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Science Capsule: Olfactory System Detects Bacteria

In addition to detecting smells, our nose conditions the air we breathe: warming, moistening, and cleaning it as it passes specialized sections of nasal lining called respiratory epithelium (RE). The RE also contains solitary chemosensory cells, which detect irritants in the air and send this message to the brain via the trigeminal nerve. One result of trigeminal activation is the ejection of the irritant via a cough or sneeze. NIDCD-supported scientists were surprised, however, to identify chemosensory cells bearing bitter taste receptors (from a group of related genes called T2R) in the RE. This discovery aroused their scientific curiosity—why would cells in the nose need to detect something that tastes bitter?

In their attempts to answer this question, the scientists may now have identified a new role for the nose: a first-line defender against disease-causing bacteria. They tested whether the bitter taste-detecting cells responded to special bitter molecules—called quorum sensing molecules—that bacteria use to let each other know when their numbers are high enough to establish a long-term infection. In some cases, the bacteria may form a biofilm, a sturdy bacterial structure that attacks respiratory tissue and is resistant to immune defenses. Using mice, they demonstrated that solitary chemosensory cells in the nose do respond to quorum sensing molecules in concentrations required for forming biofilms, and activate the trigeminal nerve. The trigeminal nerve fiber in the nose then initiates an inflammatory immune response—the blood vessels in the area become leaky to allow white blood cells to attack the bacteria, and the amount of air entering the nose is reduced—to restrict the entry of any new bacteria. In this way, the chemosensory cells in the nose set off the alarm to alert the immune system of a bacterial attack.

The current experiments tested quorum sensing molecules found in mice on mouse T2R-expressing chemosensory cells. The scientists now plan to test whether human T2R-expressing chemosensory cells respond to quorum sensing molecules from bacteria that typically invade the human nose.