Target practice: researchers shoot for new treatments against tuberculosis

Science News,  Sept 2, 2006  by Aimee Cunningham

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In the "The Three Little Pigs," the wisest pig protects himself by building the house with the strongest walls. In the world of bacteria, the microbe that causes tuberculosis employs a similar strategy. Mycobacterium tuberculosis packs extra layers of sugars and lipids into its cell wall to make a structure almost impenetrable by human--immune system defenses and by many antibiotics.

"As a general rule, they are one of the least permeable bacteria on the planet," says Clifton E. Barry III, who studies tuberculosis at the National Institute of Allergy and Infectious Diseases in Rockville, Md.

Like the wolf in the story, scientists are working to breach the microbe's tough defenses. It's a critical fight: The TB bacterium infects one-third of the world's population, and it kills nearly 2 million people every year. "More people die of TB today than [of] any other single infectious bacterium," says John S. Blanchard, a bio-chemist at the Albert Einstein College of Medicine in New York.

A cough or sneeze from a person with TB symptoms sends out droplets containing the bacteria. If someone nearby inhales the droplets, that person, too, can become infected. Researchers working with the microbe have to wear masks and protective clothing, and their laboratories have to have special air-filtration systems and safety cabinets that can contain infectious agents.

The need for new TB therapies is especially pressing because the current treatment, four drugs taken in various combinations with medical supervision over 6 months or longer, is a difficult enterprise in many parts of the world. What's more, some TB microbes remain undaunted by even that regimen.

"The incidence of multidrug-resistant tuberculosis is growing all the time and will continue to grow," says Blanchard. The World Health Organization estimates that 450,000 new multidrug-resistant TB cases occur every year. There are few back-up antibiotics to treat strains that evade the standard defense.

In the search for biochemical processes for new therapies to target, some researchers have focused on the super reinforced cell walls of the TB bacteria. They are continuing to describe not only how the microbe synthesizes that reinforced cell wall, but also other ways in which the bacterium fends off drugs. Some teams are targeting processes essential to the microbe's survival, such as the pathways for iron uptake. Aided by the complete sequence of the M. tuberculosis genome, reported in 1998, and funding boosts from public and private sources, researchers are optimistic that these efforts and others like them will soon result in new therapies for TB.

CELLWALL SECRETS The microbe's unusually well-fortified cell wall gives it several advantages. When M. tuberculosis enters a person's body, immune system cells called macrophages take it up. Whereas these cells typically destroy microbial invaders, the TB microbe manages to survive within them. Because of this hardiness, the bacterium can persist in a person's body for years.

M. tuberculosis doesn't immediately cause illness in most people. In 9 out of 10 people infected, the bacteria lie dormant in the lungs and don't produce the fever, weight loss, fatigue, and other symptoms characteristic of an active TB infection. However, AIDS patients and others with weakened immune systems are at high risk of becoming ill.

Scientists attribute the microbe's persistence--as well as its resistance to drugs--in large part to the cell wall. Like other bacteria, M. tuberculosis' inner cell membrane attaches to a wall composed of peptidoglycan, a polymer of amino acids and sugars. The TB microbe adds to that another layer of sugars, called galactans and arabinans. That's topped off by a water-repelling layer of lipids, including very long chains of fatty acids called mycolic acids. The attraction between these lipids prevents most substances from diffusing across the boundary, Barry explains.

Researchers have spent decades searching for the many enzymes that build M. tuberculosis' cell wall. Some of these enzymes may be good targets for drugs against TB. Indeed, isoniazid and ethambutol, two of the primary antibiotics now used to treat tuberculosis, interfere with cell wall biosynthesis.

Recent work has provided more details about the cell wall's machinery. Todd L. Lowary of the University of Alberta in Edmonton and his coworkers are looking for inhibitors of the enzymes that assemble galactans for the middle layer of the cell wall when the microbe replicates. Toward that goal, they've coaxed the bacterium Escherichia coli to produce one of those enzymes, called glfT, in the lab and have begun to characterize the enzyme.

With the E. coli-made glfT in hand, the team plans to work out the three-dimensional structure of the enzyme and use it to design new compounds likely to bind to and thereby block glfT. The researchers will also search existing lists of known structures for compounds that fit the bill, Lowary says.