Center for Genomic Sciences, Allegheny General Hospital, Allegheny Singer Research Institute, Pittsburgh, PA 15212, USA. gehrlich@wpahs.org
Metazoans contain multiple complex microbial ecosystems in which the balance between host and microbe can be tipped from commensalism to pathogenicity. This transition is likely to depend both on the prevailing environmental conditions and on specific gene-gene interactions placed within the context of the entire ecosystem.