Shape of Bacteria

Classification of Bacteria Based on Shape:

Bacteria are classified based on their shape, which is an essential characteristic used for identification and taxonomy. The main bacterial shapes are:

 

[1] Cocci (Spherical Bacteria)

• Cocci are round or oval-shaped bacteria.
• They can exist as single cells or in different arrangements.

Types of Cocci Based on Arrangement:

1. Monococcus (Single Coccus): A single, round bacterial cell.
2. Diplococcus: Two cocci joined together (e.g., Neisseria gonorrhoeae).
3. Streptococcus: Cocci arranged in chains (e.g., Streptococcus pyogenes).
4. Staphylococcus: Cocci arranged in grape-like clusters (e.g., Staphylococcus aureus).
5. Tetrad: A group of four cocci arranged in a square (e.g., Micrococcus species).
6. Sarcina: A cubical packet of eight or more cocci (e.g., Sarcina ventriculi).

 

[2] Bacilli (Rod-Shaped Bacteria)

• Bacilli are cylindrical or rod-shaped bacteria.
• They may be single or arranged in various forms.

Types of Bacilli Based on Arrangement:

1. Single Bacillus: One individual rod-shaped bacterium (e.g., Escherichia coli).
2. Diplobacillus: Two bacilli joined together (e.g., Klebsiella pneumoniae).
3. Streptobacillus: Bacilli arranged in chains (e.g., Streptobacillus moniliformis).
4. Coccobacillus: Short, oval-shaped bacteria that resemble both cocci and bacilli (e.g., Bordetella pertussis).

 

[3] Spiral Bacteria (Curved or Twisted Shape)

• These bacteria have a curved or helical shape and move using flagella.

Types of Spiral Bacteria:

1. Vibrio: Comma-shaped bacteria with a slight curve (e.g., Vibrio cholerae).
2. Spirillum: Rigid spiral-shaped bacteria with external flagella (e.g., Spirillum volutans).
3. Spirochete: Flexible, thin, and tightly coiled bacteria that move with axial filaments (e.g., Treponema pallidum).

 

[4] Filamentous Bacteria

• These bacteria form long, thread-like chains or filaments (e.g., Actinomyces).
• They resemble fungi in their growth patterns.

 

[5] Pleomorphic Bacteria

• These bacteria do not have a fixed shape and can change from under different conditions.
• Example: Mycoplasma species (lacks a rigid cell wall).