New Molecular Label to Help Develop Simpler and Faster Tuberculosis Tests

While chest X-rays and molecular diagnostics are highly accurate, they are not always accessible in developing nations, where TB rates are high. In these countries, TB is frequently diagnosed by culturing microbes from sputum samples, but this method has a high false negative rate and can be challenging for patients, especially children, to provide a sample. Furthermore, this test requires several weeks for the bacteria to grow, which delays diagnosis. Once the bacteria are in the lungs, their thick cell wall helps them resist the host immune system. This cell wall is largely made up of complex sugar molecules called glycans, but understanding how these glycans defend the bacteria has been difficult. One of the challenges is the lack of an easy method to label these glycans within the cells. Chemists have now overcome this challenge by demonstrating that they can label a specific glycan called ManLAM using an organic molecule that reacts with sulfur-containing sugars. These sugars are found in only three bacterial species, with Mycobacterium tuberculosis, the causative agent of TB, being the most common and problematic.

After successfully labeling the glycan, researchers from the Massachusetts Institute of Technology (MIT, Cambridge, MA, USA) showed that they could track the bacteria as they infected macrophages, immune cells involved in fighting infections. Some researchers studying TB had speculated that the bacterial cells shed ManLAM after entering a host cell and that these free glycans interacted with the host immune system. However, the MIT team discovered that the glycan remains part of the bacterial cell wall for at least the first few days of infection. Moving forward, the researchers plan to use this technique to examine the behavior of the bacteria following antibiotic treatment or immune stimulation of macrophages. This approach could also be useful in studying the assembly of the bacterial cell wall and understanding how ManLAM aids in the bacteria’s entry into macrophages and other cells.

The researchers aim to use this approach to develop a diagnostic method capable of detecting TB-associated glycans, either in cultures or urine samples. Such a test could offer a more affordable and quicker alternative to existing diagnostics. Currently, there is a diagnostic under development that uses antibodies to detect ManLAM in urine samples, but this test is most effective in patients with active TB, particularly those who are immunosuppressed due to HIV or other conditions. By using their small-molecule sensor instead of antibodies, the MIT team hopes to create a more sensitive test that can detect even small amounts of ManLAM in urine samples.

“There 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,” said Laura Kiessling, the Novartis Professor of Chemistry at MIT and the senior author of the study.