EXPERIMENTAL MODELS OF SYMBIOTIC HOST-MICROBIAL RELATIONSHIPS: UNDERSTANDING THE UNDERPINNINGS OF BENEFICENCE AND THE ORIGINS OF PATHOGENESIS

The study of beneficial associations of plants and animals with microorganisms is in its infancy. This chapter describes the very beginnings of a characterization. The understandable historical focus on determining the basis of devastating pathogenic diseases, as well as technical impediments to studying the dynamic and complex interactions between microbial communities and their hosts, have hindered one's ability to obtain an accurate understanding of the relationships of animals and plants with the microbial world. Studies of mutants in rhizobia indicate that components of the symbiont's lipopolysaccharide (LPS) are also important in inducing normal infection thread formation and the eventual release of the symbionts into host cortical cells. Overall, a remarkable number of host mechanisms used to avoid infection by pathogenic microbes, such as production of reactive oxygen species (ROS) and “antimicrobial” proteins and peptides, appear to play significant roles in the legume-rhizobium (LR) and squid-vibrio (SV) associations. Taken together, the available data on the LR and SV symbioses suggest that interactions between the partners involve molecules traditionally associated with host defense, although they appear to be modulated differently in these beneficial associations. Studies of the coevolution and coadaptation of humans and their microbial communities present an opportunity to gain new understanding about how our ancient migrations, dietary changes, and social innovations helped shaped our current biology, and how they influenced the origins of, our susceptibilities to, and the spread of various pathogenic symbioses.