Lysogeny at mid-twentieth century: P1, P2, and other experimental systems.

Most of us doing research have a preferred material, a set of well-tried techniques, a standing list of unsolved problems, ways of looking at or of doing things, which we share to a large extent with colleagues in the same laboratory and others in the same area of specialization, be they friends, former associates, or competitors. All this and more is encompassed by the concept of “experimental system” as introduced and used by HansJörg Rheinberger (76, 77), a historian of science. His concept, rather flexible and rich in metaphors, may be easily adapted to the present narrative, which proceeds from a personal view rather than from a critical historical examination. A careful restriction of material, techniques, and nomenclature allows more constructive interactions between different laboratories and different generations of scientists in the same field. Of course, carried out to excess, this process will stifle developments in new areas and defer some discoveries. Max Delbrück, who in the early forties had forcefully advocated the study of bacteriophage as the royal road to the secrets of replication and recombination, was quite outspoken about the necessity for workers on that road to use a common material (the T phages in Escherichia coli B) and precisely standardized techniques (1). Of course, the T phages are generally virulent, take-no-prisoner parasites of bacteria and thus could not instruct us about the more shadowy interactions between “weaker” bacteriophages and their bacterial hosts: about lysogeny, where infection meets heredity. The term lysogenic—generating lysis—was applied very early after the discovery of bacteriophages and was used at first in broadly descriptive, uncritical ways. On the other hand, bacterial cultures that spontaneously (i.e., in the absence of obvious infection from the outside) produced bacteriophage, yet grew well, without gross evidence of lysis, were isolated as early as 1922. Varied interpretations were debated back and forth for several years, quite fiercely so in the francophone medical research community. While the development of the concept has been well summarized (68), it deserves a serious look from the point of view of the history and philosophy of science. The flavor of the debates is conveyed by one of the participants, Paul Flu (42; see also reference 84). Eugène Wollman (90, 91) of the Institut Pasteur was one of the few who saw at the time the genetic implications of concepts of lysogeny. Genetics was then rather underdeveloped in the Latin countries of Europe, immunology being the star of the day in medical quarters. Also, the debate only rarely focused on one type of experiment with well-defined and generally available material. In the early thirties a definition of lysogeny was reached (26) that is still valid, obviously without modern molecular implications. Anlage, a German word used in embryology, was applied by Burnet and McKie (27) to what we now call prophage. The concept of Anlage or prophage was based on the fact that no one had succeeded, using a variety of methods, in demonstrating the presence of infectious phage particles inside a lysogenic bacterium. World War II interrupted much of this work. The Wollmans died, and Flu barely survived, in Nazi concentration camps. Burnet took up more medically oriented work. I encountered lysogeny in early 1949 while at Cold Spring Harbor as a research fellow in Milislav Demerec’s laboratory. I was studying spontaneous and induced mutation in a streptomycin-dependent E. coli B strain. I had never worked with bacteria before. Puzzled once by a colony that looked sectorially nibbled, I showed it to Evelyn Witkin and to the late Gus Doermann, who, fortunately for me, had their laboratories just next to my room. They suggested the obvious explanation, phage contamination, but one of them added “There is also something called lysogeny. . .” (Actually, at that time the English word in use was lysogenesis or lysogenicity.) I had never heard of lysogeny before (although I had been exposed to Paul Buchner’s [25] works on endosymbioses) and started digging for more information in the library. At about the same time, unknown to me, two important events had happened. In Madison, Esther and Joshua Lederberg, in the course of their work on the K-12 strain of E. coli, then the only bacteria—apart from Pneumococcus—known to exchange genetic material, isolated a mutant that unexpectedly lysed when in contact with the parent strain. They came to the conclusion that the original K-12 strain was lysogenic for a previously unsuspected phage, which they named lambda, thinking it might be something like Tracy Sonneborn’s kappa factor in Paramecium. This was briefly communicated in the first issue (January 1950) of Witkin’s informal Microbial Genetics Bulletin; actual publication came much later (58). In Paris, André Lwoff, a well known protozoologist and bacterial physiologist, took up the question of how phage particles are produced by lysogenic bacteria. The problem was bravely attacked by direct micromanipulation of individual, or small numbers of lysogenic bacteria, the oversized Bacillus megaterium, cultured in a droplet of broth under the microscope. At * Mailing address: 156-29 Biology Division, California Institute of Technology, Pasadena, CA 91125. Phone: (626) 395-8851. E-mail: [email protected].