External Structure of Bacteria

External Structure of Bacteria

1. Flagella

   • long filamentous appendages that propel bacteria
     • may be monotrichous, lophotrichous, or amphitrichous
       • Bacterial cells can alter the speed and direction of rotation of flagella

2. Glycocalyx

   • substances that surround cells
     • composed of polysaccharide, polypeptide, or both
     • organized and is firmly attached to the cell wall
     • a very important component of biofilm

3. Axial Filaments

   • arise at the ends of the cell beneath
     • outer sheath and spiral around the cell
       • a structure similar to flagella
         • corkscrew motion
       • enables to move effectively through body fluids

4. Fimbriae

   • at the poles of the bacterial cell or over the entire surface
     • involved in forming biofilms
       • help bacteria adhere to epithelial surfaces

5. Pili

   • involved in motility and DNA transfer
     • grappling hook model of twitching motility

6. Cell Wall

   • Composition and Characteristics
     • is a complex, semi-rigid structure
       • responsible for the shape of the cell
         • surrounds the underlying, fragile plasma membrane
           • protects the interior of the cell from adverse changes in the outside
     • composed of a macromolecular network called peptidoglycan
       • consists of a repeating disaccharide attached bypolypeptides
         • form a lattice that surrounds and protects the entire cell
           • Alternating NAM and NAG molecules are linked
             • to form a carbohydrate “backbone”
               • it always includes tetrapeptide side chains
                 • consist of four amino acids attached to NAMs in the backbone

7. Gram-Positive Cell Walls

   • contain teichoic acids
     • may bind and regulate the movement of cations into and out of the cell
       • may also assume a role in cell growth
         • provide much of the wall’s antigenic specificity

8. Gram-Negative Cell Walls

   • consist of one or a very few layers of peptidoglycan and an outer membrane
     • bonded to in the outer membrane and is in the periplasm
       • high concentration of degradative enzymes and transport proteins
         • do not contain teichoic acids
           • more susceptible to mechanical breakage
             • cell consists of lipopolysaccharides, lipoproteins, and phospholipids
             • evading phagocytosis and the actions of complement

9. Atypical Cell Walls

   • certain types of cells have no walls or have very little wall material
     • are unique among bacteria in having lipids called sterols
       • protect them from lysis (rupture)

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