Breakthrough in labeling TB glycans could lead to faster diagnosis

Tuberculosis, the world's deadliest infectious disease, is estimated to infect around 10 million people each year, and kills more than 1 million annually. Once established in the lungs, the bacteria's thick cell wall helps it to fight off the host immune system.

After labeling the glycan, the researchers were able to visualize where it is located within the bacterial cell wall, and to study what happens to it throughout the first few days of tuberculosis infection of host immune cells.

The researchers now hope to use this approach to develop a diagnostic that could detect TB-associated glycans, either in culture or in a urine sample, which could offer a cheaper and faster alternative to existing diagnostics. Chest X-rays and molecular diagnostics are very accurate but are not always available in developing nations where TB rates are high. In those countries, TB is often diagnosed by culturing microbes from a sputum sample, but that test has a high false negative rate, and it can be difficult for some patients, especially children, to provide a sputum sample. This test also requires many weeks for the bacteria to grow, delaying diagnosis.

Laura Kiessling, the Novartis Professor of Chemistry at MIT and senior author of the studyThere aren't a lot of good diagnostic options, and there are some patient populations, including children, who have a hard time giving samples that can be analyzed. There's a lot of impetus to develop very simple, fast tests."

Labeling glycans

Glycans are notoriously difficult to tag with any kind of probe, because unlike proteins or DNA, they don't have distinctive sequences or chemical reactivities that can be targeted. And unlike proteins, they are not genetically encoded, so cells can't be genetically engineered to produce sugars labeled with fluorescent tags such as green fluorescent protein.

"This is the first approach that really selectively allows us to visualize one glycan in particular," Smelyansky says.

Better diagnostics

"The bacteria still have their cell walls attached to them. So it may be that some glycan is being released, but the majority of it is retained on the bacterial cell surface, which has never been shown before," Smelyansky says.

The researchers now plan to use this approach to study what happens to the bacteria following treatment with different antibiotics, or immune stimulation of the macrophages. It could also be used to study in more detail how the bacterial cell wall is assembled, and how ManLAM helps bacteria get into macrophages and other cells.

"Having a handle to follow the bacteria is really valuable, and it will allow you to visualize processes, both in cells and in animal models, that were previously invisible," Kiessling says.

Using their small-molecule sensor instead of antibodies, the MIT team hopes to develop a more sensitive test that could detect ManLAM in the urine even when only small quantities are present.

The research was funded by the National Institute of Allergy and Infectious Disease, the National Institutes of Health, the National Science Foundation, and the Croucher Fellowship.

Source:

Massachusetts Institute of Technology

Journal reference: