Biological Sciences
Capsules
Capsules are small, membrane-bound structures found in cells that contain various substances such as nutrients, enzymes, or genetic material. They play a role in protecting and transporting these substances within the cell. In bacteria, capsules are also involved in protecting the cell from the host's immune system and in forming biofilms.
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eBook - ePubMicrobial Glycobiology
Structures, Relevance and Applications
- Anthony P Moran, Otto Holst, Patrick Brennan, Mark von Itzstein(Authors)
- 2009(Publication Date)
- Academic Press(Publisher)
Chapter 6. Bacterial capsular polysaccharides and exopolysaccharidesPaola Cescutti
Summary
Bacteria often produce an external layer of polysaccharides, characterized by a definite primary structure, which in turn is responsible for sometimes remarkable physicochemical properties. Although the number of monosaccharides which constitute the polymers is rather low, the great number of different polysaccharides defined up to now shows the capacity of the microbes to exploit the possible isomers and linkage types of the building blocks. Furthermore, variability is often introduced by the presence of non-carbohydrate groups linked to hydroxyl, carboxyl or amine functions. In this chapter, examples of polysaccharides produced by Gram-negative and Gram-positive pathogenic bacteria are given, together with a description of those polymers that are interesting for industrial and biotechnological purposes. Some discussion is also devoted to the general features of shapes that polysaccharides may adopt in solution. The biological functions of these biomolecules are discussed particularly in relation to their role in human infection processes. The structures of the polysaccharides produced by species of the Burkholderia cepacia complex is reported as an example of a current investigation devoted to the understanding of the role of these biopolymers in lung infections.Keywords: Capsular polysaccharides; Exopolysaccharides; Primary structure; Biological properties; Burkholderia cepacia complex1. Introduction
Bacterial cells are often surrounded by a polysaccharidic layer which constitutes the interface with the environment. Sometimes this structure is referred to as the “glycocalyx”. Bacterial polysaccharides are classified into two different types: capsular polysaccharides (K-antigens) (CPS) and exopolysaccharides (EPS). Thus, CPS are defined as polymers linked to the cell surface via covalent bond to phospholipid or lipid A molecules, while EPS appear to be released on the cell surface with no attachment to the cell and they are often sloughed off to form slime. However, there is no clear cut definition for CPS and EPS, because the former can also be released into the growth medium and the latter may be closely associated with the cell surface (Taylor and Roberts, 2005 ). Despite this ambiguity, these are the most common definitions found in the literature and they will be used as such in this context. It is worth mentioning thatCosterton et al
eBook - PDF- W. Russell, H. Herwald, A. Schmidt, H. Herwald(Authors)
- 2004(Publication Date)
- S. Karger(Publisher)
Functions of Bacterial Capsules As the polysaccharide capsule represents the outermost layer of the bacte-rial cell, it is not surprising that the capsule mediates interactions between the bacterium and its immediate environment. Accordingly, a number of functions has been ascribed to bacterial Capsules. Each of these functions (resistance to desiccation, adherence, resistance to nonspecific host immunity, resistance to specific host immunity) is directly relevant to pathogenicity and as such con-tributes to the role of CPS as a virulence factor. Resistance to Desiccation As CPS are highly hydrated molecules that surround the cell surface, they may protect bacteria from the harmful effects of desiccation [7]. This property is probably most relevant in the transmission and survival of encapsulated bacteria in the environment demonstrated in the cases of isolates of E. coli , Acinetobacter calcoaceticus and Erwinia stewartii , which have been shown to be more resis-tant to desiccation than their isogenic acapsular mutants [8]. Furthermore, the capsule probably provides protection during transmission from host to host. In the case of E. coli , genes encoding enzymes for the biosynthesis of capsular colanic acid have been shown to be upregulated in response to desiccation [8]. While the mechanism of regulation is unclear, it is thought that external osmo-larity is altered during desiccation, and it has been shown that expression of alginate EPS of Pseudomonas aeruginosa as well as expression of the Vi CPS of Salmonella typhi , which is essential for virulence, are increased in response to high osmolarity [9, 10]. Adherence CPS may mediate adhesion of bacteria to surfaces (both biotic and abiotic) and to each other. Adhesion to abiotic surfaces may result in the establishment of biofilms and EPS-mediated interspecies co-aggregation within biofilms can enhance colonization of various ecological niches [11]. In addition, growth of
eBook - PDF- Masao Goto(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
C H A P T E R T W O Morphology, Structure, and Composition The structure of the bacterial cell envelope is important for pathogenicity because host recognition is determined by the interactions between the host cell wall and extracellular polysaccharides or lipopolysaccharides of bacteria. In addition, the external as well as internal structures have major taxonomic value and provide essential information in the descrip-tion of bacterial taxa. 2.1 Morphology 2.1.1 SHAPE AND SIZE Eubacteria have three basic shapes, i.e., spherical, rod, and spiral, which are referred to as coccus (pi. cocci), bacillus (pi. bacilli), and spi-rillum (pi. spirilla), respectively. Most plant pathogenic bacteria are rod shaped and divide by binary fission. The size of bacterial cells varies, depending on various factors such as incubation temperature, culture medium, culture age, and the staining methods. Most plant pathogenic bacteria fall within the range of 1.0-5.0 x 0.5-1.0 μπι. Bacterial cells gradually become smaller when cultures or lesions are aged. Sometimes elongated filamentous cells may be formed, however, because cell divi-sion or the separation of divided cells is likely to be inhibited in aged cultures. Streptomyces is characterized by the formation of a highly branched 8 2.2 Structure of the Cell Envelope 9 Mesosome Reserve material / Chromosome / Cell membrane Periplasm Ribosome Capsule Fimbriae (Pili) Plasmid Fig. 2.1 Schematic structure of gram-negative bacteria. mycelium 0.5-2.0 μπ in width and the formation of chains of spores 0.5-2.0 /im in diameter at the tip of aerial hyphae. Mycoplasmalike organisms are pleomorphic and usually spherical or ovoid in shape, ranging in size from 0.3 to 2 μιη in diameter. Conspicu-ous morphological variations such as filamentous, spiral, or small spheri-cal bodies (60-100 nm) are observed in diseased plant tissue, depending on the stage of reproduction and environmental conditions.
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