What are the Structural Characteristics of Bacteria?

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Structure of a bacteria

Bacteria are the world’s most common organisms, with an estimated five nonimillion bacteria on Earth. They come in a variety of shapes and sizes. The largest bacteria is half a millimeter long and the smallest is just 0.3 microns across. General size is between 0.5 microns and 5.0 microns. The characteristics of bacteria are among the most varied in any domain of life- the relatedness between 2 species of bacteria is often much less than the relation between any 2 given metazoans, say a human and a slug. Many structural features are unique to bacteria and are not found among archaea or eukaryotes. Due to the simplicity of bacteria compared to larger organisms, the cell structure of bacteria has been well studied, revealing many biochemical principles that have been subsequently applied to other organisms. Even though they infect and can kill billions of other organisms, bacteria have a relatively simple structure.

What are the Shapes of Different Bacteria?

The most elemental structural property of bacteria is Cell morphology (shape). Cell shape is generally characteristics of a given bacteria, but can vary depending on growth conditions. Bacteria generally form distinctive cell morphologies when examined by light microscopy. Theses are often the first characteristics observed by a microbiologist to determine to identity of an unknown bacterial culture. The examples are as follows.

  • Coccus (spherical): Coccus can be used to describe any bacterium that has a spherical shape.
  • Bacillus (rod-like): Bacillus is used to describe any rod-shaped bacterium.
  • Spirillum (spiral): This refers to a bacterium with a spiral shape.
  • Filamentous: Filamentation is the growth of certain bacteria, such as E.coli, in which cells continue to elongate but do not divide.

 

What is the cell size of Bacteria?

The most obvious structural characteristics of bacteria are their small size. For example, Escherichia Coli, commonly known as E.coli cells, an average sized bacterium are about 2 micrometers and 0.5 micrometer in diameter, with a cell volume of 0.6-0.7 micrometer. Small size is very important because it allows for a large surface area-to-volume ratio which allows for rapid uptake and distribution of nutrients and excretion of wastes.

 

What is the structure of the Bacterial Cell Wall?

As in other organisms, the bacterial cell wall provides structural integrity to the cell. In prokaryotes (group of organisms that lack a cell nucleus), the primary function of the cell wall is to protect the cell from internal pressure caused by the much higher concentrations of proteins and other molecules inside the cell compared to its environment. The bacterial cell wall differs from that of all organisms by the presence of ‘peptidoglycan’, which is located immediately outside the cytoplasmic membrane (biological membrane that separates the interior of all cells from the outside environment). Peptidoglycans are large, structural molecules made of protein carbohydrate. Peptidoglycan is responsible for the rigidity of the bacterial cell wall and for the determination of cell shape. It is relatively porous. There are 2 main types of bacterial cell walls, Gram positive and Gram negative. For both gram positive and Gram negative, particles of approximately 2 nano meters can pass through the peptidoglycan. They are explained as below.

  • Gram Positive cell wall: Peptidoglycans are the structural elements of almost all bacterial cell walls. They constitute almost 95% of the cell wall in some Gram positive bacteria and as little of the cell wall in Gram negative bacteria. The cell wall of some gram positive is completely dissolved by an enzyme called lysozyme. In other gram positive bacteria, for instance, Staphylococcus aureus, the walls are resistant to the action of lysozyme.
  • Gram Negative cell wall: Unlike gram positive cell wall, the gram negative cell wall contains a thin peptidoglycan layer adjacent to the cytoplasmic membrane. In addition to the peptidoglycan layer, the gram negative layer also contains outer membrane composed by phospholipids and lipo polysaccharides (large molecules consisting of a lipid (fat molecules) and polysaccharide, a large carbohydrate molecule structures), which face into the external environment. The gram negative cell wall has an overall negative charge.

 

What is the Bacterial cytoplasmic membrane composed of?

The bacterial cytoplasmic membrane is composed of a phospholipids bi layer and thus has all the general functions of a cell membrane such as acting as a (barrier which prevents the harmful substances from passing through) permeability barrier for most molecules and serving as the location for the transport of molecules into the cell. Bacteria can have a variety of fatty acids within their membranes.

 

What are some other Bacterial surface structures?

  • Fimbrae and Pili: Fimbrae are protein that extends out from the outer membrane in many members of the major group of bacteria (Proteobacteria). They are generally short in length and present in high numbers about the entire bacterial cell surface.
  • S-layers: An S-layer (surface layer) is a cell surface protein layer found in many different bacteria. All S-layers are made up of 2 dimensional arrays of proteins and have a crystalline appearance, the symmetry of which differs between species. The function of S-layer may be the partial permeability barrier for larger molecules.
  • Capsules and Slime layers: Many bacteria secrete extracellular polymers outside of their cell walls. These polymers are usually composed either of polysaccharide, and sometimes proteins. Polymers are large molecules composed of repeating structural units.
  • Flagella: 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 movement. The arrangement of flagella about the bacterial cell is unique to the species observed. Flagella are complex structures that are composed of many different proteins. These include flagellin, which makes up the whip-like tube and a protein complex that spans the cell wall and cell membrane to form a motor that causes the flagellum to rotate. Common forms include:

    • Polar: Single flagellum found at one of the cell poles.
    • Peritrichous: Multiple flagella found at several locations about the You do not have access to view this node.
    • Lophotrichous: A tuft of flagella found at one cell pole

 

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