Researchers explain why a common bacteria might result in a serious illness

You cannot disagree with the fact that everyone is constantly sharing the germs with those around them. But even when two people have the same infection, the resulting illnesses may be dramatically different. It might be mild for one person and life-threatening or severe for the other one.

Researchers have found out two different conditions—a genetic immunodeficiency and delayed acquired immunity—which can combine to produce a life-threatening infection.

In the research paper published in Cell, Casanova and his team have given attention to the case of a young girl with a life-threatening infection caused by a very common type of bacteria. Usually, most of us carry this microbe, known as Staphylococcus aureus, in our skin and our nostrils. They might cause minor infections, but in some people, it might result in severe disease.

This girl’s illness was yet to be diagnosed as there were no known risk factors, which would lead her to develop the acute form of the disease, and none of her family members had contracted it. This set out the researchers to dig into the bottom to find the underlying cause of her disease by searching her DNA for mutation, which may have made her more susceptible to staph disease.

The main culprit was a protein known as TIRAP, which is used by specific immune cells to flag invading bacteria.

In the experiment, they have found out that TIRAP is critical for cells in the immune system’s first line of defense against invaders. These are cells that develop before we are born, with built-in recognition systems for a host of molecules that are frequently present on the surface of invaders.

“We were sure this was the explanation for the severity of her staphylococcal disease,” says Casanova. “We thought we had it all figured out.”

But things turned weren’t as simple as it looked. When the researchers analyzed the DNA of other members of the patient’s family, the result was shocking. None of them had suffered from severe staph infections, so they had normal TIRAP genes. However, they found the opposite of all seven members of her family had the same mutation as the young patient.

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There were two main questions that remained unanswered: Why did this child get the invasive disease? And why were the rest of her family seemingly immune, even when they shared her immune-compromising mutation?

They also found out that the patient lacked antibodies against a single molecule called LTA, but the levels were normal for all of her family members.

LTA is present on the surface of staphylococcal bacteria, and normally it is recognized by immune cells in both lines of defense.

The antibodies against LTA were able to restore the function of the patient’s immune cells in culture systems, and the researchers went on to confirm their hypothesis using a mouse model of the disease.

This result explains both why the patient developed a life-threatening disease and why her family members didn’t. The illness was due to the failure in both the lines of immunity. In the family, the second layer of defense compensated for genetic defects in the first. This offers insight into how two people with the same infection, and even the same DNA, can have very different illnesses.”

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