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San Antonio scientist finds how bug hurts cells

Web Posted: 04/11/2006 12:00 AM CDT
Cindy Tumiel
Express-News Staff Writer

Scientists have long puzzled over how an oddly shaped bacterium called mycoplasma pneumoniae is able to cause a wide variety of respiratory illnesses, including pneumonia, bronchitis and asthma flare-ups.

Now, after three decades of work, San Antonio microbiologist Joel Baseman says he has an answer: Once inside the body, the bug produces a toxin that injures the delicate cells of the respiratory tract.

Experts are touting the discovery as a major breakthrough that could lead to the development of vaccines, diagnostic tests and therapeutic medications.

"This is earthshaking," said Joseph Tully, retired chief mycoplasma investigator at the National Institutes of Health. "It answers a lot of questions about a whole battery of these things."

Mycoplasmas are a form of bacterium and the smallest organisms without cell walls, which enables them to fuse with cells and disrupt their functions.

A number of mycoplasma species are known to cause diseases in humans and aggravate chronic conditions. One form has been linked to pelvic diseases, and another is suspected of contributing to the onset of illness in people with HIV infections.

Mycoplasma pneumoniae is one of the most common respiratory disease agents. It is the culprit in community-acquired pneumonia, a stubborn form of the illness that spreads easily in communal living situations, such as military barracks and jails. It also has been linked to other respiratory ailments such as asthma and has been implicated in joint, liver and cardiac syndromes.

Scientists have struggled for years to understand how all mycoplasmas become virulent.

"It's been baffling for all of us who have been working in this field, plus all the people who work in infectious disease," said Baseman, chairman of microbiology and immunology at the University of Texas Health Science Center.

Baseman said his team "went fishing," using an abundant lung protein to see what molecule from the mycoplasma would grab the hook. The technique worked identifying a key protein in the mycoplasma that enabled the organism to attach to the lung protein.

"We fished out one specific protein and it turned out to be this toxin," Baseman said.

The toxic protein attacks in two ways, he said. It cuts a cell molecule called NAD into two pieces and acts to damage the cell membrane. Both actions can kill a cell, Baseman said.

The study was published Monday on the Web site of the journal Proceedings of the National Academy of Sciences.

Understanding this chemical activity opens up a range of future studies, said Dr. R. Doug Hardy, an assistant professor at the University of Texas Southwestern Medical School in Dallas.

"By discovering this toxic protein, it is possible that we will be able to make vaccines to protect people from it, which is very exciting," Hardy said. "We'll also be able to develop better diagnostic tests to look for infections."

Tully, the retired mycoplasma investigator, said other scientists who study mycoplasmas will now revisit their work to look for similar toxin production in other species of the bug.

"Everybody is going to have to go back 20 years and look at all these other organisms that we know can produce mild diseases in humans and whether that is associated with a toxin," Tully said.