January 15, 2004
Zebrafish used to study tuberculosis
In some parts of the world, tuberculosis is a growing concern because a form of the disease resistant to many antibiotics is on the rise; in other countries like India, where Dr. Lalita Ramakrishnan grew up and earned her medical degree, tuberculosis is considered an endemic disease and never stopped being a problem.
But it was only after Ramakrishnan came to the United States as a medical resident and then earned a Ph.D. in immunology at Tufts School of Medicine in Boston that she decided to focus her work on the disease.
“I really became interested in TB all over again when I was at Stanford,” she said. “We really have an urgent need for new drugs or a vaccine, but there is still a lot about how the disease works that we don’t understand.
“It is a biologically fascinating infection,” she noted, “that most often produces an immune reaction in the body that looks effective, but doesn’t always work. Along with specific information on tuberculosis, we’re seeking answers to questions about general interactions between hosts and pathogens. Why does one person get sick when another exposed person doesn’t, or in tuberculosis, develops what are called granulomas that keep the disease in check so that it is a silent infection for years?”
That silent aspect of TB infection is why the TB test is commonly used to screen populations such as hospital workers. Most active TB infections, about 85 percent, are in the lungs, Ramakrishnan said, but it is a disease that can also show up in other organs or in bones. Many people with a contained, silent, infection will never show symptoms.
In humans, TB is caused by Mycobacterium tuberculosis, a bug that is as difficult to work with as it is persistent in human populations. There have never been good animal models, in part because mice that catch tuberculosis get an acute infection and die immediately.
On the advice of Dr. Stanley Falkow, professor of microbiology and immunology at Stanford and a former UW faculty member, Ramakrishnan went looking for a “bacterial cousin,” a related organism that produces similar disease. She settled on Mycobacterium murinum, a bacterium first identified in 1926 that causes tuberculosis in fish. It grows at the low temperature, but produces a very similar disease, except that it develops much faster. It can cause a skin disease in humans, but one that is easily controlled.
At first she worked with leopard frogs, and learned how to create useful mutations in the bacteria.
By the time she moved to the UW as an assistant professor of microbiology in 2001, she had begun working with zebrafish. Zebrafish are also susceptible to Mycobacterium murinum, or “fish TB,” and they have other advantages as an organism to study. The most obvious is that for the first few weeks of their lives, zebrafish are transparent, so you can literally look right through them.
“We have been able to take the eggs and make time-lapse videos so that we can actually watch the infection develop,” she said.
Ramakrishnan will be showing several of the videos as part of the Science in Medicine Lecture at noon, Thursday, Jan. 29, in Hogness Auditorium at the Health Sciences Center. The title is “Tuberculosis: Real-Time Visualization of Host-Pathogen Interactions Using a Zebrafish Model.”
The other advantage of working with zebrafish is that they have been used for many years for basic developmental studies, so that broad-based genetic screens are available for them.
Ramakrishnan earned her medical degree from Baroda Medical College in Baroda, India, in 1983, and then the Ph.D. in immunology from Tufts. She completed a residency in medicine at New England Medical Center and then was a clinical fellow in infectious diseases at the University of California, San Francisco. In 1992, she went to Stanford as a postdoctoral fellow and was also a senior research scientist there.
In addition to her research work at the UW, she is also an adjunct assistant professor of medicine and an attending physician at UW Medical Center. She is board-certified in internal medicine and infectious disease.