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JACK DANIEL 
TANSY JULEP CUP 
Here's a most elegant way to enjoy Mr. Jack’s 
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address Telephone 615-759-7184 > 
terium cannot coast. If its flagella stop 
moving, a bacterium comes to an 
abrupt halt within about a millionth 
of its body length. The flagella work 
wonderfully well in trying circum- 
stances. 
After Berg had modified his mi- 
croscope to track individual bacteria, 
he noted that an E. coli moves in 
two ways. It may “run,” swimming 
steadily for a time in a straight or 
slightly curved path. Then it stops 
abruptly and jiggles about — a “twid- 
dle” in Berg’s terminology. After twid- 
dling, it runs off again in another di- 
rection. Twiddles last a tenth of a 
second and occur on an average of 
once a second. The timing of twiddles 
and the directions of new runs seem 
to be random unless a chemical at- 
tractant exists at high concentration 
in one part of the medium. A bac- 
terium will then move up-gradient to- 
ward the attractant by decreasing the 
probability of twiddling when a ran- 
dom run carries it in the right di- 
rection. When a random run moves 
in the wrong direction, twiddling fre- 
quency remains at its normal, higher 
level. The bacteria therefore drift to- 
ward an attractant by increasing the 
lengths of runs in favored directions. 
The bacterial flagellum is built in 
three parts: a long helical filament, 
a short segment (called a hook) con- 
necting the filament to the flagellar 
base, and a basal structure embedded 
in the cell wall. Biologists have argued 
about how bacteria move since Leeu- 
wenhoek first saw them in 1676. Most 
models assumed that flagella are fixed 
rigidly to the cell wall and that they 
propel bacteria by waving to and fro. 
When such models had little success 
in explaining the rapid transition be- 
tween runs and twiddles, some biolo- 
gists suggested that flagella might tag 
passively along and that some other 
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(and unknown) mechanism might 
move bacteria. 
Berg’s observations revealed some- 
thing surprising, hinted at and pro- 
posed in theory before, but never ade- 
quately demonstrated: the bacterial 
flagellum operates as a wheel. It ro- 
tates rigidly like a propeller, driven 
by a rotatory “motor” in the basal 
portion embedded in the cell wall. 
Moreover, the motor is reversible. E. 
coli runs by rotating the flagella in 
one direction; it twiddles by abruptly 
stopping and rotating the flagella the 
other way! 
Berg could observe the rotation and 
correlate its direction with runs and 
twiddles by following free-swimming 
bacteria in his machine, but S.H. Lar- 
sen and others, working in Julius Ad- 
ler’s laboratory at the University of 
Wisconsin, provided an even more 
striking demonstration. They isolated 
two mutant strains of E. coli — one 
that runs and never twiddles and an- 
other that twiddles incessantly. They 
“tethered” these mutant bacteria to 
glass slides, using antibodies that at- 
tach either to the hook or filament 
of the flagella and also, fortunately, 
to glass. Thus, the bacteria are affixed 
to the slide by their flagella. Larsen 
noted that the tethered bacteria rotate 
continually about their immobilized 
flagella. The running mutants turn 
counterclockwise (as viewed from out- 
side the cell), while the twiddling mu- 
tants turn clockwise. The flagellar 
wheel has a reversible motor. 
The biochemical basis of rotation 
has not yet been elucidated, but the 
morphology can be resolved. Berg pro- 
poses that the bottom end of the fla- 
gellum expands out to form a thin 
ring rotating freely in the cytoplasmic 
membrane of the cell wall. Just above 
it, another ring surrounds the flagellar 
base, but is not attached to it. This 
second ring is mounted rigidly on the 
cell wall. The lower ring (and entire 
flagellum) rotates freely, held in po- 
sition by the surrounding upper ring 
and the cell wall itself. 
Some exceptions in nature are 
dispiriting — the nasty, ugly, little facts 
that spoil great theories in Huxley’s 
aphorism. Others are enlightening and 
serve only to reinforce a regularity 
by identifying both its scope and its 
reasons. These are the exceptions that 
prove (or probe) rules — and the fla- 
gellar wheel falls into this happy class. 
Is it accidental that wheels only oc- 
cur in nature’s smallest creatures? Or- 
ganic wheels require that two parts 
46 
