The twist was revealed when Soo measured the temperature dependency of Listeria movement and thereby measured the apparent activation energy (Ea) of the rate-limiting step. He noticed that each bacterium had a different Ea. This finding is not predicted by simple polymerization-based models of Listeria movement, which assume that the rate-limiting factor (such as actin concentration) is the same for every bacterium.
Knowing the Ea range for a given population, the authors then predicted the range of speeds for that group at a given temperature. But the actual range of speeds they observed was much smaller than predicted—something was systematically speeding bacteria with high Ea so that they did not move as slowly as expected.
Polymerization-based models cannot explain this compensation simply. But Soo found that it is explained by a model that suggests that bacteria advance via the cooperative breakage of small groups of adhesive bonds. Each bond contributes both entropy and enthalpy components to the energy needed to free a bacterium. With more bonds, more thermal energy is needed to break them. But this increase is compensated by the greater entropy that is released upon their breaking.
The authors suggest that bacteria vary in the number of bonds that must break at once for the bug to move (and thus vary in Ea). “Perhaps 10 of those bonds might be stretched,” says Theriot. “If the 10 break simultaneously, the bacterium can move forward 2 or 3 nanometers.” It is then recaptured by the actin comet tail.Other models also incorporate adhesion, but assume that only one bond must break at a time. “The real insight,” says Theriot, “is thinking of things in a group.” She hopes this thinking will be applied to other force-generating elements that act in parallel, such as spindle microtubules or actin filaments at the leading edge.