Bacterial Capsule and slime layers
Some bacterial cells are surrounded by a viscous substance or extracellular polymeric substances known as capsules (discrete) or slime layers (amorphous).
It is also called glycocalyx which means sugar coat, general term used for substances that surround cell.
Bacterial glycocalyx is a viscous (sticky), gelatinous polymer that is external to the cell wall and composed of polysaccharides, polypeptide or both. Its chemical composition varies widely with the species.
If the substance is organized and is firmly attached to the cell wall, glycocalyx is known as a capsule while unorganized and are loosely attached to the cell wall, it is described as a slime layer.
Slime layer might be present in coagulase negative Staphylococci e.g. S. epidermidis, S. saprophyticus. They helps to attach to plastics. So, in surgical works, it may attach to surgical plastics and may cause disease in surgical operation.
They are similar to capsule but are not tightly bound to the cell. These layers may protect the cell against dehydration and loss of nutrients.
Capsules (Also known as K antigen)
Capsule is not essential for survival of bacteria, as number of bacteria do not have it. They are important in contributing to bacterial virulence. (Degree to which a pathogen can cause disease). They often protect pathogenic bacteria from phagocytosis by the cells of the host. [Phagocytosis is the ingestion and digestion of microorganisms and other solid particles]. Besides these there are various functions of capsule.
Capsules divided in two types
- Macro-capsule: (have thickness of 0.2µm) and visible under light (compound) microscope.
- Microcapsule: Can be visible under electron microscope and has immunological significance. Capsules which are much narrower than the true capsule & cannot be demonstrated by light microscope is called microcapsule.
Eg; N.meningitides, S. pyogens, H.influenzae are capsulated bacteria.
Composition of capsules
- Polysaccharide and
Capsule can be made up of polysaccharide or glycoprotein or a polypeptide or both. The polysaccharide may be of single type sugar (homo- polysaccharide or several types of sugar (hetero-polysaccharide)
Homopolysaccharide: Made up of only one type of sugar
Eg. In acetic acid bacteria the capsule consists of cellulose which contains glucose as the basic unit. In Leuconostoc, the capsule is homopolymer consisting of either only glucose or fructose.
Heteropolysaccharide: Made up of several types of sugar
The capsule of the Klebsiella and pneumococci etc. are hetero- polysaccharide consisting of variety of sugars such as glucose, galactose, rhamnose etc and sugar derivatives . E.g. P. aeuriginosa– has D-glucose, D-glactose, D-gluconic acid, D-mannose and D-rhamnose. Pneumococci has capsule made u of Hexose, Uronic acid, Aminosugar.
Polypeptide: The capsule of Bacillus anthracis consist of polypeptide. In B. anthracis, the capsule is composed of polymer of D-glutamic acid while streptococci has L-amino acid.
Capsulated bacteria produces smooth and mucoid colonies whereas these bacteria which do not from capsules or slime form rough colonies. This is because the amount of water in capsule is high i.e, 98% and solid material is only 2%, so it gives mucoid colony.
Some gram negative bacteria may not produce a capsule but they have smooth colonies; it is due to the presence of a lipopolysaccharide layer (Salmonella spp).
The size of the capsule varies with the species and among the strains of the species.Size of the capsule depends on
- Cultural conditions
- High degree of aeration
- High carbon to nitrogen
- State of organisms
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