#36 from R&D Innovator
Volume 2, Number 5
From a Bacterial "Contaminant"
Mendelson is professor of molecular and cellular biology at the
University of Arizona in Tucson.
recent years, interdisciplinary research has spawned a number of
hybrid specialties. For
the past 10 years, I've been working in one of the strangest
combinations of all—an offspring of bacterial genetics (my
field) and mechanical engineering of textile fibers.
study mutants of a harmless, rod-shaped bacterium called Bacillus
subtilis that display defective growth.
I hope this simple system will eventually provide greater
insight into fundamental biological processes.
When I was invited to spend the summer of 1973 at the
University of Rochester Medical School, I happily accepted so as
to flee Tucson's summer heat.
the years I've gotten to know the "habits" of B.
subtilis—I recognize its odors, the shape of its colonies on
Petri plates (colonies are the white or green clusters you've seen
on old cheese or bread), and its behavior under various culture
short, I knew enough about B. subtilis to think that my first Rochester experiment with that
familiar strain was a bust. The
colonies did not have shapes that were typical of B. subtilis. Knowing
that even adept microbiologists can allow the zoo of
microorganisms that float around in the air to contaminate their
experiments, I decided that a contamination somewhere along the
line accounted for the odd colony shapes.
before discarding the plates and repeating the experiment, I
decided to examine the cells in the colonies under the microscope,
and that's when I saw beautiful helical structures I'd never seen
before. I had no idea
what they were until I noticed small, spherical cells within the
chains of cells in the helix, and I remembered that the strain I'd
planned to use had a mutation that caused small spherical cells to
form at the ends of its normal rod-shaped cells.
that point, I realized that the plates I'd been ready to heave out
were not contaminated at all, but rather contained helical
structures probably as a result of the growth conditions of the
you see, respond to their environment in many fascinating ways,
and here was one that I, an expert, had never seen before.
Purely by accident, I had discovered a new growth form of
I spent much of the rest of that summer thinking about
helical growth, and its significance and exploitation.
What produced this structure and what could be learned from
returned to Arizona and began building models to help visualize
the origin of the helical form.
Eventually I hit upon the idea that the helical form was
not the direct result of changes in cell structure, but rather was
the result of physical, or mechanical, forces.
Because nobody had studied the possible role of mechanics
in the regulation of bacterial cells, I tried to work out the
geometrical relationships that could link cylindrical elongation
to helical form by deformation.
I was trained in genetics, microbiology (and music), I always
liked physics, math and chemistry, and thought it would be fun
learning enough engineering to analyze the helical growth.
But I first wanted to do some further experiments on the
physiology of these mutants. So I took a year's leave and worked at the Institut Pasteur
in Paris in 1976, in a laboratory that was noted for B. subtilis research. There
I learned to grow those helical structures into much larger
fibers—several millimeters in length—which I called "macrofibers."
was also startled to discover that macrofibers could be right- or
would have appreciated knowing that someone would later discover
handedness in bacterial growth in the same lab where he'd found
handedness in tartartic acid crystals.)
that year, I had many questions to answer and many skeptics to
convince. Unlike most
microbiologists, colleagues in physics and X-ray crystallography
were excited about the macrofiber phenomenon.
I had difficulty having papers accepted in leading
microbiology journals, however, and granting agencies were
unwilling to support this hybrid of physics and microbiology.
1982, a physicist colleague of mine found a book on the mechanics,
engineering, and material properties of textile fibers, and
recognized the similarity between the structure of multifilament,
twisted textile fibers like wool and cotton and my bacterial
textile monograph showed many examples of successful analyses of
problems similar to the ones I'd been grappling with using my
amateur engineering techniques.
example, I had learned that macrofiber forms could change
handedness under certain environmental conditions.
And one of the book's editors, J. J. Thwaites, had
explained similar behavior in helically twisted fibers that had
been heat set, then twisted in the opposite direction.
Thwaites showed that during the untwisting, the fibers
passed through forms similar to those I had observed when
macrofibers invert handedness.
wrote Thwaites for clarification, enclosing copies of my papers
and my layman's summary of what it was all about.
I asked if he thought we could approach the macrofiber
problem as he had dealt with textile fibers.
I'll never forget three things about his reply:
1) my mechanics interpretation was sound, 2)
even though no one in textiles had an inkling that such
forms existed in the microbial world, the textile work seemed
applicable to macrofibers, and 3) he wanted to visit Arizona to
work with me.
John Thwaites, this was an opportunity to take up a new challenge
at a late stage of his career.
For me, it was a chance to apply real engineering expertise
to bacterial growth patterns.
(It was not until years later that John admitted to me that
my black cowboy hat, silver belt buckle, and handlebar mustache
had caused him a sinking feeling when I first met him at the
collaboration has flourished in large part because Thwaites and I
have complementary skills yet speak a common scientific language.
We have confidence in each other.
Neither of us is afraid to propose ridiculous ideas, and we
enjoy challenging each other.
Important principles have resulted from this collaboration,
such as the role mechanical forces play in the self-assembly of
complex structures—and now it's getting much easier to have our
papers published and grants funded.
work has produced results that may have practical and theoretical
bacterial fibers are being examined for possible use in textiles
and insulation, and approaches devised by textile engineers are
being used to measure the material properties of bacterial walls.
my experience with bacterial fibers, I've observed that most
scientists seem resistant to truly new ideas and have few
constructive explanations for them.
I've also noticed something else, perhaps a desire to be
entertained rather than enlightened.
My audiences seemed fascinated with some spectacular
time-lapse films I had produced to show the twisting dynamics of
macrofiber assembly. Finally,
I realized that I was being invited to speak at meetings primarily
as entertainment. Having
received no useful feedback from microbiologists in the audiences,
I have shifted my attention to other scientists from whom I could
indeed learn many things.
the most important lesson I've learned in pursuing this unique
corner of the biological world is to avoid becoming discouraged by
all the things people tell you can't be done.
If they truly can't be done, then I'm happy to find that
out for myself, thank you very much.