It is largely unknown why influenza infections lead to an increased risk of bacterial pneumonia.
In a new study, researchers described important findings leading to so-called super-infections, which claim many lives around the world every year.
The findings could contribute to research on COVID-19.
The research was conducted by a team at Karolinska Institutet.
The Spanish Flu was an influenza pandemic that swept across the world in 1918–20 and unlike many other pandemics disproportionately hit young otherwise healthy adults.
One important reason for this was the so-called super-infections caused by bacteria, in particular pneumococci.
Influenza is caused by a virus, but the most common cause of death is secondary bacterial pneumonia rather than the influenza virus per se.
Pneumococcal infections are the most common cause of community-acquired pneumonia and a leading global cause of death.
A prior influenza virus infection sensitizes for pneumococcal infections, but mechanisms behind this increased susceptibility are not fully understood.
In the study, the team identified influenza-induced changes in the lower airways that affect the growth of pneumococci in the lungs.
Using an animal model, the researchers found that different nutrients and antioxidants, such as vitamin C and other normally cell-protective substances leak from the blood, thereby creating an environment in the lungs that favors the growth of the bacteria.
The bacteria adapt to the inflammatory environment by increasing the production of the bacterial enzyme HtrA.
The presence of HtrA weakens the immune system and promotes bacterial growth in the influenza-infected airways. The lack of HtrA stops bacterial growth.
The team says the ability of pneumococcus to grow in the lower airways during an influenza infection seems to depend on the nutrient-rich environment with its higher levels of antioxidants that occurs during a viral infection, as well as on the bacteria’s ability to adapt to the environment and protect itself from being eradicated by the immune system.
The results provide valuable information on how bacteria integrate with their environment in the lungs and could be used to find new therapies for double infections between the influenza virus and pneumococcal bacteria.
It is still not known if COVID-19 patients are also sensitive to such secondary bacterial infections, but the researchers think that similar mechanisms could potentially be found in severely ill COVID-19 patients.
One author of the study is Birgitta Henriques Normark, a professor at the Department of Microbiology, Tumor and Cell Biology.
The study is published in the Proceedings of the National Academy of Sciences.
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