© Wikimedia Commons, GNU Free Documentation License. Author: Greg L., 25 August 2006.
Molecules in a gas move around in a manner that is essentially random. Moving at a velocity determined by their temperature, the molecules travel in a straight line until they scatter off of another molecule or the wall of the container holding them. The average distance a gas molecule travels before scattering and changing direction is called the mean free path. The motion of individual bacteria resembles the motion of molecules in a gas when the bacteria do not display collective behavior.