Microbial cells outnumber human cells 10 to one, including many that are commensal and live in harmony with our cells. But sometimes these commensal microbes can cause disease, in particular when they leave their normal environment - such as the gut or pharynx - and spread to other sites.

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. He has two National Institute of Health awards, one from the National Institute of Allergy and Infectious Disease and the other from the National Institute on Deafness and Other Communication Disorders, to support his investigations of Kingella kingae and Haemophilus influenzae, respectively.

Both 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," Dr. St. Geme said.

H. influenzae and K. kingae are members of the normal bacterial flora; H. influenzae colonizes the nasopharynx, and K. kingae colonizes the posterior pharynx. H. influenzae does not cause influenza, but is instead associated with invasive infections and localized respiratory tract disease. K. kingae is an emerging cause of bone and joint infections in young children.

"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."

CHOP researchers Katherine Rempe, Brad Kern, and Eric Porsch, as well as Duke University's Sue Grass, Jessica McCann, and Kim Starr, have contributed to the H. influenzae and K. kingae projects.