GUEST COMMENTARY Bacterial Growth: Constant Obsession with dN/dt

One of life’s inevitable disappointments—one felt often by scientists and artists, but not only by them—comes from expecting others to share the particularities of one’s own sense of awe and wonder. This truth came home to me recently when I picked up Michael Guillen’s fine book Five Equations That Changed the World (4) and discovered that my equation—the one that shaped my scientific career—was not considered one of the five. I met this equation in the winter of 1952–1953 when Emanuel Suter, the bacteriology and immunology instructor in the integrated Medical Sciences program at Harvard Medical School, brought three very young colleagues to help teach the instructional laboratory of this innovative course. In this way we 20 privileged students met Boris Magasanik, Marcus Brooke, and H. Edwin Umbarger and were plunged into bacterial physiology. They had designed a laboratory experience to introduce us to contemporary issues and cutting-edge techniques in 1950s bacterial physiology. As I remember, one objective was to study diauxic growth by varying the limiting amount of glucose added to a minimal medium containing a secondary carbon source and inoculated with an enteric bacterium. A second objective was to construct the steps in a biosynthetic pathway by examining the abilities of various compounds to satisfy the nutritional needs of auxotrophic mutants. Both experiments required measuring the growth of bacteria, the former as a kinetic process. For me, encountering the bacterial growth curve was a transforming experience. As my partner and I took samples of the culture at intervals to measure optical density and plotted the results on semilogarithmic paper, we saw, after the lag period, a straight line developing. . .beautiful in precision and remarkable in speed. As the line extended itself straight-edge true, I imagined what was happening in the flask—living protoplasm being made from glucose and salts as the initial cells (Klebsiella aerogenes, they were called then) grew and divided. The liquid in the flask progressed from having a barely discernible haze to a milky whiteness thick with the stuff of life, all within a very brief Boston winter afternoon. Mutably specific autocatalysis, the physicist Erwin Schrödinger had declared a few years earlier (28), was the defining characteristic of living systems, and I had just witnessed the working out of the mathematical statement of that property, dN/dt 5 kN (where N is the number of cells or any extensive property thereof, t is time, and k is the first-order rate constant [in reciprocal time units]). I had never before seen such a clear display of autocatalysis. Its mathematical elegance and simplicity—but more importantly, its invitation to explore—affected me profoundly. The first-order rate constant k in the growth equation seemed to me the ideal tool by which to assess the state of a culture of cells, i.e., the rate at which they were performing life, as it were. I elected to pursue my Ph.D. studies with Boris Magasanik, studying the molecular basis of diauxic growth. Over the ensuing half-century, close analysis of growth curves was to be a central feature of my work, as I followed my intense curiosity (read obsession) about the processes that form living matter from salts and sugar. Catabolite repression, the growth rate-related regulation of stable RNA synthesis, the isolation and use of temperature-sensitive mutants in essential functions (particularly aminoacyl-tRNA synthetases), and the molecular responses of bacteria to heat and other stresses—all these studies depended on inferences and deductions from the growth behavior of bacterial cultures. For anyone interested in the synthesis of protoplasm, bacteria are the system to study (reviewed in reference 17). With four billion years of practice they have perfected the art of growing in many environments, and they outclass all other known forms of life in their rate of metabolism geared for autocatalysis. The first-order rate constant k is most conveniently expressed in minutes or hours for bacteria rather than in days, months, or years (as for most eucaryotes). Little matter that k is not a constant for long during batch cultivation of bacteria in the laboratory or fermentation vat or that its value may vary continuously in any given natural population. Suffice it that k for bacteria is very large, probably the largest for all Earth creatures. Of course, these organisms are appropriately studied for properties other than growth. Bacteria have evolved a dazzling array of capabilities along with rapid growth. As a group, they utilize almost any chemical source to harness energy for growth and maintenance and have mastered photosynthesis as well. They assess their chemical and physical environment with great sensitivity. They move with purpose. They communicate with each other. They employ devilishly clever strategies for colonizing eucaryotic hosts, both plant and animal. (It is through this adeptness that most humans have come to know and respect bacteria.) The spore-formers in particular are famous for enduring long periods of nongrowth under conditions hostile to life (high or low temperature, dryness, and atmospheric pressures from a vacuum to many bars of hydrostatic pressure). Even non-spore-forming bacteria differentiate from a growing form to a form remarkably able to survive prolonged periods inimical to growth. Each of these properties is the focus of contemporary studies, intensified in many cases by useful hints supplied by knowledge of a score or more of completely sequenced bacterial genomes. In particular, the quest to understand the molecular details of the conversion of bacterial cells from growth to stationary phase has attracted the attention of scores of bacterial physiologists, as Roberto Kolter has highlighted in his * Corresponding author. Mailing address: Department of Microbiology and Immunology, University of Michigan, Ann Arbor, MI 481090620. Phone: (734) 763-1209. Fax: (734) 764-3562. E-mail: fcneid @umich.edu.