For the past two decades, Eduardo Groisman has been unraveling the chains of signals and commands that help bacteria survive when conditions in the environment change and, in some cases, cause disease. He was drawn to this area of research by a growing interest in genetic engineering, a discipline not taught in his native Argentina when he attended university. Instead, he learned about it on his own, after an auspicious trip to the United States.
While an undergraduate in biochemistry at the University of Buenos Aires, Groisman decided he "wanted the cultural experience of learning about the United States," he recalls. He visited a cousin who was attending medical school in Chicago, and while he was there he enrolled in a couple of biology courses at the university.
After returning to Argentina to complete his degree, Groisman subscribed to the magazine Scientific American and bought several science textbooks in English. "I started learning things that our professors did not teach us," he says. Through his readings, he became fascinated with genetic engineering and realized that to pursue this newly found interest he would have to seek a Ph.D. abroad.
After convincing his fiancé to move to the United States, he became a graduate student in the laboratory of Malcolm Casadaban at the University of Chicago. There, he developed a clever "trick" for isolating, or cloning, genes from bacteria. At the time, gene cloning was difficult to do and required the use of enzymes that cleave DNA at specific sequences. Instead of using these restriction enzymes, Groisman's new method took advantage of bacteriophages—viruses that naturally infect bacteria—to "pull out" bacterial genes. "The method had been tried by previous graduate students but without success. When I tried it worked right away," says Groisman, who first published the technique in 1984.
Groisman then looked for a postdoctoral position where he could apply his innovative in vivo cloning method to identify interesting bacterial genes. He learned that Fred Heffron at the Scripps Research Institute in California had identified several mutant Salmonella bacteria with unusual characteristics. So Groisman decided to join Heffron's lab and clone the genes affected in these mutants.
Salmonella causes several potentially life-threatening diseases, from gastroenteritis to typhoid fever. What makes the bacteria so dangerous is their ability to live in the intestines of people, an inhospitable environment for most other bacteria, as well as inside macrophages, blood cells whose job it is to destroy infecting microbes. Heffron's mutant Salmonella did not survive in macrophages and were not as deadly as regular Salmonella when injected in mice.
Groisman proceeded to isolate the gene mutated in these "less deadly" bacteria, which was called phoP. In a 1989 Science paper, he explained that Salmonella strains with mutations that inactivate phoP become sensitive to defensins, microbe-destroying peptides that are present in macrophages. After obtaining a faculty position at Washington University in St. Louis—once again having had to convince his wife to move, this time from sunny San Diego—he set out to learn more about the protein PhoP's function.
He discovered, for instance, that PhoP is a regulatory protein that is activated by another protein called PhoQ. PhoQ's activity, in turn, is regulated by the amount of magnesium in the environment surrounding Salmonella. Because magnesium is a near-ubiquitous element in the environment, no one had suspected that it could act as a signal to regulate molecular pathways. When the PhoP/PhoQ system is activated, it modifies the expression of a plethora of genes, some of which help Salmonella survive and multiply inside the macrophage and become more deadly.
Typically, scientists thought of gene regulation as a linear process whereby one sensor protein responds to a signal by activating another protein, such as a transcription factor, which binds to the promoter of a gene, thereby altering its expression. Groisman discovered that the PhoP/PhoQ system could use "connectors" to activate different regulatory systems, each of which would turn on several genes, in response to the signal detected by the PhoQ sensor protein. Such connectors could allow complex networks of genes to be activated at once.
For example, the PhoP/PhoQ system uses the connector protein PmrD to turn on the PmrA/PmrB two-component system, which renders Salmonella resistant to several antimicrobial proteins produced in the body. Groisman discovered this process occurs in bacteria, but the same principles are likely to apply to other organisms.
Another intriguing insight into gene regulation came in 2006 when Groisman engineered the PhoP protein to be constantly synthesized in Salmonella. When he injected these engineered bacteria in mice, instead of being more virulent the microbes were less so. To explain the puzzling results, Groisman discovered that, during infection with normal Salmonella, PhoQ triggers a "surge" of PhoP activity, which wanes as PhoP reaches a steady level of activity. That surge in PhoP activity is needed to kick-start Salmonella's disease-causing program. "If Salmonella don't have a surge of PhoP activity, the infection will not kill a mouse," says Groisman. "We now need to follow this up, to try to understand the mechanism for the surge."
In 2010, Groisman moved again, this time to the Yale School of Medicine, where he also joined the Yale Microbial Diversity Institute.
Throughout his career Groisman seems to have had a knack for discovering the unexpected—something he attributes to being a good listener. "Bacteria are always talking," he says. "If you listen carefully, you can get the answers to fundamental questions."
