Saturday, 23 January 2021

Classification of Microorganisms Based on Requirements for Molecular Oxygen

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 classifled in to four groups. 

 1. Aerobes 

 2. Anaerobes 

 3. Facultative anaerobes

 4. 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 

   Anaeroble bacteria are those which do not require molebular oxygen for growth. They can be further classifled as

a). Non stringent or tolerant anaerobes and

b). Stringent or strict unaerobes.


- 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. 

Patterns of growth show by different oxygen types of bacteria in broth media. (1) Pellicle growth where oxygen is maximum available. (2) Growth beneath the surface where oxygen availability is limiting. (3) Dispersed growth through the medium. (4) Growth at the bottom of the tube where anaerobic conditions exist.


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. 



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