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 are found in procaryotic cells. Not all structures are found in every genus. Also, 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
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
· S- layers are very common among Archaea bacteria, 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.
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, pH fluctuations, osmotic stress, enzymes, or the predacious bacterium Bdellovibrio
- protection against bacterial viruses and detergents
- protect some pathogens against complement attack and phagocytosis, thus contributing to their virulence
