The motile prokaryotes enjoy a selective advantage over their non-motile counter parts under certain environmental conditions. When they encounter gradient of physical and chemical agents in nature, the flagellar machinery functions to develop a response towards the stimulus.
- In a positive response, bacteria move towards the stimulus e.g. nutrients, light. temperature, etc.
- In a negative response the bacteria move away from the stimulus.
The movement of prokaryotes towards or away from the stimulus or the environment is called taxis or tactic response. There are several types of tactic responses.
1]. Chemotaxis
- Movement of an organism towards the chemical attractant or away from the repellants is called chemo taxis.
- Prokaryotic cell surface possesses special proteins, called chemo receptors.
- Chemo receptors may be located either in the periplasmic space or the plasma membrane.
- The tactic responses develop due to gradients of concentration of various chemicals.
2]. Phototaxis
- The tactic responses exhibited by phototrophic (photosynthetic) prokaryotes to a gradient of light Intensity is called phototaxis.
- They accumulate towards intense allowing most efficient photosynthesis.
- Like the chemo receptors, bacteria possess photoreceptor.
3]. Magnetotaxis
- The movement of bacteria towards the Earth's magnetic field or to local magnetie fields (magnets placed near the bacterial culture) is called magnetotaxis.
- It has been observed in Aquaspirillum magnetotacticum.
- It is due to the presence of a chain of magnetite (Fe3O4) inclusions (or magnetosomes) within the cell, which facilitates the positioning of bacteria as a magnetic dipole.
- Magnetotactic bacteria move toward the oxygen-deficient sediments in the aquatic environment.
4]. Osmotaxis
- Movement of prokaryotes in response to the osmotic gradient is called osmotaxis.
- It is related to the concentration gradient of disolved solutes.
