Fibers From a Bacterial "Contaminant"
by N. H. Mendelson, Ph.D.

Dr. Mendelson is professor of molecular and cellular biology at the University of Arizona in Tucson.

In 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. 

I 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.  

Over 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 conditions.

In 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. 

But 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.

A New Structure

At 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 experiment.  Bacteria, 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 B. subtilis.  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 studying it?

I 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.

Bacterial Engineering

Although 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." 

I was also startled to discover that macrofibers could be right- or left-handed.  (Pasteur 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.) 

After 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.

In 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 macrofibers.  The textile monograph showed many examples of successful analyses of problems similar to the ones I'd been grappling with using my amateur engineering techniques.

For 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.

Textile bacteriology?

I 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.

For 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 Tucson airport.)

Our 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.

Practically Speaking

This work has produced results that may have practical and theoretical applications.  The 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.

From 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.

Perhaps 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.