In the spotlight. Mycobacteria growing in tiny, shallow channels allowed researchers to watch single cells growing and dividing under the microscope.


Why Tuberculosis Is So Hard to Cure

When microbes divide, you usually get more of the same: A cell splits up and creates two identical copies of itself. But a new study shows that's not true for mycobacteria, which cause tuberculosis (TB) in humans—and that may explain why the disease is so difficult to treat. Mycobacteria divide asymmetrically, generating a population of cells that grow at different rates, have different sizes, and differ in how susceptible they are to antibiotics, increasing the chances that at least some will survive. Researchers hope the findings will help them develop drugs against those cells that are especially hard to kill.

"It is incredible that we are finding such basic things out only now," says immunologist Sarah Fortune of at the Harvard School of Public Health in Boston, the paper's lead author. "But it reflects the fact that mycobacteria are relatively understudied."

More than a third of the world's population is estimated to be infected with Mycobacterium tuberculosis. Most people's immune system can keep the bacteria in check, but there is a lifetime chance of 1 in 10 that the dormant infection will progress to TB; the disease still kills 4000 people every day. Tuberculosis treatment is a combination of antibiotics taken for half a year or more—a major drawback, because patients often quit therapy prematurely, increasing the risk of drug-resistant strains emerging. Scientists have assumed that mycobacteria are so hard to kill because dormant cells exist even in patients with active disease and these cells are far less susceptible to antibiotics than metabolically active bacteria.

But Fortune and her colleagues found a second, more surprising mechanism. They cultured M. smegmatis, which is closely related to M. tuberculosis but faster growing, in a tiny chamber with a constant flow of nutrients, allowing them to watch single live cells growing and replicating. Unlike other rod-shaped bacteria, such as E. coli, mycobacterial cells divided asymmetrically, creating a tapestry of cell types with widely different sizes and growth rates, the team reports online today in Science.

By labeling the cell wall of the mycobacteria with a fluorescent dye and observing the new, unstained cell wall growing at the poles, the researchers found that daughter cells mainly grow at their "old" pole. As the new end, created by the cell division, grows older, it matures and the cell elongates faster. And as the cells go through numerous divisions, cells with poles of many different "ages" emerge, leading to the wide variety in growth rates.

Importantly, the cells also differed in their susceptibility to antibiotics: While "older," fast-growing cells were more susceptible to the drugs isoniazid and cycloserine; younger, slower-growing cells were more susceptible to rifampicin. "When I started working on mycobacteria, the assumption was that all the bacteria are indistinguishable. This is the first mechanistic insight into why the cells are phenotypically different," says Fortune. The asymmetry is a way for mycobacteria to keep their population diverse, she says, just like viruses create diversity by mutating frenetically.

"This is an important study, because it shows that our way of thinking that populations are the sum of equal organisms is incorrect," says immunologist Stefan Kaufmann of the Max Planck Institute for Infection Biology in Berlin. "As we look at individual microbes, we find diversity." Kaufmann cautions, however, that most of the experiments were done with M. smegmatis and need to be verified with M. tuberculosis. "But this could explain, at least in part, why tuberculosis is so hard to treat," he says. "And it could pave the way for a rational search for new combination therapies composed of drugs that attack the different types of bacteria."