Infectious microbes have developed sophisticated means to invade host cells, outwit the body’s defenses and cause disease. Although researchers have tried to understand the complex interactions between microorganisms and the host cells they infect, one aspect of the disease process is often ignored – the physical forces that affect host-pathogen interactions and disease outcomes.
In a new study, relevant authors Cheryl Nickerson, Jennifer Barilla and colleagues showed that under conditions of low shear fluid fluids that mimic those found in microgravity culture during space flight, the food pathogen Salmonella infects 3-D models of human intestinal tissue significantly high levels. , and causes unique changes in gene expression.
This study promotes previous work by the same group, which shows that the physical shear forces of fluids, acting on both the pathogen and the host, can alter the landscape of infection.
Understanding this subtle host-pathogen interaction during infection is critical to astronaut health, especially on long space missions. Such studies also shed new light on the still largely mysterious processes of infection on earth, as low shear fluid forces are also found in some tissues of our body that pathogens infect, including the intestinal tract.
Although the team broadly described the interaction between traditionally grown Salmonella cultures of Typhimurium in cultured flasks and 3-D gut models, this study marks the first time S. Typhimurium was grown in low-shift, simulated fluid imitation and then used to infect a 3-D model of human intestinal epithelium cultured in conjunction with macrophage immune cells, key cell types targeted by salmonella during infection.
The 3-D joint intestinal culture model used in this study more accurately replicates the structure and behavior of the same tissue in the human body and is more predictable of response to infection compared to conventional laboratory cell cultures.
The results showed significant changes in intestinal 3-D gene expression after infection with both wild-type and mutant S. Typhimurium strains grown under simulated microgravity conditions. Many of these changes have occurred in genes that are known to be closely linked to the grand ability of S. typhimurium to invade and colonize host cells and avoid observation and destruction by the host’s immune system.
“The main problem limiting human space exploration is the lack of a full understanding of the impact of space travel on crew health,” Nickerson says. “This challenge will negatively affect both deep space exploration by professional astronauts and civilians involved in the rapidly expanding commercial space market in low Earth orbit. Because microbes accompany people wherever they travel and they are needed to control the balance between health and disease, understanding the relationship between space flight, immune cell function and microorganisms will be very important for understanding the risk of infectious diseases for humans ”.
Nickerson, who co-led the new study with Jennifer Baryl, is a researcher at the Center for Fundamental and Applied Microbiome Biodesign, and a professor at the ASU School of Life Sciences. The study is published in the current issue of the journal The boundaries of cellular and infectious microbiology
A life-changing force
Life on earth has diversified into an almost incomprehensibly wide range of forms, evolving in wildly dissimilar environmental conditions. But one parameter remained unchanged. During the 3.7 billion history of life on Earth, all living organisms evolved under the influence of Earth’s gravity and respond to it.
For more than 20 years, Nickerson has pioneered research into the effects of reduced space gravity microgravity on a number of pathogenic microbes and the effects on interactions with human cells and the animals they infect. She and her colleagues persistently conducted this study in both ground and space flight, the results of which helped lay the groundwork for a rapidly growing field of research, mechanobiology of infectious diseases, studying how physical forces affect infection and disease outcomes.
Among their important findings is that low-shear fluid conditions associated with low-gravity spaceflight and analog spaceflight culture are similar to those encountered by pathogens within an infected host, and that these conditions can cause unique changes in abilities Pathogenic microbes such as Salmonella aggressively infect host cells and exacerbate a disease property known as virulence.
The infectious agent studied in a new study, Salmonella Typhimurium, is a bacterial pathogen responsible for gastrointestinal diseases 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. Bacteria-contaminated foods are the main source of most of these diseases.
Salmonella infection usually causes diarrhea, fever, and stomach cramps that begin 6 hours to 6 days after infection. Sickness from the disease usually lasts 4 to 7 days. In severe cases, hospitalization may be required.
Probability of “shift”?
Cells of mammals, including humans, as well as cells of bacteria that infect them, are exposed to extracellular fluid flowing through their outer surfaces. Just as a soft flow downstream will affect the stones in the downstream of the flow differently than a raging flow, so the force of the fluid sliding over the cell surface can cause changes in the affected cells. This liquid isolation of cell surfaces is known as shear fluid.
Because spaceflight experiments are rare and access to the space exploration platform is currently limited, researchers often simulate the low-shear fluid conditions encountered by microbes during culture in spaceflight by growing cells in a liquid growth medium known as a device. rotating vessel bioreactor, or RWV. . When the cylindrical reactor rotates, the cells are kept in suspension, gently and continuously rotating in their culture medium. This process mimics the low shear fluid conditions of microgravity experienced by cells during culture in space flight.
The team also found that this level of fluid shift is relevant to the conditions that microbial cells encounter in the human gut and other tissues during infection, causing changes in gene expression that may help some pathogens better colonize host cells and avoid immune system efforts. destruction. them.
Portrait of an attacker
The study found significant changes in both gene expression and the ability to infect three-dimensional gut models with Salmonella bacteria cultured in the RWV bioreactor. Two strains of S. typhimurium, one strain of unchanged or wild type and one strain-mutant participated in these experiments.
Otherwise, the mutant strain was identical to the wild-type but lacked an important protein known as Hfq, the major regulator of the response to stress in salmonella. In previous studies, Nickerson and her team found that Hfq acts as a major regulator of salmonella infection in both space flight and analog spaceflight culture. They later discovered additional pathogens that also use Hfq to regulate their response to the same conditions.
Unexpectedly in the current study, the mutant hfq strain was still able to attach, penetrate, and survive in three-dimensional tissue models at a level comparable to the wild-type strain. According to this finding, many genes responsible for the ability of salmonella to colonize human cells, including those associated with cellular adhesion, motility, and invasion, were still activated in the mutant strain under simulated microgravity despite Hfq removal. .
From a host perspective, the 3-D intestinal joint culture model responded to salmonella infection by activating genes involved in inflammation, tissue remodeling, and wound healing at higher levels when bacteria were grown under simulated microgravity prior to use in infection studies. This was observed for both wild-type and mutant strains of the hfq pathogen.
The data from this new analogue space flight study corroborate the team’s previous findings in 2006, 2008 and 2010. In particular, a 2010 flight experiment aboard the Space Shuttle Discovery called STL-IMMUNE used the same wild-type S. Typhimurium strain to infect a 3-D model of human intestinal tissue made from the same epithelial cells used in new study. .
Several commonalities were observed between host cell responses to infection in a new analogous space flight study and those previously reported when the infection occurred in a true space flight during the STL-IMMUNE experiment. These results further reinforce RWV as an analog space flight culture system that mimics key aspects of the microbial response to true space flight culture.
“During STL-IMMUNE, we found that infection of the 3-D model of human intestinal epithelium with salmonella during space flight causes key transcriptional and proteomic biosignatures corresponding to increased infection with the pathogen,” says Baryla. “However, due to technical issues related to in-flight contamination, we have not been able to determine whether bacteria are attaching and invading tissues at higher levels. Using the RWV bioreactor as an analog space flight culture system in our current study has been a powerful tool. which allowed us to explore this experimental issue on a deeper level. “
Astronauts face a double risk of contracting infectious diseases during their missions far from Earth. The combined difficulties of space flight weaken their immune system. At the same time, some pathogens, such as salmonella, can be caused by low fluid shear conditions caused by microgravity to become more effective infectious agents.
Due to the longer space flights in the advanced planning stages and the emergence of civil space flights that are fast emerging, the protection of space travelers from infectious diseases is very important.
Studies such as the current one also help push back the veil of the infection process by revealing fundamental details that are of great importance for disease control on Earth and beyond.