Microbiology. We get by with a little help from our (little) friends.

T o understand diseases of the heart, kidney, lung, and other organs, medical students first learn how these systems function under normal conditions. The principal exception to this health-before-disease approach is the study of the relationship between humans and microorganisms. Remarkably, we know far less about the thousands of species that make up our intrinsic microbiota than we know about the few dozen microbes that cause disease (1). Biologists are becoming increasingly aware that animals have co-evolved, and continue to coexist, with diverse assemblages of microorganisms that are required for normal health and development (2, 3). This awareness has opened our eyes to a new biological frontier, the subject of a recent workshop that explored the influence of beneficial bacteria on animal host biology (4). We Are Not Alone Genomic and evolutionary analyses show us that we are not the single " individuals " that we think we are. Instead, we and other complex organisms are composed of an interconnected ecosystem of eukaryotic and prokaryotic cells whose interactions can best be understood in the context of community ecology. Two workshop participants , showed how mathematical mod-eling can predict the forces that shape and maintain microbial diversity in the host. In particular, activities of both the host and its microbiota create spatial and temporal niche heterogeneity, allowing complex and stable microbial consortia, such as those present in the intestinal tract, to coexist in stable assemblages. Ecological modeling may also predict how the microbial community reacts to natural processes, such as the normal developmental succession of microbiota that occurs during the matura-tion of an animal's digestive tract. Jo Handelsman (Univ. Wisconsin, Madison) and others described how the microbial communities found in insects are typically less complex than those in vertebrates. Nonetheless, these invertebrate hosts have evolved highly integrated, species-specific associations with microbes. As such, invertebrate alliances with bacteria can serve as experimentally tractable models of vertebrate host-microbe communities. Trifling with Mother Nature The tremendous impact of humans on global ecosystems such as rainforests and coral reefs is well documented, but little is known about how our activities are influencing the internal ecosystems of animals. Mounting evidence suggests that social practices and medical intervention in industrialized nations have profound and lasting repercussions. For example, Lora Hooper (Univ. Texas Southwestern Medical Center) explained the importance of indigenous mi-crobiota in the normal development of the mammalian digestive and immune systems, and how disruption of this …