News: Understanding the biophysics of TB transmission

A new simulation system could become the standard for research.

IMES

Developed by an interdisciplinary team led by Prof. Lydia Bourouiba at MIT and Dr. Martin Gengenbacher at Center for Discovery and Innovation, Hackensack Meridian Health, a new system could become a global research standard to study diseases transmitted through the air.

Tuberculosis (TB) claims more lives each year than any other single infectious agent with 1.23 million people killed by TB in 2025 according to the World Health Organization’s (WHO) Global tuberculosis report.

Surpassed only by COVID-19 during the early phase of the pandemic, TB remains one of the top 10 causes of death worldwide.

This lethality is caused, in large part, by the ability of Mycobacterium tuberculosis (Mtb), the bacterium that causes TB, to maintain its virulence while spreading from person to person through the air.

Taxing treatments

Billions of dollars of investments in vaccines and drug research have focused on the last phase of the TB transmission process, the host’s immunological response, after the Mtb has been transported deep into the lungs and is starting the infection process. 

However, current understanding of the physical, chemical, and biological conditions enabling Mtb’s successful journey through the air remains surprisingly limited, as are current TB treatment options.

Drug regimens used to treat TB are taxing and require months-long compliance, and they depend largely on antibiotics, the efficacy of which is affected by the rise of antibiotics resistance, among other global health and societal challenges.

Highly improbable yet very successful

TB transmission is a long, complex process. 

“Mtb is typically lodged very deep in the lungs and can stay dormant for years within asymptomatic individuals,” says Lydia Bourouiba, the Japan Steel Industry Professor at MIT and a core faculty member of MIT’s Institute for Medical Engineering & Science (IMES), and co-lead of the Fluids & Health Network where the research was conducted at MIT.  

“Then, upon emission from the respiratory tract of an infected individual, the Mtb has to go through the ambient air again, be inhaled by somebody else, make it very deep into their lungs and overcome finally the immune response to cause infection. This is a priori a highly improbable and difficult process for an organism to pull off,” Bourouiba says, “and yet Mtb is very successful at doing exactly that.” 

Due to the technical challenges in replicating this transmission process effectively in a laboratory setting, past studies on Mtb transmission have mostly overlooked the physiological conditions of transmission through the air, something the team is seeking to change by building standardized toolkits to study the complex steps of pathogen transmission through the air.

Building off their collaborative program which studies the role of Mtb’s genome during transmission, the team led by Dr. Nathan and Prof. Bourouiba identified hundreds of genes that Mtb actively engages to adapt and survive through its complex journey between hosts. Prof. Bourouiba and Dr. Gengenbacher sought to further open the black box of Mtb transmission.

Redefining processes and terminology

Characterizing this complex process has started with rethinking the foundations of the concept of “transmission through the air”.

The foundations of transmission through the air, including association of small exhaled respiratory microdroplets (below 100 micron) with potential of airborne transmission, were introduced by TB researchers William and Mildred Wells, and their team, back in the 1930s, but were not widely accepted at the time.

Decades later, rethinking the terminology of air transmission itself became necessary. In 2024, a WHO guidance document which Bourouiba helped develop as part of a WHO expert team, put forth new definitions of key terminology used to describe the transmission of pathogens through the air. 

This reflects a new, shared understanding of how respiratory infectious pathogens move from one person to the next. It also paves the way for more collaborative solutions to air transmission challenges, including appropriate animal models of infection.

Source of variability: tools or biology?

Current animal exposure and infection experiments done for treatment development typically use full-body inhalation exposure. 

"This is essentially injecting infectious particles into a chamber occupied by the animals and waiting for the animals to become infected,” Bourouiba clarifies. “This design is inherently poorly controlled with respect to the aerosolization process, what size of particles and injection efficacy it introduces, how long the infectious particles can stay in the air, and where they may linger, let alone where they land and what the animals actually inhale.” 

When repeated, these experiments have shown variable infection rate and lung lesions in the animals. Whether that was due to how the experiments are set up, with their limited control of how the particles evolve in the chamber and how they are inhaled by the animals, or simply to variations in the biology and immunity of the animals, has been hard to decipher.

“High level of variability means reduced clarity on mechanisms and less consensus built, thus, more repeats of experiments ensue, with all the additional animals, time, and funding such repeats entail,” Bourouiba adds.

The Transmission Simulation System (TSS)

Using fundamental fluid physics calculations as a foundation, the researchers designed a Transmission Simulation System (TSS). At its heart, the TSS relies on fluid fragmentation and dispersal processes fine-tuned to the rheological properties of the carrying liquid and inhalation physiology of the target animals. The system is coupled with  airflow condition control and in-flight sampling of the Mtb carrying particles. 

“Essentially, the Mtb-containing particles are delivered consistently and only to the animals’ inhalation zone, rather than dispersed across a enclosure.” says Bourouiba. “Our goal was to maximize repeatability and control, minimize variability, and adapt the aerosol production to the properties of the carrying fluid phase.”

Respiratory fluids are complex. They contain cells, salts and proteins in varied proportions that cause the fluids to respond unexpectedly to stretching and shearing, making the control of their fragmentation and nebulization not trivial.

“We were able to control the nebulization process because of our team’s expertise in fundamental fluid fragmentation, high-speed imaging and quantification, combined with physics expertise in out-of-equilibrium fluid instabilities,” says Bourouiba. “Our TSS is able to generate aerosols of these complex fluids in a much more controlled way, in terms of selection of droplet sizes and their content, and with equal distribution of the particles to all the animals.”

And the results indeed reflect this. The TSS generated Mtb-carrying aerosol concentration and size distribution comparable to those produced by TB patient coughs, and in a more consistent way than those produced by full-body inhalation exposure systems.

Continuum of viewpoints

The design of the TSS involved experts in animal model experiments, engineers, and fluid physicists with expertise in fluid fragmentation and spray dynamics.

“Transmission from a fluid physics and biophysical point of view has been my research focus for more than fifteen years. With the Fluids & Health Network at MIT, we have built an interdisciplinary and integrated team that can deconstruct the complex physiological processes of microorganism transport from host to host and from the microorganism’s viewpoint. This involves multiple scales of respiratory physiology, biophysics of mucosalivary rheology and atomization, and their interaction with the carried microorganisms and pathogens,” says Bourouiba. 

“The team involved expertise across multiple disciplines to make the TSS possible as a new standard and enable us to now conduct the key science of transmission with it,” adds Thomas Heldt, Richard J. Cohen (1976) Professor in Medicine and Biomedical Physics and Associate Director of IMES, and collaborator on this work. “And when it works out well, as it has, that's very rewarding not just from an intellectual standpoint but also from a clinical and public health impact standpoint.”

Solving hard problems today requires such an integrated multi-disciplinary approach. “The key to such collaborative success is hard to quantify, but what seems to matter most is the curiosity and open mind of the researchers, and their collective drive to solve difficult and important open problems that can affect millions of lives,” says Heldt.

A potential new standard

“Our paper shows how well the TSS performs against the standard full body exposure system, increasing infection efficacy and reducing its variability. And that is a key result for the field,” says Bourouiba. Such encouraging benchmark results now set the team up to conduct the next stage of studies, with more physiologically accurate fluids.

With the TSS, Bourouiba, Gengenbacher, and their team of collaborators are opening up possibilities for more efficient development and testing of novel airborne pathogen transmission blocking treatments and preventive measure that could have industrial and clinical implications for building management and drug R&D, well beyond TB. 

“By decreasing the noise in the data emerging from TB infection experiments, our TSS could significantly accelerate the rate of discovery of the actual mechanisms that underlie TB’s tremendously effective transmission through the air, and that of other airborne pathogens as well,” Bourouiba concludes.

This work was supported, in part, by: 

• The National Institute of Allergy and Infectious Diseases of the National Institutes of Health: P01AI159402 and R01AI161013
• The Weill Cornell Medicine Abby and Howard P. Milstein Program in Chemical Biology and Translational Medicine, 
• INDITEX
• NSF Center for Analysis and Prediction of Pandemic Expansion (APPEX) and NSF DBI 2412115This reflects a new, shared understanding of how respiratory infectious pathogens move from one person to the next. It also paves the way for more collaborative solutions to air transmission challenges, including appropriate animal models of infection.