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:
- Morphological Classification (Based on Shape)
- Composition of the cell wall
- Mode of respiration
- 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:
- Spheres (coccus)
- Round-ended cylinders (bacillus).
- Helically twisted cylinders (spirochetes)
- Cylinders curved in one plane (selenomonads)
- Unusual morphologies (such as the square, flat box-shaped cells of the archaean genus).
• Arrangements include pairs, tetrads, clusters, chains and palisades.
- Coccus (Pleural – Cocci):
• 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.
- 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).
- 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.
- Bacteria have Other Shapes Such as:
• Coccobacilli – Elongated spherical or ovoid form.
• Filamentous – Bacilli that occur in long chains or threads