Whether or not a microbe is successful at establishing an infection depends both on the microbe and the host. Scientists from Duke found that a single DNA change can allow Salmonella typhi, the bacteria that causes typhoid fever, to invade cells. That single genetic variation increased the amount of cholesterol on cell membranes that Salmonella and other bacteria use as a docking station to attach to a cell to invade it. They also found that common cholesterol-lowering drugs protected zebrafish against the infection.
The researchers think the discovery of this mutated gene that helps Salmonella infect cells may be part of the reason why some people get certain infections like typhoid fever, while some people — like Typhoid Mary — carry a bacteria in their body that never causes an infection. They believe that genetic variation that causes changes to our cells may be the path of investigation that will lead us to unravel the differences in infection susceptibility.
The research findings, by senior author Dennis C. Ko of Duke University School of Medicine and his colleagues, were published August 21 in the journal PNAS.
Few people are unfamiliar with Typhoid Mary. Mary Mallon was born in Ireland and emigrated to the US in 1883 or 1884. There, she found work as a cook for a wealthy family. That's when her trouble started. Six of the 11 people present in the household of Charles Henry Warren, her employer — a wealthy New York banker — became ill with typhoid fever. At that time, this bacteria killed 10% of those infected — usually, those people were of lower socioeconomic groups.
A sanitary engineer by the name of George Sober investigated the outbreak, concluded it was due to bad clams and published the results of his investigation in June 1907, in JAMA. Eventually, he came to realize that typhoid cases were following Mary's presence and he became the first author to describe a "healthy carrier" of Salmonella typhi in the US. By 1907, about 3000 New Yorkers had contracted typhoid fever, and scientists blamed Mary for the outbreaks. They never explained her carrier status to her. Instead, she was forced by the N.Y. Department of Health to give blood and fecal samples for testing. She tested positive for Salmonella typhi and authorities quarantined her for two years where they treated her with a barrage of preparations that included Hexamethylenamin, laxatives, Urotropin, and brewer's yeast.
After her confinement ended, against warning from the Health Commissioner, she found work as a cook in Sloane Maternity in Manhattan. In three months, she infected at least 25 people, including doctors, nurses, and staff. Two of them died. It was during this time that she became known as "Typhoid Mary." Mary was again confined on North Brother Island and remained there until her death. By the time Mary died in 1938, New York health officials had identified more than 400 other healthy carriers of Salmonella typhi. But, no one besides Mary was forced into confinement or treated as an outcast.
Modern science has taught us about disease carriers. A person who carries Salmonella typhi, has bacteria present in their stool or urine and sometimes a wound or other tissues. They can spread typhoid fever to anyone else who contacts those bodily fluids or if bacteria there finds its way into our mouth, through food, or water. About 2 to 5% of people who ingest Salmonella typhi become carriers. Ko and his team searched for a genetic reason to explain why some people are more susceptible to contracting an infectious disease, while others are not.
To find a genetic needle in a haystack, the researchers used hundreds of cells from healthy human volunteers. The scientists exposed the human cells to the same dose of Salmonella typhi. The bacteria had been tagged with a green fluorescent marker so that they could easily visualize where the bacteria was. The scientists looked to see which cells had the highest rates of bacteria entering them; then they looked for genetic differences in those cells.
One change to the individual "letters" of DNA — called nucleotides — in a single gene called VAC14 was associated with the level of bacterial invasion in cells. When cells were genetically modified to lack the gene, the bacteria invaded more easily, and the researchers saw the cells glow green as they accumulated glowing bacteria. When the VAC14 gene had a single mutation, cholesterol on cell membranes allowed Salmonella to attach to and enter the cell.
A researcher studying typhoid fever in Vietnam, Sarah Dunstan, provided her expertise to investigate the gene in humans. She tested DNA from about 1000 Vietnamese people, half of whom had typhoid fever and half of whom did not. Of those people, those with the VAC14 gene variant had an increased risk — about 38% higher — of having typhoid fever, than those without the gene mutation. Variation in the VAC14 gene increased the amount of cholesterol on the human's cell membranes. The more cholesterol present, the more bacteria invaded the cells. That finding prompted the researchers to study whether cholesterol-lowering drugs could slow the invasion. Monica Alvarez, a graduate student in Ko's lab, worked with zebrafish to test cholesterol-lowering drugs on them. She added the drugs to the fish waster, then injected the fish with Salmonella Typhi. Twenty-four hours later, 30% of the fish treated with the cholesterol-lowering drug ezetimibe were free of bacteria, compared to only 8% of the untreated fish.
Why Mary didn't develop typhoid fever from the bacteria she carried in her body or why only some of the people she came in contact with got the disease, has been a mystery for more than a century. Ko and his team may have figured out part of the puzzle. The VAC14 gene makes people more susceptible to becoming infected with Salmonella typhi. The relationship between the VAC14 gene and cholesterol may give us both a place to look for susceptibility to other infections and a target to stop those infections.
"This is just the first step. We need to try this approach in different model organisms, such as mice, and likely with different pathogens, before we can consider taking this into the clinic. What's so exciting is that our study provides a blueprint for combining different techniques for understanding why some people are more susceptible to disease than others, and what can be done about it", said Ko in a press release.
The study authors noted that entry of Ebola, chlamydia, hepatitis C virus, and malaria into cells is regulated by cholesterol on the host's cell membranes, too. The findings from this study may be applied in future work to see if the gene variant can predict the risk of other infections and whether cholesterol-lowering drugs might help prevent those diseases, too. Finding a genetic difference in people, that translates into a reason someone's cells are more susceptible to an infection, also sheds some light on why Typhoid Mary could carry Salmonella Typhi in her body but not become infected.