Ultrastructure of bacteria- External structures-glycocalyx & capsule

BACTERIAL CELL STRUCTURE

Procaryotic cells are in general smaller, grow extremely rapidly and lack the complex vesicular transport systems in comparison to eucaryotic cells. A variety of structures is found in procaryotic cells. Not all structures are found in every genus. Furthermore, gram- negative and gram-positive cells differ, particularly with respect to their cell walls. Despite these variations procaryotes are consistent in their fundamental structure and most important components.

Procaryotic cells are bounded by a chemically complex cell wall. Inside this wall, and separated from it by a periplasmic space is the plasma membrane. The genetic material is localized in a discrete region, the nucleoid and is not separated from the surrounding cytoplasm by membranes. Ribosomes and larger masses called inclusion bodies are scattered about in the cytoplasmic matrix. Both gram-positive and gram-negative cells can use flagella for locomotion. In addition, many cells are surrounded by a capsule or slime layer external to the cell wall.

Structure	Functions
Plasma membrane	Selectively permeable barrier, mechanical boundary of cell, nutrient and waste transport, location of many metabolic processes  (respiration, photosynthesis), detection of environmental cues for chemotaxis
Gas vacuole	Buoyancy for floating in aquatic environments
Ribosomes	Protein synthesis
Inclusion bodies	Storage of carbon, phosphate, and other substances
Nucleoid	Localization of genetic material (DNA)
Periplasmic space	Contains hydrolytic enzymes and binding proteins for nutrient processing and uptake
Cell wall	Gives bacteria shape and protection from lysis in dilute solutions
Capsules and slime layers	Resistance to phagocytosis, adherence to surfaces
Fimbriae and pili	Attachment to surfaces, bacterial mating
Flagella	Movement
Endospore	Survival under harsh environmental conditions

 

External cell structures

Glycocalyx

·         All polysaccharide containing substances found external to cell wall

·         Thickest capsule to thinnest slime layer

Capsules

·         Protective structure secreted by the organism; seen outside the cell wall

·         Capsules -clearly visible in the light microscope using negative stains or special capsule stains; also with the electron microscope

·         Well organized & not easily washed off

·         Chemical composition unique to the organism

·    Usually composed of polysaccharides; may be constructed of other materials. Eg., Bacillus anthracis - poly- D-glutamic acid capsule

·  Pathogens usually are capsulated; prevents phagocytosis by host phagocytic cells eg., Streptococcus pneumoniae -Without a capsule, more vulnerable to destruction & less likely to cause disease, whereas the capsulated variant is pathogenic

·         Capsules contain a great deal of water; protect bacteria against desiccation

·         Protection against bacterial viruses and hydrophobic toxic materials such as detergents also

Slime Layers

·         A slime layer is a zone of diffuse, unorganized material that is removed easily.

·         Capsules and slime layers usually are composed of polysaccharides

S-Layers

·         Many gram-positive and gram-negative bacteria have a regularly structured layer called an S-layer on their surface- S layer has a pattern like floor tiles and is composed of protein or glycoprotein

·         S- layers also are very common among Archaea, where they may be the only wall structure outside the plasma membrane.

·    In gram-negative bacteria the S-layer adheres directly to the outer membrane whereas it is associated with the peptidoglycan surface in gram-positive bacteria.

·     It may protect the cell against  pH fluctuations, osmotic stress, enzymes, or the predacious bacterium Bdellovibrio

·         The S-layer also helps maintain the shape and envelope rigidity of at least some bacterial cells.

In general, glycocalyx, capsules, slime layers, s-layers

• helps maintain the shape and envelope rigidity of at least some bacterial cells
• aids bacterial attachment to surfaces of solid objects in aquatic environments or to tissue surfaces in plant and animal hosts
• protect bacteria against desiccation
• protect the cell against pH fluctuations, osmotic stress, enzymes, or the predacious bacterium Bdellovibrio
• protection against bacterial viruses and hydrophobic toxic materials such as detergents
• protect some pathogens against complement attack and phagocytosis, thus contributing to their virulence