First the bad news: you're probably carrying the bacteria that cause pneumonia in your throat and nose right now.

But here's the good news: for most people, the Streptococcus pneumonia bacteria is completely harmless. Well, to be clear, it's harmless as long as it stays in your nose and throat. However, if it reaches your ears, lungs or bloodstream, then you need to watch out – that's when the bacteria becomes virulent and people get sick.

Why is Streptococcus pneumonia harmless in the nose but dangerous in the lungs? A team of scientists from the University at Buffalo (UB) and other institutions set out to find an answer. Their research was published last month in the open-access journal mBio.

"We are looking to find ways to interfere with the transition to disease," Anders P. Hakansson, assistant professor in the UB School of Medicine and Biomedical Sciences, explained in a recent press release. "Few have looked at the specific mechanism that suddenly makes these bacteria leave the nose where they typically prefer to reside and travel into the lungs or the middle ear where they cause disease. If we can understand that process, then maybe we can block it."

So what's that process? It all starts in the nose. In an earlier paper, the researchers showed that when the bacteria colonize the nose, they form stable tower-like structures called biofilm. This normally keeps the bacteria in place and in a harmless state. But these biofilms can break down under the right conditions, sending the bacteria to other parts of the body.

In a lab, the researchers grew biofilms on top of human epithelial cells (the same types of cells that line the inside of your nose and other body cavities). They then introduced the influenza A virus, infecting the bacteria. They also exposed the bacteria to the same conditions they would experience if the host human had a bad case of the flu: high temperature, increased levels of the stress hormone norepinephrine, and higher concentrations of an energy molecule called ATP. These conditions all triggered the biofilm to suddenly release the bacteria, freeing the Streptococcus pneumonia to enter other parts of the body.

The researchers say this is an example of interkingdom signaling, a process first discovered about 10 years ago, where microorganisms (in this case the bacteria) and their hosts communicate via hormones. They say this interkingdom signaling, in which the bacteria responds to conditions in the human host, is the key to understanding why and when Streptococcus pneumonia can become a virulent disease. More importantly, as Hakansson says in the UB press release, "If we can find ways to interrupt that signaling, we might be able to prevent disease."

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