How Typhoid Mary Stayed Healthy

Some people carrying chronic diseases can breeze through life, showing no symptoms of the microbes within—only to infect the people they come into contact with, occasionally with deadly results. The most infamous case of an asymptomatic disease-carrier was Typhoid Mary. In the early 1900s, Mary Mallon worked as a cook in prominent New York households. After arriving in a kitchen, however, a plague of typhoid would often sweep the household. Eventually, Mallon was forced into exile on North Brother Island on the East River, but only after she had infected around 50 people, killing three of them.

Now, researchers have come one step closer to understanding what makes some people able to carry otherwise deadly microbes without so much as a sniffle. Bacteria such as the salmonella that Mary Mallon carried, researchers found, may “hack” some of the hosts’ cells, the Los Angeles Times reports, occasionally producing asymptomatic infection.

The trickery, revealed in experiments with mice, involves a receptor protein that affects how macrophages — the body’s Pac-Man gobblers of foreign pathogens — get the energy required to survive. The team found that the bacteria tend to hang out with a mellower macrophage associated with the later stages of infection. Enough of the bacteria survive the more aggressive wave of attackers during the inflammatory phase of the immune response to settle in with the more placid anti-inflammatory cells, according to the study. Once inside, the bacteria essentially hack the genetic programming that sets off production of glucose for the host cell, and its own survival.

The researchers suspected that this protein may be involved in bestowing some patients with  asymptomatic infection, since mice infected with typhoid tend to have higher levels of those molecules. Salmonella, the researchers think, quietly invades the macrophages, then forces those cells to jump into a hospitable anti-inflammatory state and, additionally, lend the bacteria a metabolic boost. The LA Times explains the research supporting these hypotheses:

Mice whose genes were altered to be deficient in production of the transcriptional protein were a lot like Typhoid Mary — infected, but not sick. Six weeks later, levels of the tell-tale protein were nearly undetectable.

With a a better understanding of this mechanism, the researchers think, a potential therapy could be developed to block the bacteria’s ability to produce asymptomatic typhoid, which afflicts around 16 million people worldwide each year, the researchers report. Between 1 and 6 percent of those people will develop  asymptomatic typhoid. 

While this doesn’t provide a complete explanation for asymptomatic typhoid and other macrophage-targeting diseases in humans, it does hint at a potential answer for how typhoid Mary inadvertently brought havoc to New York kitchens a century ago. 

More from Smithsonian.com:

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Scientists Unleash Bacteria into a Boston Subway to Study Bioterrorism 

Rachel Nuwer | | READ MORE

Rachel Nuwer is a freelance science writer based in Brooklyn.