The Bacterial Cell Surface
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Fimbriae usually function to facilitate the attachment of a bacterium to a surface e. A few organisms e. Myxococcus use fimbriae for motility to facilitate the assembly of multicellular structures such as fruiting bodies. Pili are similar in structure to fimbriae but are much longer and present on the bacterial cell in low numbers. Pili are involved in the process of bacterial conjugation where they are called conjugation pili or " sex pili ".
Type IV pili non-sex pili also aid bacteria in gripping surfaces. An S-layer surface layer is a cell surface protein layer found in many different bacteria and in some archaea , where it serves as the cell wall.
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All S-layers are made up of a two-dimensional array of proteins and have a crystalline appearance, the symmetry of which differs between species. The exact function of S-layers is unknown, but it has been suggested that they act as a partial permeability barrier for large substrates. For example, an S-layer could conceivably keep extracellular proteins near the cell membrane by preventing their diffusion away from the cell.
Monitoring Surface Chemical Changes in the Bacterial Cell Wall
In some pathogenic species, an S-layer may help to facilitate survival within the host by conferring protection against host defence mechanisms. Many bacteria secrete extracellular polymers outside of their cell walls called glycocalyx. These polymers are usually composed of polysaccharides and sometimes protein.
Capsules are relatively impermeable structures that cannot be stained with dyes such as India ink. They are structures that help protect bacteria from phagocytosis and desiccation. Slime layer is involved in attachment of bacteria to other cells or inanimate surfaces to form biofilms. Slime layers can also be used as a food reserve for the cell. Perhaps the most recognizable extracellular bacterial cell structures are flagella. Flagella are whip-like structures protruding from the bacterial cell wall and are responsible for bacterial motility i.
The arrangement of flagella about the bacterial cell is unique to the species observed. Common forms include:. The bacterial flagellum consists of three basic components: a whip-like filament, a motor complex, and a hook that connects them. The bundle is held together by a cap and may or may not be encapsulated. The motor complex consists of a series of rings anchoring the flagellum in the inner and outer membranes, followed by a proton-driven motor that drives rotational movement in the filament. In comparison to eukaryotes , the intracellular features of the bacterial cell are extremely simple.
Bacteria do not contain organelles in the same sense as eukaryotes. Instead, the chromosome and perhaps ribosomes are the only easily observable intracellular structures found in all bacteria.
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They do exist, however, specialized groups of bacteria that contain more complex intracellular structures, some of which are discussed below. Unlike eukaryotes , the bacterial DNA is not enclosed inside of a membrane-bound nucleus but instead resides inside the bacterial cytoplasm. This means that the transfer of cellular information through the processes of translation , transcription and DNA replication all occur within the same compartment and can interact with other cytoplasmic structures, most notably ribosomes.
The bacterial DNA is not packaged using histones to form chromatin as in eukaryotes but instead exists as a highly compact supercoiled structure, the precise nature of which remains unclear. Borrelia burgdorferi. Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmids that often encode for traits that are advantageous but not essential to their bacterial host. Plasmids can be easily gained or lost by a bacterium and can be transferred between bacteria as a form of horizontal gene transfer.
So plasmids can be described as an extra chromosomal DNA in a bacterial cell. In most bacteria the most numerous intracellular structure is the ribosome , the site of protein synthesis in all living organisms. The 70S ribosome is made up of a 50S and 30S subunits. These rRNA molecules differ in size in eukaryotes and are complexed with a large number of ribosomal proteins, the number and type of which can vary slightly between organisms.
While the ribosome is the most commonly observed intracellular multiprotein complex in bacteria other large complexes do occur and can sometimes be seen using microscopy. While not typical of all bacteria some microbes contain intracellular membranes in addition to or as extensions of their cytoplasmic membranes. An early idea was that bacteria might contain membrane folds termed mesosomes , but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy.
Intracellular membranes are also found in bacteria belonging to the poorly studied Planctomycetes group, although these membranes more closely resemble organellar membranes in eukaryotes and are currently of unknown function. Used primarily for photosynthesis, they contain bacteriochlorophyll pigments and carotenoids. The prokaryotic cytoskeleton is the collective name for all structural filaments in prokaryotes. It was once thought that prokaryotic cells did not possess cytoskeletons , but recent advances in visualization technology and structure determination have shown that filaments indeed exist in these cells.
Cytoskeletal elements play essential roles in cell division , protection, shape determination, and polarity determination in various prokaryotes. Most bacteria do not live in environments that contain large amounts of nutrients at all times. To accommodate these transient levels of nutrients bacteria contain several different methods of nutrient storage in times of plenty for use in times of want. For example, many bacteria store excess carbon in the form of polyhydroxyalkanoates or glycogen.
Some microbes store soluble nutrients such as nitrate in vacuoles. Sulfur is most often stored as elemental S 0 granules which can be deposited either intra- or extracellularly. Sulfur granules are especially common in bacteria that use hydrogen sulfide as an electron source.
The Bacterial Cell Wall
Most of the above-mentioned examples can be viewed using a microscope and are surrounded by a thin nonunit membrane to separate them from the cytoplasm. Inclusions are considered to be nonliving components of the cell that do not possess metabolic activity and are not bounded by membranes. The most common inclusions are glycogen, lipid droplets, crystals, and pigments. Volutin granules are cytoplasmic inclusions of complexed inorganic polyphosphate.
These granules are called metachromatic granules due to their displaying the metachromatic effect; they appear red or blue when stained with the blue dyes methylene blue or toluidine blue. Gas vacuoles are membrane-bound, spindle-shaped vesicles , found in some planktonic bacteria and Cyanobacteria , that provides buoyancy to these cells by decreasing their overall cell density. Positive buoyancy is needed to keep the cells in the upper reaches of the water column, so that they can continue to perform photosynthesis.
They are made up of a shell of protein that has a highly hydrophobic inner surface, making it impermeable to water and stopping water vapour from condensing inside but permeable to most gases. Because the gas vesicle is a hollow cylinder, it is liable to collapse when the surrounding pressure increases.
Natural selection has fine tuned the structure of the gas vesicle to maximise its resistance to buckling , including an external strengthening protein, GvpC, rather like the green thread in a braided hosepipe. There is a simple relationship between the diameter of the gas vesicle and pressure at which it will collapse — the wider the gas vesicle the weaker it becomes. However, wider gas vesicles are more efficient, providing more buoyancy per unit of protein than narrow gas vesicles.
Different species produce gas vesicle of different diameter, allowing them to colonise different depths of the water column fast growing, highly competitive species with wide gas vesicles in the top most layers; slow growing, dark-adapted, species with strong narrow gas vesicles in the deeper layers. The diameter of the gas vesicle will also help determine which species survive in different bodies of water.
Deep lakes that experience winter mixing expose the cells to the hydrostatic pressure generated by the full water column. This will select for species with narrower, stronger gas vesicles. The cell achieves its height in the water column by synthesising gas vesicles. As the cell rises up, it is able to increase its carbohydrate load through increased photosynthesis. Too high and the cell will suffer photobleaching and possible death, however, the carbohydrate produced during photosynthesis increases the cell's density, causing it to sink.
The daily cycle of carbohydrate build-up from photosynthesis and carbohydrate catabolism during dark hours is enough to fine-tune the cell's position in the water column, bring it up toward the surface when its carbohydrate levels are low and it needs to photosynthesis, and allowing it to sink away from the harmful UV radiation when the cell's carbohydrate levels have been replenished.
An extreme excess of carbohydrate causes a significant change in the internal pressure of the cell, which causes the gas vesicles to buckle and collapse and the cell to sink out. Bacterial microcompartments are widespread, membrane-bound organelles that are made of a protein shell that surrounds and encloses various enzymes. These "polyhedral organelles" localize and compartmentalize bacterial metabolism, a function performed by the membrane-bound organelles in eukaryotes.
Carboxysomes are bacterial microcompartments found in many autotrophic bacteria such as Cyanobacteria, Knallgasbacteria, Nitroso- and Nitrobacteria. It is thought that the high local concentration of the enzymes along with the fast conversion of bicarbonate to carbon dioxide by carbonic anhydrase allows faster and more efficient carbon dioxide fixation than possible inside the cytoplasm. Magnetosomes are bacterial microcompartments found in magnetotactic bacteria that allow them to sense and align themselves along a magnetic field magnetotaxis. The ecological role of magnetotaxis is unknown but is thought to be involved in the determination of optimal oxygen concentrations.
Magnetosomes are composed of the mineral magnetite or greigite and are surrounded by a lipid bilayer membrane. The morphology of magnetosomes is species-specific. Perhaps the best known bacterial adaptation to stress is the formation of endospores. Endospores are bacterial survival structures that are highly resistant to many different types of chemical and environmental stresses and therefore enable the survival of bacteria in environments that would be lethal for these cells in their normal vegetative form.
It has been proposed that endospore formation has allowed for the survival of some bacteria for hundreds of millions of years e. It differs from reproductive spores in that only one spore is formed per cell resulting in no net gain in cell number upon endospore germination.
Bacterial Antigens - Creative Diagnostics
The location of an endospore within a cell is species-specific and can be used to determine the identity of a bacterium. Archaeologists have found viable endospores taken from the intestines of Egyptian mummies and also from sediment of over seven thousand years old taken from Minnesota's Elk Lake. From Wikipedia, the free encyclopedia.
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