NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study was supported by Contract No. N01-OD-4-2139 (Task Order #153) between the National Academy of Sciences and the National Institutes of Health. The content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products or organizations imply endorsement by the U.S. Government.

Cover image: Confocal micrograph depicting a common behavior of bacteria (green) gathering in mucus shed by host epithelial cells (red). Such behavior has been noted in both beneficial and pathogenic associations with microbial partners. This image shows the gathering of the microbial symbiont Vibrio fischeri during its colonization of tissues of the host squid Euprymna scolopes. As described in the report, models such as the squid-vibrio system promise to provide insight into the mechanisms underlying the reciprocal dialogue between the hosts and their microbial partners, whether the relationship results in health or disease. Image courtesy of Laura Sycuro and Margaret McFall-Ngai.