Classification of Microorganisms based on requirements for molecular oxygen
Oxygen is essential for energy metabolism of the organisms, However, bacteria differ in their requirement for molecular Oxygen. Accordingly, they can be classified in to four groups.
- Aerobes
- Anaerobes
- Facultative anaerobes
- Microaerophiles
1. Aerobes
These are the bacteria which can grow only in the presence of free molecular oxygen. When grown in broth media, these bacteria, usually give pellicle growth. This is because of their aero tactic movement.
Bacteria move towards surface medium due to the surface tension. Hence, they grow on the surface of medium to give pellicle growth, e.g. Bacillus.
2. Anaerobes
Anaerobic bacteria are those which do not require molecular oxygen for growth. They can be further classified as
- Non stringent or tolerant anaerobes and
- Stringent or strict anaerobes.
- Non stringent anaerobes are those, which can tolerate low levels of molecular oxygen.
- Stringent anaerobes are those, which cannot tolerate even low levels of gaseous oxygen and are killed on exposure to oxygen in air.
e.g. Certain Clostridia. In broth media, they grow at the bottom of tube.
3. Facultative anaerobes
The bacteria, which can grow both in presence or absence of molecular oxygen or air, are called facultative anaerobes. They can grow in absence of oxygen. But if it is available, they can use molecular O₂ during their respiratory metabolism and grow well.
e.g. E. coli. In broth media, their growth develops uniform turbidity.
4. Microaerophiles
These are the bacteria which require low levels of oxygen for growth. However, they cannot tolerate high concentration of O₂ present in air and are killed. In broth media, they grow beneath the surface, where oxygen availability is limiting.
No comments:
Post a Comment