This month we had the opportunity to interview Professor Howard Berg, the Herchel Smith Research Professor of Physics and Professor of Molecular and Cellular Biology at Harvard University. As you will hear, Professor Berg studies the movement of bacteria. Luckily for us all, he does so with an adventurer's eye and a storyteller's style. His four thoughtful replies to our questions follow.
(1) Where does the drive toward your research stem from? Was there a certain moment in your upbringing that led you to where you are today in your research?
Well, it is a long story. When I was 10, during WWII, I became fascinated by radios. My grandfather, who left Sweden and settled in Nebraska at the age of 16, was apprenticed as a harness maker. The tractor put him out of that business, so by the time I knew him he had moved to a town called Wahoo and set up a shoe-repair shop. My father was the first of seven siblings. In the evening, my grandfather would smoke his pipe in the cellar, the only part of the house where smoking was allowed. I enjoyed watching him. There was a room nearby where my uncle Paul had accumulated a number of 1920's battery-operated radios. Amazing devices with bulbous glass tubes, tuning coils, waxed capacitors, wires everywhere. I loved to take them apart. In high school I worked in a radio repair shop and learned how to put them back together again. Warm and friendly, and you could understand how they worked. Nowadays, its integrated circuits; you don't need to know how they work, just how to connect one to another.
I grew up in Iowa City, Iowa, where my father was a professor of biochemistry. I went to a university elementary and high school, where two of my teachers, one in political science and the other in geometry, were spectacular. We developed the sense that there was no limit to what one might learn.
I wanted to become an electrical engineer, so I went to Caltech. I began my studies in physics, which was more fundamental, but switched to chemistry, having been inspired by Linus Pauling, who taught the introductory chemistry course. (Another spectacular teacher.) Along the way, I tried to learn more biology and got the idea that to appreciate biochemistry, one needed to know something about medicine.
I was admitted to the Harvard Medical School but had misgivings, so spent a Fulbright year in Copenhagen, studying protein chemistry, in a lab across the street from the Carlsberg brewery. (Free beer.) Work as a medical student proved very disappointing: don't think, just memorize. So, after two years I withdrew, moved to Cambridge, and began study for a Ph.D. in the physics department. I spent two years taking classes to catch up, and then began work with Norman Ramsey, on the atomic hydrogen maser. I converted his lab from vacuum tubes to transistors. I spent a fair amount of time in the instrument shop building components for a new maser. The foreman, Charlie Chase, who had worked for Percy Bridgman, taught me how to do precision work with machines that were not very precise. In general, I wanted to understand how things worked and to build instruments useful for that task.
Finally, I spent three years in Harvard's Society of Fellows, where you could work on any problem you liked. I finished my thesis, began studies on cell membranes, and moved back toward biology. One of the senior fellows was Ed Purcell, famous for his work on nuclear magnetic resonance. He had an idea of how one might separate and determine some physical parameters of molecules of high molecular weight. I thought of ways to make the method practical, so we spent a few years developing what became known as sedimentation field-flow fractionation. We wanted to use the method on biological material, such as proteins, but this required interaction with surfaces, such as those of centrifuge rotors, to which proteins adsorb. Rather than trying to master such problems, we set the work aside, and I went back to biology. (Purcell was another spectacular teacher, with a penetrating mind). In an effort to understand some of the things we talked about, I wrote a small book for Princeton University Press called "Random Walks in Biology" which discusses, in elementary terms, the statistical behavior of large molecules or small cells as they move about in aqueous media.
Then I stumbled upon a problem, worthy of all this armament, with which I have been absorbed since 1968: the motile behavior of bacteria. Bacteria are the smallest free-living organisms, which given a few minerals and small organic molecules, such as sugars, can synthesize everything they need. They tend to be rod-shaped cells, about a micron in diameter (1/10000th of a centimeter), two or three times longer than wide, that swim about 30 diameters per second.
(2) In your current research, which project do you consider to be your favorite and why?
My group found that a bacterium -- my favorite is Escherichia coli (E. coli, for short), about which more is known than any other living thing -- tend to swim in a random walk, along a relatively straight path (called a run), followed by a short period of erratic motion (called a tumble), which leads to another run, generally in a new direction. Runs are about 1 sec long, and tumbles are about 0.1 sec long. A cell counts molecules of interest as it goes along. If the molecule is tasty (an attractant), tumbles are suppressed and runs get longer. If the molecule is of little interest, the cell tries a new direction anyway, roughly at the same time as it would were the medium free of attractants or repellents. Thus, the random walk is biased and the bias is positive: if life is getting better, enjoy it more; if life is getting worse, don't worry about it. E. coli is an optimist.
We learned these things by tracking cells in three dimensions, one at a time. I built a microscope that would do this, giving an output of position (x, y, z) as a function of time (t). These data were recorded by an incremental tape recorder and taken to the computer center with a stack of punched cards. With luck, an output (a printout and more punched cards) appeared the next day. This was before video recorders, laptop computers, and commercial data-acquisition and analysis software.
Then we found, from an analysis of existing evidence, that the flagella that propel bacteria do not wave or beat, as the tails of sperm, but rather rotate rigidly, each helical filament driven at its base by a reversible rotary motor. In E. coli, there are about four such motors located at random positions along the sides of the cell. Their filaments are left-handed helices, with a pitch of about 2.5 microns (the length of the cell) and a diameter of about 0.5 microns (about half the diameter of the cell). If the motors all turn counterclockwise (CCW, as viewed from the outside of the cell), their filaments form a bundle that extends several microns from the back of the cell, pushing it forward. Remarkably, the filaments just rotate side by side. If a motor happens to switch to clockwise (CW) its filament changes shape (to a right-handed helix of about half the previous pitch but the same diameter) which disrupts the bundle, initiating a tumble. Such helices, called semi-coiled, then relax to right-handed ones of half the original pitch and half the original diameter, called curly. When the motor driving such a filament goes back to turning CCW, the filament goes back to the normal left-handed form, the bundle reforms, and the cell begins another run.
The molecular machinery that makes this chemotaxis possible, has been worked out in the main by bacterial geneticists and biochemists. We get together every other year at a meeting called BLAST, which stands for bacterial locomotion and sensory transduction.
More recently, we found that motors remodel. If the external load is increased, they add more torque-generating units (called stator units) to accommodate this change and continue to run at about 100 Hz (revolutions per second).
Our current research is focused on how motors remodel, and more fundamentally, on how they work. Recently, an atomic structure has been obtained of a stator unit by a technique known as cryo-em tomography ("em" for electron microscopy). One purifies stator units (large protein complexes containing five copies of one protein, MotA, and two copies of another, MotB) and looks at large numbers of them at different angles in an electron microscope, refining this process until reaching atomic resolution. This effort has been led by a group in Copenhagen with the assistance of another in Berlin. One of my postdoctoral fellows and I were asked to join this group as consultants on physics. This has been a lot of fun. It appears that MotA and MotB comprise a rotary motor all by themselves. Five copies of MotA form a pentamer that rotates around two copies of MotB in response to the passage of protons from the outside to the inside of the cell. The complex is embedded in the inner cell membrane and powered by a transmembrane electrochemical gradient called a protonmotive force. The Mot B are anchored to the rigid framework of the cell wall, while the MotA engage the periphery of the rotor of the flagellar motor. So, the motor is driven by a rotary gear train. When fully assembled, each flagellar motor contains about ten such stator units. The bacterial rotary motor is a remarkable machine.
(3) What life process within bacteria do you appreciate the most and why?
As just enumerated, how bacteria are able to move about and find regions in their environment that they deem more favorable. Swimming, powered by flagella, is only one means of locomotion. Spirochetes, long thin helical microorganisms, swim via the action of one or more internal flagella that rotate between the rigid inner helical cell body and the fluid outer cell membrane. Gliding bacteria have rotary engines that drive adhesins that move along spiral tracks on the cell surface. When an adhesin happens to adhere to a solid substrate, the cell moves forward. There are other appendages called pili. One type, type IV, are assembled, attached via their distal tips to an external substrate, and then disassembled, pulling the cell forward, in a process called twitching motility. Wonderful variety!
(4) What microorganism is your favorite and which one do you think you relate to the most and why?
E. coli, because so much is known about its structure, biochemistry, and physiology, including its motile behavior. And also because it is a friend (with rare exceptions) that we carry around in our guts. E. coli has had many millions of years to change its genetic makeup, by trial and error -- if a new feature is programmed that works, keep it; if it does not work, throw it away. In a rich medium at the temperature of our gut, E. coli copies itself in about 20 minutes. It grows twice as long and divides in the middle. So, we are dealing with a very large number of generations. There are still many surprises ahead as we try to learn how E. coli works.