existing commensals can cause eventual bone loss even in the absence of infection with P. gingivalis, but P. gingivalis does accelerate bone loss (i.e., when P. gingivalis is present, bone loss occurs at 6 weeks versus 16 weeks). Additionally, Hajishengallis et al. (2011) showed that both P. gingivalis-induced and natural bone loss require complement.3 Complement receptor knockout mice showed no signs of bone loss even in the presence of P. gingivalis. Together, these results suggest that P. gingivalis accelerates natural bone loss by exploiting and modulating naturally existing commensal interaction with complement.
Recent, unpublished data in mice underscore the important role that commensals play in periodontal disease (Zenobia et al., manuscript in preparation). The data indicate that CXCL1 (a mouse analog of human IL-8) is expressed in both conventionally reared and germ-free mice, but that expression of CXCL2 (another mouse analogue of human IL-8) requires the presence of commensals. In mice, both CXCL1 and CXCL2 are needed for the “normal” neutrophil migration that characterizes a healthy oral environment. In humans, IL-8 is believed to be a key mediator in tissue production. Darveau concluded that we “need to know more concerning oral commensal bacteria contribution to health.” For example, which oral commensal bacteria contribute to neutrophil migration in health? Can modulation of commensal bacteria improve health in certain individuals?
Medical students today are learning how to think about microbes in a different way from when Vincent Young was a student. “Find the bug, find the drug, because the only good bug is a dead bug” was the mantra, a way of thinking that originated with Koch’s postulates (1882).5 However, microbes play a much more complex role in human health and disease than previously thought. Today, Young is teaching his students to think not about “bad” and “good” bugs, but rather good and bad communities of microbial organisms.
3 The complement system comprises about 25 proteins that work together to assist, or complement, the action of antibodies in destroying bacteria.
4 This section summarizes the presentation of Vincent Young.
5 Young’s rendition of Koch’s postulates was that the pathogen must be found in all cases of disease, the pathogen must be isolated from the host and grown in culture, the pathogen must re-create disease when given to a susceptible host, and the pathogen must be re-isolated from the experimental host.