An Interview with Ann and Manuel Varela about Joshua Lederberg: Bacteria Can Mate? Who Would have thought?

Nov 9, 2020 by

Joshua Lederberg

Michael F. Shaughnessy

1) Joshua Lederberg was born in Montclair, New Jersey—but it did not take his family and him long to relocate to the massive cultural center of New York City. When exactly was Lederberg born, and where did he go to school?

Nobel Laureate Joshua Lederberg is a pioneer of molecular biology. He is famous for his bacterial mating studies, called conjugation, and for his classic studies of genetic recombination. Although Lederberg was born in Montclair, New Jersey, on May 23, 1925, his family moved to Washington Heights, Manhattan, when he was six months old. Lederberg’s father, Zvi, was an Orthodox rabbi and his mother, Esther, was a homemaker. They were Palestinian immigrants. Lederberg had two younger brothers. He was interested in science from the young age of seven. Lederberg said that he wanted to discover a few theories in science. Though his father wanted him to follow his example and pursue a career with a religious calling, the two agreed that following a science path would lead to devotion and insight. Therefore, science was a commendable pursuit.

Lederberg attended Public School 46 and Stitt Junior High School 164. Lederberg was enrolled in Stuyvesant High School, an all-male public high school specializing in science and technology. During his formative years at Stuyvesant, Lederberg conducted experiments in cytochemistry. In 1941, Lederberg graduated at the young age of 15. After graduation, Lederberg continued his experiments at the American Institute Science Laboratory, which provided selected high school students laboratory space and equipment. This experience’s significance is that it offered Lederberg some scientific researchers’ skills, namely how to prepare and stain tissue samples with formaldehyde, dyes, and other chemicals.

Lederberg attended Columbia University by taking advantage of a $400 scholarship. Starting in 1941, he conducted biochemical and genetic studies on the orange bread mold Neurospora crassa in Francis J. Ryan’s laboratory. Lederberg certainly wanted to make significant strides in the areas of cancer and neurological malfunctions. His major was zoology. Due to military service requirements, Lederberg worked as a hospital corpsman while on active duty in the United States Naval Reserves. In 1943, he worked in the clinical pathology laboratory at St. Albans Naval Hospital on Long Island, New York. His job was to analyze sailors’ blood and stool samples for malaria. This preliminary work influenced his later philosophy about the life cycle of bacteria. In 1944, Lederberg completed his undergraduate degree in zoology and pre-medicine.

2) When young, he read a book at the Washington Heights Public Library—The book—The Microbe Hunters—by Paul De Kruif was influential. How did it impact him for the rest of his life?

Hundreds of books, which he read at the New York Public Library, inspired Lederberg. He explored mathematics, philosophy, history, and fictional works. One of the most notable books he read was The Microbe Hunters by Paul de Kruif. Some sources report that after hearing a radio broadcast of War of the Worlds by H. G. Wells, Lederberg was captivated by the possibility of life on other planets.

3) He began his medical studies at the Columbia University College of Physicians and surgeons. What transpired there?

Attending Medical School at Columbia’s College of Physicians and Surgeons was Lederberg’s next objective because it allowed him to continue his research endeavors. Lederberg was inspired by Oswald Avery’s discovery of the importance of DNA, so much so that he continued research as a first-year medical student, which was not customary. His emphasis was supplementary studies on the genetics of bacteria. His premise was that bacteria were “Schizomycetes,” primitive organisms reproduced by cell division and consequently produced progeny that were genetically undifferentiated from one another. Due to difficulties with experimental success, Lederberg decided to break from medical school and work in partnership with Edward L. Tatum at Yale University. This collaboration lasted for two years and culminated with Lederberg’s Ph.D. from Yale in 1947. His thesis title was Genetic recombination in Escherichia coli.

4) Over his illustrious career—he was involved in microbial genetics, artificial intelligence, and the space program in the U.S. Can you provide insights into each of these?

Let us consider Lederberg’s contributions to each of these areas. Lederberg’s most significant discoveries in microbial genetics dealt with viral transduction, bacterial conjugation, and genetic recombination. Before his return to medical school at Columbia, Lederberg received an offer from the University of Wisconsin at Madison for an assistant professorship in genetics. Despite reservations about halting his medical career, he accepted because the position offered an exceptional opportunity to pursue basic genetic research without a teaching obligation. During the next twelve years, Lederberg and his wife, Dr. Esther Zimmer, a microbiologist and a few postgraduate students, published numerous papers containing original experimental results from a laboratory in the genetics department.

In 1957, Lederberg became chairperson of a newly created department at the University of Wisconsin, the Department of Medical Genetics. Previously, Lederberg and his colleagues were part of the university’s School of Agriculture. Though temporary, this position paved his path to chairing the Department of Genetics at Stanford University’s School of Medicine the following year. Additionally, his Nobel Prize award’s announcement came soon after he decided to move to Palo Alto, California.

Lederberg studied bacterial genetics in his Stanford laboratory. Human health and biology were now his main interests, specifically; as director of Stanford’s Joseph P. Kennedy Jr. Laboratories for Molecular Medicine, he embarked on studies into the genetic and neurological source of intellectual disabilities. Amid his lectures on artificial intelligence (AI), Lederberg had astutely speculated that machines would be increasingly able to read the literature and spend time living in the real world. Perhaps, he reasoned, smart devices would evolve as we fed them more data.

Being a Nobel laureate did have its advantages. Lederberg extended his scientific interests further. The liftoff of the Soviet Sputnik satellite in 1958 encouraged him to consider the biological implications and hazards of space travel. Lederberg gained a place for biologists in the burgeoning U.S. space program when he publicly warned against the dangers of the moon’s contamination and other planets by spacecraft carrying microbes from the Earth. He investigated the possibility of interplanetary life as a member of the National Academy of Sciences’ Space Science Board from 1958 to 1974. Lederberg served on lunar and planetary mission boards for NASA. For example, as part of the National Aeronautics and Space Administration’s 1975 Viking mission to the planet Mars, he helped design and create tools to detect possible microbial traces on the planet’s surface.

4) Lederberg coined the term “exobiology” back in the day—What was he referring to here?

Lederberg was referring to the so-called cosmic distribution of life, life in outer space. The study of the phenomenon of life outside the physical realms of Earth, he called exobiology. Recently, the term exobiology has been revised to astrobiology. Lederberg was anticipating the new exploration of space by man and machines. He spoke of the possibilities of human-crewed spaced flight and the retrieval of lunar samples. He pondered what potential microbial aliens might be brought back along with astronauts returning from the moon after a moonwalk or from orbiting Earth during a spacewalk. Lederberg speculated about the roles of biology, chemistry, medicine, genes, and evolution concerning the realm of outer space. Exobiologists (astrobiologists) are grappling with the question of whether life outside the confines of Earth exists. Such living beings are considered extraterrestrial life, i.e., alien life. If so, astrobiologists are much-interested in knowing how to detect such life convincingly. These scientists are concerned with acquiring evidence for biochemical activities of extraterrestrial life, such as metabolism. Astrobiologists are interested in solar system locations suitable for life. Such scientists are curious about understanding the conditions that support extraterrestrial life.

Lederberg reasoned that water, temperature, and the presence of life’s building blocks, such as nucleic acids or amino acids, were all factors that needed an astrobiologist’s attention. Further, Lederberg considered the origin of life on Earth and whether such forces responsible 4.3 billion years ago could be at play in outer space.

Another issue concerned microbes. On Earth, microbes that could be isolated and studied with specific abilities intrigued Lederberg and the astrobiologists. For example, some microbes can live without air, an ability known as anaerobic. Furthermore, some microbes can live in frigid temperatures, a psychrophilic process, or “cold-loving.” Lastly, some microbes survive the extreme toxic effects of ionizing radiation and doses lethal to humans. Thus, it was reasonable to argue that if microbes can survive lack of oxygen, intense cold, and radioactive exposure, all of which are normal conditions of outer space, then microbes would be excellent candidates searching for life in outer space. Many astrobiologists are convinced that the first aliens from outer space that we Earthlings will encounter will be microbial.

5) Lederberg received the Nobel Prize at the tender young age of 33. What exactly did he discover or investigate to receive such this award?

Lederberg earned the Nobel in physiology and medicine in 1958 for his discoveries of genetic recombination in bacteria and the bacterial genetic apparatus organization. He shared the Nobel with Edward Tatum and George Beadle, both of whom are considered in separate chapters in this book. Tatum and Lederberg found that bacteria can mate by conjugation and transfer genetic information along the way. When Lederberg started this Nobel work as a graduate student under the tutelage of Ed Tatum at Yale, virtually little was known regarding the nature of the gene and bacterial mating. In fact, up to the early 1940s, it was generally believed that bacteria had no genes or mating behavior whatsoever.

Lederberg and Tatum studied mutant strains of the bacterium Escherichia coli. These types of mutants are termed auxotrophs because of their inability to make a needed nutrient. Thus, to grow these mutants in the lab, one must supply the required nutrients in the culture media artificially. One could provide minimal media with a few salts, perhaps ammonia for nitrogen, and some sugar for a carbon source. Under these conditions, a so-called wild-type microbial cell can grow. However, the auxotroph cannot increase its numbers because it cannot make specific biomolecules. Thus, the growing auxotroph mutant will require the necessary ingredient in the minimal medium.

Lederberg and Tatum had in the Yale lab several auxotroph mutants. We will call one of these mutants “strain A.” The strain is defective in making the vitamin biotin (bio¯) and the amino acid methionine (met¯). This bacterial mutant was denoted bio¯ met¯. To grow the mutant strain in minimal media, Lederberg and Tatum had to supply biotin and methionine. Likewise, Lederberg and Tatum had a second auxotroph, called “strain B,” that could not make the amino acids proline (pro¯) and threonine (thr¯). Therefore, strain B Escherichia coli was denoted as pro¯ thr¯, and this mutant required the supplementation of proline and threonine in the minimal medium for its growth on plates in the laboratory.

Lederberg and Tatum then mixed the two autotrophs, strains A and B, together in one test tube for a brief time. They plated onto minimal media lacking vitamins and amino acids. The results were startling.

The individual strains, each by themselves, strain A and strain B, could not grow on the minimal agar plates. No bacterial colonies appeared. However, surprisingly, when the mixture of the two mutant strains was plated onto minimal agar plates, bacterial colonies appeared!

Remember, separately, neither strain could grow colonies, but together, bacterial growth was possible. For growing to be likely at all, the growing cells must have produced the necessary vitamins and amino acids. Thus, only bacteria that were bio+ met+ pro+ thr+ could have grown on the minimal plates without vitamins or amino acids.

Lederberg and Tatum reasoned that the two auxotroph mutants exchanged their genetic material to produce the genotype. The genetic exchange created a new variant that made the missing vitamins and amino acids, thus allowing them to grow. That is, the bacteria underwent genetic recombination by transferring genetic material between them!

Furthermore, their data and those from follow-up experiments indicated that the bacteria were mating to transfer genetic material and recombine it to produce viable bacterial strains. Their genetic material complemented each other to create a feasible variant.

The process of bacterial mating is called conjugation. The donor of the genetic material is called the male. The bacterium that receives the genetic material is called the female or the conjugant.

File:Bacterial Conjugation en.png

Figure 33. Overview of bacterial conjugation.

In modern times, the conjugative process of bacterial mating has been worked out at the cellular level. See Figure 33. In step one, the cell on the left is the donor cell, which has transferable genetic material in a plasmid DNA-encoding conjugation machine, such as a pilus. In step 2, the two mating bacteria, the male (DNA donor) and female (DNA recipient), bind to each other via the male’s pilus. In step 3, additional conjugative machinery functions after bacterial attachment. One conjugation machinery component is called the relaxosome. It prepares donor DNA. Another conjugative element is the transferosome, which is a DNA delivery system. Together, the pilus, the relaxosome, and the transferosome mediate the genetic transfer from donor to recipient. Once the donor’s DNA has been acquired, the recipient becomes a donor. Then the conjugative process can begin again with another new recipient cell.

6) At one point, he was President of Rockefeller University—in the City of New York—what were his accomplishments there?

In 1978, Lederberg was selected President of Rockefeller University and continued his research. From 1990, he was Professor Emeritus of Rockefeller University. He served on several government advisory boards and wrote a weekly column, Science and Man, for the Washington Post.

He wrote reflective pieces based on his life’s many scientific contributions. He continued to serve the U.S. government by holding several scientific advisory responsibilities, some of which he had begun in 1950. Some of these advisory roles were at the U.S. president’s level. Lederberg lent his scientific expertise to defense and cancer issues. One of these issues pertained explicitly with the so-called Gulf War syndrome. This illness emerged in U.S. military personnel during the 1990s, after the terroristic events of September 11, 2001.

7) As with many scientists, he was mentored and mentored others—who were some of the stellar researchers with whom he worked?

Dr. Joshua Lederberg worked with many prominent scientists. In the 1940s, Lederberg and Edward Tatum studied bacterial conjugation and genetic recombination. This collaboration with Tatum would mark the Nobel accolade to Lederberg at a very young age, 33 years, for having conducted a graduate thesis that provided evidence of bacterial mating and genetic recombination.

At Columbia University, he studied under Francis Ryan in 1946. Together Ryan and Lederberg examined mutant fungi of the genus Neurospora, which was George Beadle’s model organism for his Nobel work. Ryan and Lederberg found that the Neurospora mutants readily reverted to its so-called prototrophic form, a reverse-mutation process.

Interestingly, Lederberg collaborated with his wife, Dr. Esther Miriam Zimmer Lederberg, who had earned her M.S. from Stanford under George Beadle, and her Ph.D. from the University of Wisconsin. See Figure 34.

Figure 34. Esther Lederberg, in her research laboratory.

The Lederbergs also worked with Luigi Cavalli-Sforza. The trio discovered the so-called fertility factor, F, a plasmid-linked genetic element that encodes conjugation pili production and associated machinery. The F factor permits the transfer of donor DNA from F+ cells to F¯cells, lacking the F factor, to create new conjugants.

Esther and Joshua Lederberg also collaborated in the 1950s to develop an advantageous molecular biological technique called replica plating. See Figure 35 for an example of the method.


Figure 35. Replica plating.

In Figure 35, mutants that have lost a function, like antibiotic resistance, or phage resistance in the case of the Lederbergs, can be studied. In these cases, the mutants are susceptible to antibiotics or phages. These types of “loss-of-function” mutants are kept on a master plate, where they can grow without worrying about losing their viability. Then the mutants can be plated onto two different plates via replica plating.

The process was ingenious—one end of a wood cylinder is covered with sterile velvet material—one merely touches the master plate, picking up the mutants onto the velvet. Then the Lederbergs would use the velvet block to inoculate two plates, one containing antibiotic (Figure 35) or phage (Lederbergs) and another plate lacking the antibiotic (Figure 35) or phage. The mutants that have lost the function, i.e., antibiotic resistance or phage resistance, can grow on the plates without the inhibitory agents (i.e., antibiotic or phage). The two dishes are replicas of each other. As long as one ensures the plates’ proper orientations, they knew where susceptible mutants on one Petri dish are on its replicate dish. The sensitive mutant can be picked up from the plate lacking the inhibitory substance, grown, and studied further.

It was an excellent method for studying mutants that have lost a biological function. The Lederbergs used replica plating for studying T1 phage resistance and susceptibility mutants to provide evidence for the hypothesis that mutations are random events, similar to those found by Max Delbrück.

Norton Zinder studied under Lederberg, who was at the University of Wisconsin. Together, they examined a process called transduction. The biological function of transduction involves introducing viral genetic material into a new cellular host, like bacteria. Zinder and Lederberg studied phages that were specific for Salmonella enterica.

8) Apparently, he was ahead of his time—warning others of the threat of bioterrorism and new and reemerging human infectious diseases. Given that we are struggling with COVID-19—he had some insights!

In the early 1990s, Lederberg acquired an active interest in the subject of bioterrorism. The term refers to disease-causing microbes as agents of disease in a hostile manner as biologic-based weapons. The targets of bioterrorism weapons are believed to be humans, food plants, and farm animals. Attacking the human food supply is known today as agroterrorism.

In 2001, Lederberg testified in a hearing sponsored by the Committee on Foreign Relations devoted to bioterrorism threats and infectious disease spread. He testified that bioterrorism weapons could readily be developed and difficult to detect. Further, these biological agents could be easily mass stockpiled incognito and delivered efficiently to their intended targets. Moreover, Lederberg continued, biological weapons could wreak havoc in the field before authorities could respond effectively, exacerbating the threat.

Lederberg testified that creating weaponized microbes needed rudimentary expertise. Thus, rouge amateurs could just as quickly be involved as irresponsible governments of the state. Lederberg intensely worried that while a worldwide treaty to ban biological weapons was invoked during the early 1970s, verification was problematic.

While pointing out problems, Lederberg also pointed out solutions. He suggested more knowledge must be gained regarding the terrain between the politics versus bioweapon developers. Lederberg proposed that responsible governments invoke ingeniously designed sanctions for harborers of bioterrorists. Lederberg also recommended developing early detection methods, such as biosensors, for surveillance of attacks. He advocated that medical officials consider inventions for early diagnosis of diseases caused by a bioterror attack. Lederberg expressed the need for funding to train research and medical personnel and response capabilities, such as isolation, decontamination, and treatment. Critically, he declared that an adequate response policy was lacking, as evidenced by the incident related to September 11, 2001, involving the surprise anthrax attack.

For today’s bioterrorism-related issues, several large countries have implemented treaties and laws prohibiting biological weapon use. The development of weaponized microbes as bioterrorism agents can potentially affect humans, food crops, or farm animals.

Biological weapons that target humans are a chief concern. Such microbial threats include high-risk level agents like anthrax from the Bacillus anthracis bacterium, smallpox from variola virus, and the plague from Yersinia pestis bacteria. These agents and others like it are a high priority threat. Biological threats of moderate risk to humans include Q fever from the bacterium Coxiella burnetii, brucellosis from Brucella bacteria, gas gangrene from the Clostridium perfringens bacteria, and food-borne gastrointestinal illnesses from Escherichia coli, Shigella dysenteriae, and Salmonella enterica. Low-risk microbial weapon threats to humans include yellow fever and encephalitis caused by flaviviruses, tuberculosis by Mycobacterium tuberculosis, influenza by the influenza virus, and rabies from the rabies virus.

Weaponized pathogens of food plants include fungi that target grains. Such plant pathogens can also widely contaminate surrounding soils and would be difficult to discern from naturally occurring pathogens. This area of bioterrorism is not as fully developed as that for human threats. High priority microbial threats to food animals include the notorious foot-and-mouth disease caused by a virus of the genus Aphthovirus, a member of the Picornaviruses family.

9) Sadly, he died at the Columbia University College of Physicians and surgeons—an almost fitting tribute to a Nobel Prize Winner. What did he die of, and when did he die? What have I neglected to ask?

Lederberg is reported to have died of pneumonia in New York City, New York, on February 2, 2008.

Lederberg married his first wife, Esther Miriam Zimmer, in 1946. The marriage ended in divorce in 1966. His second marriage was in 1968 to Dr. Marguerite Stein Kirsch, which lasted until his death from pneumonia at age 82. The couple had two children, David Kirsch and Anne Lederberg.

Lederberg published over 300 scientific articles. Lederberg had frequently visited the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts, to study and teach. Lederberg was inducted into the National Academy of Sciences in 1957. He received a Fulbright Visiting Professorship to go to Melbourne University in Australia. In 1958, Dr. Lederberg was given the Nobel Prize in Physiology or Medicine for discovering the mechanisms of genetic recombination in bacteria with George W. Beadle and Edward L. Tatum. Lederberg was given the National Medal of Science (1989) and Presidential Medal of Freedom (2006).

For additional information regarding this Nobel Laureate and pioneer of molecular biology, visit:

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