Mary Mallon (a.k.a. "Typhoid Mary") didn't mean any harm to anybody. An Irish immigrant, she made her living for several years about a century ago by cooking for better-off families in the New York City area. Strangely, the people she cooked for kept on coming down with typhoid fever - but not Mary.
Mallon, alas, turned out to be a chronic asymptomatic carrier of Salmonella typhi, the bacterial strain that causes typhoid fever. Typhoid is a deadly disease that, while no longer a huge problem in the United States, infects tens of millions - and kills hundreds of thousands - of people around the world every year.
"She didn't know she had it," says Stanford microbiologist Denise Monack, PhD. "To all outward appearances, she was perfectly healthy."
Salmonella strains, including one called S. typhimurium, also cause food poisoning in people and pets, taking an annual human toll of 150,000 globally. While S. typhi infects only humans, closely related S. typhimurium can infect lots of mammals.
Between 1 and 6 percent of people infected with S. typhi become chronic, asymptomatic typhoid fever carriers. Nobody has known why this happens, but it's a serious public-health issue. To address this, Monack has developed an experimental mouse model that mimicks asymptomatic typhoid carriers. In a new study published in Cell Host & Microbe, she and her colleagues put that model to good effect, showing that Salmonella has a sophisticated way of messing with our immune systems. The bacteria set up house inside voracious attack cells called macrophages (from the Greek words for “big eater”). Macrophages, are known for their ability to engulf and digest pathogens and are called to the front lines of an immune assault against invading microbes. Ornery critters that they are, macrophages would seem like the last thing bacteria bent on long-term survival would want to meet.
But, as I wrote in my release about this study, a macrophage has two faces, depending on its biochemical environment:
“Early in the course of an infection,” [Monack] said, “inflammatory substances secreted by other immune cells stir macrophages into an antimicrobial frenzy. If you’re not a good pathogen, you’ll be wiped out after several days of causing symptoms.” But salmonella is one tough bug. And our bodies can’t tolerate lots of inflammation. So, after several days of inflammatory overdrive, the immune system starts switching to the secretion of anti-inflammatory factors. This shifts macrophages into a kinder, gentler mode. Thus defanged, anti-inflammatory macrophages are more suited to peaceful activities, such as wound healing, than to devouring microbes.