CHEMOTAXIS is the process by which bacteria travel to higher concentrations of attractant or lower concentrations of repellant. Its study has attracted wide interest as an example of the stimulus-response network since bacteria are the most primitive of living creatures. Behaviour in peritrichous bacteria is indeed simple: bacteria alternately swim smoothly and tumble, which results in random reorientation for the next swim1. Chemotaxis occurs by the increasing tendency of bacteria to tumble when headed in the 'unfavourable' direction and by increasing inclination to swim when the bacteria are headed in the 'favourable' direction (such as towards higher attractant concentrations)1,2. Tumbling is caused by clockwise rotation of flagella and swimming by counter-clockwise rotation3. Addition of repellent to bacteria causes tumbling; addition of attractant causes swimming4-6. Understanding how the switch that controls direction of flagellar rotation is controlled and how the controlling parameter is regulated by changes in attractant or repellent concentration is necessary to understand chemotaxis. Here I present evidence that for Bacillus subtilis the free concentration of internal Ca2+ ion controls direction of flagellar rotation (high concentrations bringing about tumbling; low concentrations, swimming) and that repellents, which cause tumbling when added to bacteria, work by increasing the flow of Ca2+ ion across the plasma membrane.
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