. coli K92 is able to produce colanic acid at temperatures ranging from 19 °C to 42 °C, but it predominates at around 20 °C.[6]

Colanic acid is typically produced at a low pH to protect bacteria from the acidic environment. A study was conducted to determine the minimal pH that E.coli could withstand. It was concluded that the production of colanic acid can range from a pH of 2 to a pH of 8; with the initial response to acidity occurring at a pH of 5.5.[7]

Colanic acid production in E. coli is dependent on both lipopolysaccharide structure and glucose availability, because important nucleotide-sugar precursors are needed and provided by both.[8]

Activation and regulation

Activation

At least two positive protein regulators, RcsA[9] and RcsB,[10] are involved in the transcription of the operon for capsule (cps) gene expression in E. coli. The activation of colanic acid is due to an initial response to an environmental stimulus such as osmotic shock. This stimulus is relayed to the MdoH gene[11] which is tied to the biosynthesis of MDOs. Unstable MDO levels due to changes within the environment, triggers the RcsC[12] sensor to directly or indirectly relay the signal to the RcsB gene, which is a main activator of cps expression.[13] The RcsA gene activates its own expression.[13]

Regulation

The cps colanic acid operon can control the biosynthesis of colanic acid.[14] It is composed of one large transcriptional unit that contains a ugd gene right outside the cps operon. It has been shown that the transcriptional antiterminator rfaH promotes said cps transcription. It does so by mediating the cps operon and promoting ugd expression.[15]

A study was conducted to test whether RfaH was able to enhance cps colanic acid transcription for colanic acid production. E. coli K92 wild-type and rfaH mutant strains were grown and analyzed. It was observed that the deletion of rfaH had dramatically decreased colanic acid production in both.[15]

References

  1. ↑ "Chemical Synthesis of a Colanic Acid Hexasaccharide". Organic Letters 24 (42): 7779–7783. October 2022. doi:10.1021/acs.orglett.2c03116. PMID 36240128. 
  2. ↑ 2.0 2.1 2.2 "Colanic acid". Proceedings of the National Academy of Sciences of the United States of America 49 (4): 464–471. April 1963. doi:10.1073/pnas.49.4.464. PMID 13963285. Bibcode: 1963PNAS...49..464G. 
  3. ↑ "Pathway: colanic acid building blocks biosynthesis". https://biocyc.org/ECOLI/NEW-IMAGE?type=PATHWAY&object=COLANSYN-PWY. "Escherichia coli K-12 substr. MG1655" 
  4. ↑ (in en) Biotechnology of Microbial Exopolysaccharides. Cambridge University Press. 1990-10-18. ISBN 978-0-521-36350-1. https://books.google.com/books?id=WV1ngzp7e0UC&dq=Composed+of+polyanionic+heteropolysaccharides+with+hexasaccharide+repeating+units&pg=PP9. 
  5. ↑ 5.0 5.1 "Role of capsular colanic acid in adhesion of uropathogenic Escherichia coli". Applied and Environmental Microbiology 69 (8): 4474–4481. August 2003. doi:10.1128/AEM.69.8.4474-4481.2003. PMID 12902231. Bibcode: 2003ApEnM..69.4474H. 
  6. ↑ "Temperature has reciprocal effects on colanic acid and polysialic acid biosynthesis in E. coli K92". Applied Microbiology and Biotechnology 82 (4): 721–729. March 2009. doi:10.1007/s00253-008-1840-4. PMID 19139876. 
  7. ↑ "Role of colanic acid exopolysaccharide in the survival of enterohaemorrhagic Escherichia coli O157:H7 in simulated gastrointestinal fluids". Letters in Applied Microbiology 42 (6): 642–647. June 2006. doi:10.1111/j.1472-765X.2006.01875.x. PMID 16706906. 
  8. ↑ "Colanic acid biosynthesis in Escherichia coli is dependent on lipopolysaccharide structure and glucose availability". Microbiological Research 239: 126527. October 2020. doi:10.1016/j.micres.2020.126527. PMID 32590169. 
  9. ↑ Ebel, W.; Trempy, J. E. (January 1999). "Escherichia coli RcsA, a positive activator of colanic acid capsular polysaccharide synthesis, functions To activate its own expression". Journal of Bacteriology 181 (2): 577–584. doi:10.1128/JB.181.2.577-584.1999. PMID 9882673. 
  10. ↑ Gervais, F. G.; Phoenix, P.; Drapeau, G. R. (June 1992). "The rcsB gene, a positive regulator of colanic acid biosynthesis in Escherichia coli, is also an activator of ftsZ expression". Journal of Bacteriology 174 (12): 3964–3971. doi:10.1128/jb.174.12.3964-3971.1992. PMID 1597415. 
  11. ↑ Ebel, W.; Vaughn, G. J.; Peters, H. K.; Trempy, J. E. (November 1997). "Inactivation of mdoH leads to increased expression of colanic acid capsular polysaccharide in Escherichia coli". Journal of Bacteriology 179 (21): 6858–6861. doi:10.1128/jb.179.21.6858-6861.1997. ISSN 0021-9193. PMID 9352941. 
  12. ↑ Ferrières, Lionel; Clarke, David J. (December 2003). "The RcsC sensor kinase is required for normal biofilm formation in Escherichia coli K-12 and controls the expression of a regulon in response to growth on a solid surface". Molecular Microbiology 50 (5): 1665–1682. doi:10.1046/j.1365-2958.2003.03815.x. PMID 14651646. https://pubmed.ncbi.nlm.nih.gov/14651646/. Retrieved 4 September 2023. 
  13. ↑ 13.0 13.1 "Escherichia coli RcsA, a positive activator of colanic acid capsular polysaccharide synthesis, functions To activate its own expression". Journal of Bacteriology 181 (2): 577–584. January 1999. doi:10.1128/JB.181.2.577-584.1999. PMID 9882673. 
  14. ↑ Stout, V (July 1996). "Identification of the promoter region for the colanic acid polysaccharide biosynthetic genes in Escherichia coli K-12.". Journal of Bacteriology 178 (14): 4273–4280. doi:10.1128/jb.178.14.4273-4280.1996. PMID 8763957. 
  15. ↑ 15.0 15.1 "Transcriptional control of RfaH on polysialic and colanic acid synthesis by Escherichia coli K92". FEBS Letters 588 (6): 922–928. March 2014. doi:10.1016/j.febslet.2014.01.047. PMID 24491998. 



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