Chemotaxis: a physiological adapation

A dramatic example of rapid adaptation is the process of "chemotaxis", i.e. the ability to move toward nutrients (positive chemotaxis) and away from noxious compounds (negative chemotaxis).

This is a valuable trait in bacteria, since it enages them to swim to sources of nutrients and away from toxic chemicals. 

Chemotactic responses are very rapid, they do not require active gene expression. 

How is it that bacteria move directionally?  One way that some types of bacteria move involves organelles known as flagella. 

These are rotary motors powered by ion gradients across the plasma membrane.  This motor drives a helical propeller.  The basic motor can rotate in either a clockwise or a counterclockwise wise direction.

Because their three dimensional structure, when the flagellum rotates in one direction (counterclockwise), the bacteria moves forward; when rotating in the opposite direction, the bacteria tumbles.

Tumbling allows the bacteria to change direction.  When the motor switches back to the counter clockwise direction, the bacteria swims off in a new direction. 

The ability to control chemotaxis involves controlling the probability that the motor will switch its direction of rotation. 

In bacteria, motor rotation switching is controlled by a receptor system that senses changing concentrations of attractants (inhibiting switch) and repellants (induce switching). 

There are two possible ways to do this.  One would be to measure the difference in concentration between the two ends of the bacterium.   This is difficult, since the bacteria is very small (1-2 um long) and the differences in concentration between two points separated by 1-2 um is very small (perhaps we could calculate this?)  

An alternative approach is to measure the concentration around the bacteria as a function of time.  If it is increasing, the bacteria is moving toward the source of the chemical; if it is decreasing, it is moving away from thge source. 

• Define the rules a bacteria would use to move toward a food molecule; away from a toxic molecule?  What components would you need to assume are present to get the bacterium to be able to do both behaviors? 
• READ the very, very short paper by Park et al (2003) Motion to form a quorum and describe how they deduced the nature of chemoattractant involved.