Morphological classification of bacteria

Morphological classification of bacteria

Bacteria can be classified into various categories based on their features and characteristics.
The classification of bacteria is mainly based on the following characters:

  1. Morphological Classification (Based on Shape)
  2. Composition of the cell wall
  3. Mode of respiration
  4. Mode of nutrition

Morphological Classification (Based on Shape)
• Bacteria and Archaea are classified by direct examination with the light microscope according to
their morphology and arrangement.
• The basic morphologies are:

  1. Spheres (coccus)
  2. Round-ended cylinders (bacillus).
  3. Helically twisted cylinders (spirochetes)
  4. Cylinders curved in one plane (selenomonads)
  5. Unusual morphologies (such as the square, flat box-shaped cells of the archaean genus).
    • Arrangements include pairs, tetrads, clusters, chains and palisades.

  1. Coccus (Pleural – Cocci):
    Spherical bacteria
    • may occur in pairs (diplococci)
    • in groups of four (tetracocci)
    • in grape-like clusters (Staphylococci)
    • in chains (Streptococci)
    • in cubical arrangements of eight or more (sarcinae).
    Example: Staphylococcus aureus, S. pyogenes.
  2. Bacillus (Pleural–Bacilli): Rod-shaped bacteria; for example – Bacillus cereus, Clostridium
    • generally occur singly
    • but may occasionally be found in pairs (diplo-bacilli)
    • chains (streptobacilli).
  3. Spirillum (Pleural–Spirilla): Spiral-shaped bacteria.
    • Spiral bacteria can be sub-classified on the basis of number of twists per cell, cell thickness,
    cell flexibility, and motility.
    Spirilla, Spirochetes, vibrios
    For example – Spirillum, Vibrio, Spirochete species.
  4. Bacteria have Other Shapes Such as:
    • Coccobacilli – Elongated spherical or ovoid form.
    • Filamentous – Bacilli that occur in long chains or threads