Infectious microbes have developed sophisticated means to invade host cells, overcome the body’s defenses, and cause disease. Although researchers have tried to find out the complicated interactions between microorganisms and infecting host cells, one facet of the disease process has often been overlooked: the physical forces that affect host-pathogen interactions and the pathogens. disease outcomes.
In a new study, the authors Cheryl Nickerson, Jennifer Barrila and colleagues show that in conditions of low fluid shear strength that simulate those found in microgravity culture during space flight, the pathogen transmitted by food Salmonella infects 3D models of human intestinal tissue at much higher levels. , and induces unique alterations in gene expression.
This study advances the previous work of the same team which shows that the physical shear forces of the fluid acting on both the pathogen and the host can transform the landscape of the infection.
Understanding this subtle interaction of the host and pathogen during infection is critical to ensuring the health of astronauts, especially on prolonged space missions. This research also sheds new light on the still largely mysterious infection processes on earth, as low fluid shear forces are also found in certain tissues of our body that infect pathogens, including the intestinal tract.
Although the team has extensively characterized the interaction between cultures in Salmonella Typhimurium smoothie flasks and conventionally grown 3-D intestinal models, this study marks the first time that S. Typhimurium has been grown under shear conditions. low simulated microgravity fluid and then used to infect a 3-D model of cocultured human intestinal epithelium with macrophage immune cells, key cell types targeted by Salmonella during infection.
The intestinal 3D co-culture model used in this study more accurately replicates the structure and behavior of the same tissue within the human body and is more predictive of responses to infection, compared to laboratory cell cultures. conventional.
The results showed dramatic changes in gene expression of 3-D intestinal cells after infection with wild-type S. typhimurium strains and mutants cultured under simulated microgravity conditions. Many of these changes occurred in genes known to be closely involved with S. Typhimurium’s prodigious ability to invade and colonize host cells and escape surveillance and destruction by the host’s immune system.
“A major challenge limiting human exploration of space is the lack of a full understanding of the impact of space travel on the health of the crew,” says Nickerson. “This challenge will have a negative impact on the exploration of deep space by professional astronauts, as well as civilians participating in the rapidly expanding commercial space market in low Earth orbit. Since microbes accompany humans wherever they travel and are essential to controlling the balance between health and disease, understanding the relationship between spaceflight, the function of immune cells and microorganisms will be essential to understanding the risk of infectious diseases to humans. “
Nickerson, who co-led the new study with Jennifer Barrila, is a researcher at the Center for Fundamental and Applied Microbiomics Biodesign and is also a professor at the ASU School of Life Sciences. The research appears in the current issue of the journal Frontiers in Cellular and Infection Microbiology
Force that alters life
Life on earth has diversified into an almost incomprehensibly wide variety of forms, evolving into very different environmental conditions. However, one parameter has remained constant. Over the course of 3.7 billion years of life on Earth, all living organisms evolved under and are attracted to Earth’s gravity.
For more than 20 years, Nickerson has been a pioneer in exploring the effects of the reduced microgravity environment of spaceflight on a number of pathogenic microbes and the impact on interactions with human and animal cells that infect. She and her colleagues have conducted this research stubbornly in both terrestrial and space flight environments, the results of which helped lay the groundwork for the rapidly growing field of research, the mechanobiology of infectious diseases. the study of how physical forces affect infection and disease outcomes.
Among its important findings are that the low fluid shear conditions associated with the low-gravity environment of space flight and the analog culture of space flight are similar to those found by pathogens within the infected host, and that these conditions can induce unique changes in the ability of pathogenic microbes such as salmonella to aggressively infect host cells and aggravate the disease, a property known as virulence.
The infectious agent explored in the new study, Salmonella Typhimurium, is a bacterial pathogen responsible for gastrointestinal disease in humans and animals. Salmonella is the leading cause of death from foodborne illness in the United States. According to the CDC, Salmonella bacteria cause about 1.35 million infections, 26,500 hospitalizations, and 420 deaths in the United States each year. Foods contaminated with bacteria are the main source of most of these diseases.
Salmonella infection usually causes diarrhea, fever, and stomach cramps, starting between 6 hours and 6 days after infection. The onset of the disease usually lasts for 4 to 7 days. In severe cases, hospitalization may be necessary.
Probability of “cutting”?
Cells in mammalian organisms, including humans, as well as bacterial cells that infect them, are exposed to extracellular fluid flowing through their outer surfaces. Just as a gentle downstream current will affect the pebbles of the underlying bed in a different way than a raging torrent, so the force of the fluid sliding over the cell surfaces can cause changes to the affected cells. This liquid abrasion of cell surfaces is known as fluid shear.
Because spaceflight experiments are rare and access to the space research platform is currently limited, researchers often simulate the low fluid shear conditions encountered by microbes during culture in spaceflight by increasing cell growth. Cells in liquid growth media within a device known as a rotating wall vessel bioreactor or RWV. . As the cylindrical reactor rotates, the cells remain in suspension, rotating gently and continuously in the surrounding culture medium. This process mimics the low shear conditions of microgravity fluid that cells experience during culture in spaceflight.
The team has also shown that this level of fluid shear is relevant to the conditions that microbial cells encounter in the human gut and other tissues during infection, causing changes in gene expression that can help some pathogens to better colonize host cells and evade the immune system’s efforts to destroy them. they.
Portrait of an intruder
The study found significant changes in both gene expression and the ability to infect 3-D intestinal models by Salmonella bacteria cultured in the RWV bioreactor. These experiments involved two strains of S. typhimurium, an unaltered or wild-type strain, and a mutant strain.
The mutant strain was identical to the wild-type strain, but lacked an important protein known as Hfq, an important regulator of the stress response to Salmonella. In previous research, Nickerson and his team found that Hfq acts as a major regulator of the Salmonella infection process in both spaceflight and spaceflight analog cultures. They later discovered additional pathogens that also use Hfq to regulate their responses to these same conditions.
Unexpectedly, in the current study, the hfq mutant strain was still able to bind, invade, and survive within 3-D tissue models at levels comparable to the wild-type strain. According to this finding, many genes responsible for Salmonella’s ability to colonize human cells, including those associated with cell adhesion, motility, and invasion, were still activated in the mutant strain under microgravity conditions. simulated, despite the removal of Hfq.
From a host perspective, the 3-D intestinal coculture model responded to Salmonella infection by regulating genes involved in inflammation, tissue remodeling, and wound healing at higher levels when bacteria were cultured under simulated microgravity conditions before being used in infection studies. This was observed for both wild-type and hfq mutant strains of the pathogen.
Data from this new analog spaceflight study reinforce previous findings from the team’s 2006, 2008, and 2010 space shuttle experiments. In particular, the flight experiment conducted in 2010 aboard the space shuttle Discovery, called STL-IMMUNE, used the same wild-type strain of S. typhimurium to infect a 3-D model of human intestinal tissue made from the same epithelial cells used in the new study. .
Several commonalities were observed between host cell responses to infection in the new analog spaceflight study and those previously reported when infections occurred in actual spaceflight during the STL experiment. -IMMUNE. These results further reinforce RWV as a predictive ground-based analog space flight culture system that mimics key aspects of microbial responses to true space flight culture.
“During STL-IMMUNE, we discovered that infection of a human 3-D intestinal epithelial model by Salmonella during space flight induced key transcriptional and proteomic biosignatures that were consistent with pathogen-enhanced infection,” says Barrila. “However, due to the technical challenges of performing infections in flight, we were unable to quantify whether the bacteria were actually adhering and invading the tissue at higher levels. The use of the RWV bioreactor as an analog culture system “Space flight in our current study has been a powerful tool that has allowed us to explore this experimental issue at a deeper level.”
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