Why Can Commensal Organisms Cause Infections?

The human body is a very crowded place. In addition to human cells — which number approximately 37.2 trillion, according to a recent Annals of Human Biology paper — there are many, many more microbial cells that live in our bodies. Indeed, the NIH’s Human Microbiome Project notes “microbial cells are estimated to outnumber human cells ten to one,” including many that are commensal and live in perfect harmony with our own cells.

But sometimes these commensal microbes can cause disease, in particular when they leave their normal environment and spread to other sites. One Children’s Hospital researcher, Joseph W. St. Geme, III, MD, CHOP’s Physician-in-Chief and Chair of the Department of Pediatrics at the University of Pennsylvania, has spent much of his career working to better understand how this transition can happen.

For many years Dr. St. Geme has been examining host-pathogen interactions, with a particular focus on the bacterium Haemophilus influenzae. Despite its somewhat misleading name, H. influenzae does not cause influenza, but is instead associated with invasive infections and localized respiratory tract disease. More recently, Dr. St. Geme has initiated studies of Kingella kingae, an emerging cause of bone and joint infections in young children.

H. influenzae and K. kingae are members of the normal bacterial flora, as H. influenzae is located in the nasopharynx and K. kingae in the posterior pharynx. “Up to 70 percent or so of children in their first few years of life are colonized at some point with each of these organisms,” Dr. Geme noted.

Both Dr. St. Geme’s H. influenzae and K. kingae research projects are “investigations of host-pathogen interactions and focus on understanding how bacteria that are common, commensal organisms, usually not associated with disease, in some circumstances produce disease,” he said.

Despite the fact that K. kingae is common in young children, the bacterium has only been appreciated as an important pathogen within the last fifteen years, and is a leading cause of bone and joint infections in children younger than 3 or 4 years of age, Dr. St. Geme said. A recent award from the National Institute of Allergy and Infectious Disease is allowing Dr. St. Geme to build on his earlier K. kingae research, with investigations that he hopes will lead to “an improved understanding of the pathogenesis of disease … and will lay the foundation for developing a capsule-based vaccine.”

“If we understand the bacterial determinants of the initial stages of infection, which are fundamental to the development of disease, we can use this information to develop new vaccines to prevent disease” Dr. St. Geme said. “Alternatively, we can use this information to develop novel antimicrobials that target bacterial factors required for infection.”