Bacterial Structure

Morphology

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Bacteria exhibit considerable variety in shape, size, and colonial arrangement. Bacterial cells have an average size of about 1 um. Cocci have a circumference of 1 um, and rods may have a length of 2 um with a width of 1 um. But that's just an average.

One of the most important ways to describe bacteria is by the shape: cocci/coccus, rod/bacillus, vibrio, spirillum, spirochete and branching filaments. Bacterial cells can also be categorized according to arrangement, or style of grouping. The greatest variety in arrangement occurs in cocci, in tetrads (groups of four), in irregular clusters (as in staphylococci and micrococci), or in chains of a few to hundreds of cells (streptococci). An even more complex grouping is a cubical packet of eight or more cells called sarcina. These trains are usually typical for genus and are diagnostically useful.

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Cell Structure

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Bacterial cells have a cytoplasmatic membrane, cytoplasm, ribosomes, a cytoskeleton, and one (or a few) chromosome(s); the majority have a cell wall and a surface coating called glycocalyx. Specific structures are found in some but not all bacteria, such structures are flagella, pili, fimbriae, plasmids, inclusions, endospores and microcompartments.

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External Structures

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Flagella: A basal body anchored in the plasma membrane and cell wall, they are slender, rigid structures, about 20 nm across and up to 15 or 20 um long. The primary function of flagella is to confer motility, or self-propulsion. Bacterial species often differ in their patterns of flagella distribution, the number and arrangement of flagella on the cell are diagnostically useful.

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Fimbriae: Small, bristlike fibers sprouting off the surface of bacterial cells. They may be responsible for the mutual clinging of cells that lead to biofilms and other thick aggregates on the surface of liquids and for the microbial colonization of inanimate solids.

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Pili/Pilus: Long, rigid tubular structure made of pilin. Bacterias use pilus for drawing another bacteria close in order to transfer DNA to it (Conjugation).

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S layers:  Single layers of thousands of copies of a single protein linked together like tiny chain mail. They are often called "the armor" of a bacterial cell. This layers are produce  when they are in a hostile environment. Some bacteria use S layers to aid in attachment, as well.

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Glycocalyx: This protects the cell and, in some cases, helps it adhere to its environment. Some bacteria are covered with a loose shield called a slime layer that evidently protects them from loss water and nutrients. A glycocalyx is called capsule when it is bound more thightly to the cell tan a slime layer. Capsules produce mucoid colonies on agar and often are visible in negatively stained preparations.

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Cell envelope

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The majority of bacteria have chemically complex external covering, termed the cell envelope, that lies outside of the cytoplasm. This cell envelope is composed of two or three basic layers which are describe below: 

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Outer membrane: Serves as a partial chemical sieve by allowing only small molecules to penetrate.  The outer membrane (OM) contains specialized types of polysaccharides and proteins. The uppermost layer of the OM contains lipopolysaccharide (LPS), The polysaccharide chains extending off the surface function as antigens and receptors. The lipid portion of LPS has been referred to as endotoxin because stimulates fever and shock reactions in gram-negative infections.

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Cell wall: Helps to determine the shape of a bacterium, and it also provides structural support necessary to keep a bacterium from bursting or collapsing because of changes in osmotic pressure. 

 

Cell walls of most bacteria gain their quality from the peptidoglycan (PG). This compound is composed of a repeating framework of long glycan chains cross-linked by short peptide fragments to provide a strong but flexible support framework. The cell walls of many pathogens have components that contribute to their pathogenicity. The wall can protect a cell form toxic substances and is the site of action of several antibiotics. Based on the Gram-stain procedure bacteria are divided into two major groups, Gram-positive and Gram-negative.

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Several bacteria groups lack the cell wall structure of gram-positive or gram-negative, and some bacteria have no cell wall at all. These forms can stain positive or negative in the Gram-stain, Mycobacterium species contain peptidoglycan and stain gram-positve, but their wall is composed of unique types of lipids (mycolic acid), these lipids are responsible for a high degree of resistance to certain chemicals and dyes, they also contribuites to the pathogenicity of this group.

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Some bacteria that ordinarily have a cell wall can lose it during part of their life cycle. These wall-deficient forms are referred to as L forms or L-phase variants.

 

Cytoplasmic membrane: Provides a site for functions such as energy reactions, nutrient processing, synthesis., and regulate transport. Bacterial cytoplasmic membranes  contain primarly phospholipids (30%-40% of the membrane mass) and proteins (contributing 60%-70%). Major exceptions to this description are the membranes of mycoplasmas, which contain high  amounts of sterols.

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Internal Structures

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Cytoplasm: Its major component is water (70%-80%), which serves as a solvent for the cell pool, a complex mixture of nutrients including sugars, amino acids, salts. These components serve as building blocks for cell synthesis or as a source of energy. 

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Ribosomes: A bacterial cell contains thousands of tiny ribosomes, the site of protein synthesis, they are dispersed throughout the cytoplasm that often occur in chains called polysomes. Many are attaches to the cytoplasmic membrane.

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Inclusions: Bacteria manufacture inclusion bodies to respond to their environmental conditions. The can store nutrients in this way to respond to periods of low food availability.

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Nucleoid/Chromosome: Most bacteria hereditary material exists in the form of a single circular strand of DNA known as the bacterial chromosome. Their DNA is aggregated in a dense area of the cell called the nucleoid.

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Plasmid: Although the chromosome is the minimal requirement for bacterial survival, many bacteria contain plasmids. These tiny strands exist as separate double-stranded circles of DNA, although they can become integrated into the chromosome. They can be duplicated and passed on to related nearby bacteria (Conjugation). They confer resistance to drugs and produce toxins and enzymes.

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Endospores: Gram-negative and gram-positive bacteria produce endospores. The endospores are dominant bodies produced by bacteria such as Bacillus and Clostridium. These bacteria have a two-phase life cycle, a vegetative cell and an endospore. The vegetative cell is metabolically active and growing entity that can be induce by enviromental conditions to undergo endospore formation (sporulation). The endospore exists initially inside the cell, but eventually the cell disintegrates and the endospore is on its own. 

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Microcompartments: Their outer shell are made of protein , arranged geometrically, and are packed full of enzymes that are designed to work together in biochemical pathways.

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