Fundamentals of Microbiology

1.2.3 Chemotaxis and Motility

Chemotaxis and Motility

Chemotaxis is the movement of an organism towards or away from a chemical. Positive chemotoxis is movement towards a chemical (attractant); negative chemotaxis is movement away from a chemical (repellent). Bacterial movement is controlled by the presence of these compounds such that where there is no gradient of attractant or repellent in the environment, the organism moves in a random way. However, in the presence of a concentration gradient, the net movement of the bacterium is in one direction. Bacteria detect the presence of a gradient through the action of membrane-bound chemoreceptors. Bacterial movement is characterized by runs and tumbles. Random movement is marked by runs and tumbles in no one particular direction. But when an attractant (gradient of a chemical attractant) is present it is marked by larger runs and less frequent tumbles.

Bacterial endospores

Many living organisms like filamentous fungi produce structures called spores. Their function is a reproductive one. The bacterial endospore is not a reproductive structure. Bacterial cell produces only one endospore, which is resistant to harsh environmental conditions including high temperatures and toxic chemicals. Bacteria of the genera Bacillus and Clostridium produce endospores.

The structure of the endospore is more complex than the vegetative cell which produces it. Dipicolimic acid (DPA) with high concentrations of calcium ions are found in all endospores. A complex of calcium and DPA is responsible for their heat resistance.

Sporulation occurs in the environment (in soil and on nutrient media) but not in human / animal tissues.

The sporulation process occurs in four success stages:

1. Preparatory stage

2. Forespore stage

3. Stage of cell wall formation

4. Maturation stage

Under unfavourable conditions (eg. Nutrient depletion) structural changes take place inside the cell. It is characterized by thickening of the cytoplasm in a certain region and the formation of a forespore, which becomes surrounded by a thick poorly permeable multilayered wall. The rest of the cell gradually disappears and a spore is produced.

Other structures:

Inclusion bodies may be present in prokaryotes which are meant for storage of materials. The most common storage materials are:

1. Poly-β – hydroxybutyric acid (PHB), a lipid like molecule surrounded by proteins.

2. Granules of polyphosphate, sometimes called volutin granules or metachromatic granules which are used by the cells for generation of ATP and other cell constitutions.