Christina Faherty, PhD
Christina Faherty, PhD, an Assistant Investigator in the Mucosal Immunology and Biology Research Center and Assistant Professor of Pediatrics at Harvard Medical School, has focused her research career on Shigella—a pathogenic bacteria that infects the human digestive tract.
Like a detective meticulously recreating a crime scene, Faherty has used creative research techniques to capture two key elements of Shigella infection:
• The changes in gene expression and function that occur within the bacteria as it journeys through the intestinal tract
• The conditions within the digestive tract that make some individuals more vulnerable to infection and severe disease
A better understanding of both could help in developing the first-ever vaccine for Shigella, a goal that could save hundreds of thousands of young lives worldwide.
A Persistent Pathogen
In the United States and other high resource settings, most Shigella infections cause a bad case of diarrhea, fever and stomach cramps that can last for up to a week. It’s not pleasant, but it is typically not fatal.
Still, Shigella infection persists and has been readily detected in contaminated food and circulating in homeless populations throughout the country.
Worldwide, however, Shigella is a much bigger problem. It is responsible for 200,000 deaths, mostly among children under the age of five years in developing countries with limited resources and medical care.
A vaccine against Shigella could not only save lives, but also reduce the need to use antibiotics and limit the spread of treatment-resistant strains of Shigella, which have been increasing at alarming rates in recent years.
Charting a Path Towards a Shigella Vaccine
Faherty believes that the path to a successful Shigella vaccine requires a better understanding of the conditions that occur at the point of infection.
The Shigella that enters the body through the mouth, for example, likely undergoes some changes as it travels through the digestive tract, particularly after being exposed to bile in the small intestine.
Previous studies have documented Faherty’s efforts to track these changes, for example, by exposing lab-based versions of Shigella to bile to see how the bacteria changes its gene expression and function as a result.
In a recent study published in the journal Gut Microbes, Faherty and colleagues demonstrate that it is also possible to recreate the second part of the equation by using organoids to replicate the malnourished intestinal lining where Shigella often does the most damage.
Making Malnourished Organoids
First author Meryl Perlman, MD, played a key role in developing the malnourished organoids.
The intestinal lining, or epithelium, is the single layer of cells that lines the inside of the small and large intestine.
These cells form a barrier that protects the body against foreign substances.
The epithelium also plays an important role in sampling the antigens (food, pathogens, medications and allergens) that enter the digestive system through the mouth and deciding if the body should tolerate them or mount an immune response.
Intestinal organoids are miniaturized versions of organs that are created by taking adult stem cells from the intestinal lining, placing them in a petri dish and surrounding them with nutritious cell media.
The cells will then replicate and differentiate into the different types of cells that can be found in the intestinal epithelium. They will also naturally form into 3D structures that replicate the structure and function of the organ itself.
It took researchers decades of trial and error before finding the right combination of nutrients needed to sustain this process and create viable intestinal organoids, which were first introduced in the early 2000s.
“Healthy” organoids have been a valuable tool for helping to understand the structure and function of the intestinal epithelium, and Faherty and team have been using them to study infection by Shigella and other bacterial pathogens.
However, these healthy models were not providing investigators with insights into the changes that occur in cases of undernutrition.
In their quest to create malnourished organoids, Faherty and team focused on strategically reducing macronutrients in the cell media such as sugar, carbohydrates and amino acids.
The team used three sets of organoid lines from different donors and analyzed the organoids of each line in three different types of media—100%, 75% and 50% formulations.
After some trial and error, they demonstrated that media solutions containing 75% and 50% of the macronutrients typically used to support healthy organoids would preserve cell viability despite the reduction in nutrients.
Faherty and team also crosschecked their malnourished models against known markers of malnutrition in human patients to verify the cells were accurately representing malnourished conditions.
Challenging the Malnourished Cells with Shigella
Smriti Verma, PhD, captured the immunofluorescence image of the organoid shown above.
Next, to study how malnourished conditions affected the barrier formation of the epithelium and susceptibility to Shigella infection, the investigators took cells from the malnourished organoids and challenged them with Shigella bacteria in two-dimensional transwells designed to replicate the intestinal barrier.
The single epithelial layer along the intestines is designed to be a first line defense or barrier to prevent unwanted antigens from entering the body.
They found clear and measurable differences in the permeability of the malnourished epithelial cells, demonstrating reduced integrity of barrier formation.
The malnourished conditions significantly enhanced the ability of the bacteria to invade the epithelial cells, especially the cells in the 75% media.
The 50% model also showed increased susceptibility to infection, though it was harder to quantify the results due to breakdowns in barrier integrity that made it hard to determine how much of the Shigella invaded the cells through the typical pathways of infection.
The team is now exploring ways to improve upon those results, potentially by testing a 60% solution.
What's Next
Faherty notes that the research team purposely created organioids derived from the duodenum or upper part of the small intestine.
Even though Shigella typically infects the colon, the researchers were curious to see if the duodenum would be more susceptible to infection in a malnourished state.
“Since we did see that increase, the data suggest that Shigella can access other parts of the intestine in malnourished children, which is important to consider for therapeutic development for a global population.”
Faherty and team also believe the malnourished organoids can also be used to study other intestinal diseases where malnutrition may play a role.
“Our model offers a proof-of-concept approach to alter the nutritional contents of organoid media and study the subsequent effects on the epithelium,” they write.
“Our malnourished media formulations can be applied and adapted to a wide range of studies for communicable or non-communicable diseases, and we look forward to helping researchers advance this approach.”
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