The bacterium that causes tuberculosis could well be the most successful pathogen on Earth. It infects fully a third of the human population—more than two billion people—and kills around two million a year. Yet, despite 120 years of study, scientists had little idea what made Mycobacterium tuberculosis tick.
“Here you have a disease that has been killing millions of people for centuries, and we knew almost nothing about its biology even 10 years ago,” says Christopher Sassetti, a microbiologist at the University of Massachusetts Medical School. “At the molecular level, it was a black box.”
Sassetti, a self-described “gear-head” who likes to tinker, builds genetic tools to pry open the black box and peer inside. Early in his career, while working in the lab of Eric Rubin at the Harvard School of Public Health, Sassetti helped engineer a system that exposes the genetic innards of M. tuberculosis. This system, called TraSH, has led to fundamental discoveries about how the organism acquires the nutrients to keep itself alive for years or decades inside the human body. Such discoveries open the way for new ideas on how to treat tuberculosis.
TraSH stands for transposon site hybridization, and it deploys transposons—genetic elements that leap through the genome—to randomly trash genes in the tuberculosis bacterium. In practice, the system produces libraries of hundreds of thousands of bacteria, each with different mutations. Because tuberculosis has only 4,000 genes, in a library this size every one of those genes will be mutated in many different individual cells. When the library is exposed to particular conditions—say, those mimicking the human lung—comparing the bacteria that live with those that die reveals all the genes essential for survival in those conditions.
TraSH quickly outlines whole networks of genes involved in key aspects of how the bacterium operates. “It allows us to rapidly ask what set of genes is required for adapting to any particular condition,” says Sassetti.
After perfecting TraSH, Sassetti decided to use it to understand how the bacterium survives during infection. While residing in its host, the bacterium lives inside immune cells in the lung, apparently surrounded by membranes of the host. While many pathogens live in a similar niche, it is unclear how any microbe can acquire nutrients while sequestered inside a host membrane. Sassetti solved that mystery for M. tuberculosis by identifying a cluster of proteins the bacterium uses to siphon cholesterol from its host organism and feed itself. Interfering with this cholesterol pilfering—or interfering with other metabolic pathways Sassetti is uncovering—might provide bold new approaches to treating tuberculosis, which is notoriously difficult to kill.
Sassetti has used similar approaches to examine latent tuberculosis infections. The vast majority of tuberculosis infections fall into that category, and during this period the bacterium hides in an apparently dormant state, causing no symptoms. Latent infections can persist for years before symptoms emerge, but no one knew how the bacterium assumes and maintains this apparently quiescent state. Sassetti discovered a variety of cellular functions that are necessary for the bacterium to perform this feat. Strikingly, bacteria lacking these functions succumb to drugs much more readily than usual, providing another approach that pharmaceutical companies might be able to exploit in their quest for better tuberculosis drugs.
“While we have drugs to treat most strains of tuberculosis, what’s a problem for most of the world is that it takes six months, nine months, a year to actually cure someone," says Sassetti. “Perhaps we can speed up therapy by manipulating the genes that control whether the bacteria sleep.”
Sassetti now plans to investigate the interaction between HIV and tuberculosis. In Africa, in particular, tuberculosis kills many HIV-infected individuals with weakened immune systems. Sassetti wants to uncover the molecular interactions between these pathogens that drive this deadly synergy.
In the meantime, he takes satisfaction in watching the tools he’s built spread across the scientific community. “That's one of my favorite things,” he says. “I really like building things, and I really like seeing other people use them. We've spread a lot of genetic tools around. It's really gratifying to see that.”