Tuberculosis exists and flourishes in the lungs. When the germs responsible for the illness are expelled into the atmosphere, they are thrust into a relatively unfriendly setting, facing significant alterations to their surrounding pH and chemistry. Understanding how these germs endure their airborne transit is crucial for their survival, yet there is limited knowledge regarding how they defend themselves while drifting from one host to another.
Recently, researchers from MIT and their associates have identified a gene family that becomes crucial for survival particularly when the pathogen is subjected to air exposure, likely shielding the bacterium during its airborne trajectory.
Numerous genes in this family were previously regarded as nonessential, given that they appeared to have no impact on the bacteria’s role in inducing disease when injected into a host. However, this new research indicates that these genes are indeed vital, though more for transmission than for proliferation.
“We have overlooked the aspect of airborne transmission in terms of how a pathogen can thrive amid these abrupt changes while circulating in the air,” asserts Lydia Bourouiba, the leader of the Fluid Dynamics of Disease Transmission Laboratory and an associate professor of civil and environmental engineering and mechanical engineering, who is also a key faculty member at the Institute for Medical Engineering and Science at MIT. “Through these genes, we are beginning to understand what strategies tuberculosis employs for self-protection.”
The findings from the team, featured this week in the Proceedings of the National Academy of Sciences, may offer new targets for tuberculosis treatments that both address the infection and impede its transmission.
“If a medication were to focus on the products of these particular genes, it could efficiently treat an individual, and even before the person is fully cured, it might prevent the infection from spreading to others,” remarks Carl Nathan, chair of the Department of Microbiology and Immunology and R.A. Rees Pritchett Professor of Microbiology at Weill Cornell Medicine.
Nathan and Bourouiba are co-senior authors of the investigation, which includes MIT co-authors and Bourouiba’s mentees in the Fluids and Health Network: co-lead author postdoc Xiaoyi Hu, postdoc Eric Shen, along with student mentees Robin Jahn and Luc Geurts. The research also involves collaborators from Weill Cornell Medicine, the University of California at San Diego, Rockefeller University, Hackensack Meridian Health, and the University of Washington.
Pathogen’s viewpoint
Tuberculosis is a respiratory ailment induced by Mycobacterium tuberculosis, a bacterium primarily affecting the lungs and transmitted via droplets that an infected person exhales into the atmosphere, frequently through coughing or sneezing. Tuberculosis stands as the foremost cause of death from infectious diseases, with the exception of significant global pandemics triggered by viruses.
“In the last century, we’ve witnessed the 1918 influenza, the HIV AIDS crisis starting in 1981, and the 2019 SARS CoV-2 pandemic,” Nathan remarks. “Each of those viruses resulted in the deaths of countless individuals. As they settled, we are left with a ‘permanent pandemic’ of tuberculosis.”
A significant portion of tuberculosis research focuses on its pathophysiology — the processes through which the bacteria invade and infect a host — as well as methods for diagnosing and treating the illness. In their recent study, Nathan and Bourouiba concentrated on tuberculosis transmission from the bacterium’s perspective, to explore the defenses it might use to survive airborne transmission.
“This represents one of the initial efforts to examine tuberculosis from the airborne angle, considering what happens to the organism at the level of being safeguarded against these abrupt changes and severe biophysical challenges,” Bourouiba explains.
Vital defense
At MIT, Bourouiba investigates fluid dynamics and the ways droplet behavior can disperse particles and pathogens. She collaborated with Nathan, who studies tuberculosis and the genes that bacteria depend on throughout their lifecycle.
To understand how tuberculosis can endure in the atmosphere, the team aimed to replicate the conditions that the bacteria face during transmission. They initially sought to formulate a fluid akin in viscosity and droplet sizes to that which an ill patient might cough or sneeze into the air. Bourouiba points out that much prior experimental work on tuberculosis relied on a liquid solution used by scientists to cultivate the bacteria. However, the team determined that this liquid had a chemical makeup that significantly differs from the fluid expelled by tuberculosis patients.
Moreover, Bourouiba emphasizes that the fluid typically obtained from tuberculosis patients is based on sputum that a patient spits for diagnostic purposes. “The fluid is thick and viscous, commonly regarded in the tuberculosis field as a representation of what is occurring in the body,” she explains. “However, it’s exceedingly ineffective for spreading to others as it’s too sticky to form inhalable droplets.”
Using Bourouiba’s expertise in fluid and droplet physics, the team established the realistic viscosity and probable size distribution of tuberculosis-laden microdroplets likely to be transmitted through the air. They also characterized the droplet compositions based on analyses of infected lung tissue samples. Subsequently, they developed a more authentic fluid, tailored to have a composition, viscosity, surface tension, and droplet size that closely resembles what would be exhaled into the atmosphere.
Afterward, the researchers applied various fluid mixtures onto plates in tiny individual droplets and meticulously measured their evaporation behaviors and the residual internal structures they left behind. They discovered that the new fluid tended to protect the bacteria at the core of the droplet during evaporation, in contrast to traditional fluids where bacteria were more exposed to the air. The more realistic fluid also retained more moisture.
In addition, the team infused each droplet with bacteria that had genes with various knockouts, to determine if the absence of specific genes would influence the bacteria’s survival as the droplets evaporated.
Through this approach, the researchers analyzed the activity of over 4,000 tuberculosis genes and identified a family of several hundred genes that seemed to become significant specifically when the bacteria adapted to airborne settings. Many of these genes are associated with repairing damage to oxidized proteins, such as those exposed to air. Other activated genes relate to degrading damaged proteins that cannot be repaired.
“What we discovered is a profoundly lengthy candidate list,” Nathan states. “There are numerous genes, some more significantly implicated than others, that may be critically involved in enabling tuberculosis to endure its transmission phase.”
The team recognizes that the experiments do not fully replicate the biophysical transmission of the bacteria. In reality, tuberculosis is transported in droplets that traverse the air, evaporating in the process. To conduct their genetic analyses, the researchers worked with droplets positioned on plates. Under these limitations, they imitated the droplet transmission as closely as they could by placing the plates in an extremely dry chamber to hasten the droplets’ evaporation, analogous to what they might experience in flight.
Moving forward, the researchers have begun experimenting with platforms that permit them to observe droplets in flight under various conditions. They intend to concentrate on the new gene family in even more realistic tests to verify whether these genes genuinely protect Mycobacterium tuberculosis during airborne transmission, potentially paving the path for weakening its airborne defenses.
“The notion of waiting to identify someone with tuberculosis, then treating and curing them, is a highly inefficient strategy to halt the pandemic,” Nathan states. “Most individuals who exhale tuberculosis do not yet have a diagnosis. Therefore, we must interrupt its transmission. But how do you achieve that without knowledge of the process itself? We now have some insights.”
This research was funded, in part, by the National Institutes of Health, the Abby and Howard P. Milstein Program in Chemical Biology and Translational Medicine, the Potts Memorial Foundation, and the National Science Foundation Center for Analysis and Prediction of Pandemic Expansion (APPEX), along with support from Inditex, the NASA Translational Research Institute for Space Health, and Analog Devices, Inc.