The Structure of Bacteria.

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The Structure of Bacteria

Bacteria

Bacteria are unicellular prokaryotic organisms which multiply by binary fission.  Bacteria lack the membrane-bound nuclei of eukaryotes; their DNA is in the form of a loop known as a nucleoid.  Bacteria can be classified according to their cell shape, Gram-stain reaction, motility, oxygen and nutrient requirements, biochemical properties, phage susceptibility and genetic composition (DNA) analysis. When arranged according to cell shape, bacteria almost invariably take one of three forms: rod (bacillus), sphere (coccus), or spiral (spirilla and spirochetes).  Rods that are curved are called vibrios.

Cell Envelope Structure

The bacterial cell wall is a unique structure which surrounds the cell membrane.  Although not present in every bacterial species, the cell wall is very important as a cellular component.  Structurally, the wall is necessary for:

  1. Providing attachment sites for bacteriophages.
  2. Maintaining the cell’s characteristic shape.
  3. Preventing osmotic lysis.
  4. Providing a rigid platform for surface structures including flagella and pili.

The cell wall is composed of peptidoglycan which is a polymer consisting of a disaccharide repeating unit of two different N-acetylated amino sugars, one of which is attached to a short peptide chain.  Individual glycan strands are cross-linked through peptide bonds between the peptide chains.  Gram-negative bacteria contain an outer layer surrounding the cell wall, which is similar to the structure of the phospholipid bilayer in the cytoplasmic membrane but consists of lipopolysaccharide (LPS) molecules.  This component of the cell wall of most Gram-negative bacteria contains sugars that function as antigens and Lipid A, an endotoxin responsible for most of the pathogenic affects associated with harmful Gram-negative bacteria.  Also present are porins which control the movement of small molecules through the outer membrane.  Polysaccharides which extend out from the bilayer also contribute to the toxicity of the LPS which together with lipoproteins and polysaccharides forms the outer membrane.  In addition, these organisms have an inner cytoplasmic membrane on the inside of the cell wall, which is in contact with the cytoplasm.  It consists of phospholipids and proteins.  This membrane serves as the selective permeability barrier between the cytoplasm and the cell environment.  It is the site at which many of the important cellular functions occur and the target site for many antibacterial agents.  Gram-positive bacteria have a much thicker layer of peptidoglycan than Gram-negative but they do not have an outer membrane which prevents toxic substances from entering the cell.  For this reason Gram-negative bacteria generally have a greater resistance to antimicrobial agents.

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Nutrition

In terms of carbon utilization, bacteria may be either herterotrophic or autotrophic.  Heterotrophs obtain their carbon source from organic compounds such as amino acids and sugars.  Most heterotrophic bacteria are saprophytes, meaning that they obtain their nourishment from dead organic matter.  Autotrophs use carbon dioxide as their sole source of carbon for growth and obtain their energy from light (photoautotrophs) or from the oxidation of inorganic compounds (chemoautotrophs).

Growth Requirements

Temperature

All bacteria have an optimal growth temperature range.  Those that grow best between 20 and 40°C are referred to as mesophiles ...

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