Population level virulence in polymicrobial communities associated with chronic disease

Renewed studies of chronic infection have shifted the focus from single pathogens to multi-microbial communities as culture-independent techniques reveal complex consortia of microbes associated with chronic disease. Despite a general acceptance that some chronic diseases are caused by mixed microbial communities, areas of research exploring community interactions as they relate to the alteration of virulence are still in the early stages. Members of the NIH Human Microbiome Project have been actively characterizing the microbial communities of the skin, nasal, oral, gastrointestinal, and urogenital cavities of healthy adults. Concomitantly, several independent studies have begun to characterize the oral, nasal, sinus, upper and lower respiratory microbiomes in healthy and diseased human tissue. The interactions among the members of these polymicrobial communities have not been thoroughly explored and it is clear there is a need to identify the functional interactions that drive population-level virulence if new therapeutic approaches to chronic disease are to be developed. For example, multiple studies have examined the role of quorum sensing (QS) in microbial virulence, and QS antagonists are being developed and tested as novel therapeutics. Other potential targets include the Gram-negative type III signaling system (T3SS), type IV pili, and two component regulatory systems (TCRS). Initial results from these studies indicate limited efficacy in vivo, further suggesting that the interactions in a heterogeneous community are complex and poorly understood. If progress is to be made in the development of more effective treatments for chronic diseases, a better understanding of the composition and functional interactions that occur within multi-microbial communities must be developed.