E. coli tumbling

[From Bill Powers (970826.1015 MDT)]

Hans Blom (970826) --

I've been re-reading the Koshland book on chemotaxis, and fortunately find
that my memory of the observations is accurate. With respect to the
specific question you raised, we find on p.57, "... the absolute amount of
chemoeffector had no influence on behavior." This was an experiment in
which a suspension of bacteria in a medium containing a chemoeffector was
rapidly mixed with another solution of the same chemoeffector at the same
or different concentrations. Behavior was observed after mixing was
complete (a matter of a few seconds). The above observation was made when
the second solution had the same concentration as the first. There was no
difference in behavior at concentrations of 10e-7, 10e-6, or 10e-5 M of
aspartic acid and other chemoeffectors tested. Tumbling behavior was
suppressed or enhanced only when the concentration increased or decreased,
respectively.

The time-scales of our E. coli simulations are misleading. The real
bacterium is about one micron long (1 micrometer), and when swimming moves
at about 20 microns per second. When there is a large step-increase in
concentration of an attractant, tumbling is suppressed (swimming continues)
for as long as 2 minutes (!). Unfortunately, these data are averages across
many bacteria, so individual behavior would probably vary over a wide
range. Stroboscopic photographs of normal E. coli show that they swim for
about 2 seconds between tumbles when the concentration is constant,
traveling 30 to 60 microns. This is the baseline rate of tumbling, with
changes in temporal gradient increasing or decreasing the delay to the next
tumble. When repellents are tested, the same relationships are observed in
reverse.

Koshland reports that the efficiency of this mode of "steering" is
_measured_ as being about half of the maximum possible efficiency (moving
straight up the gradient). It's possible to get higher efficiencies in
simulations.

An incidental observation. These bacteria do not coast when their flagellae
stop: they stop immediately. It is calculated that if all the flagellae
stopped rotating at the same instant, the bacterium would continue moving
by only about one-millionth of its own body length. Viscosity effects at
these micron scales are huge.

Best,

Bill P.