The human microbiome co-evolutionary continuum

Humans enter into a range of symbioses with resident and transiently colonising microbes, which span a dynamic continuum from antagonistic to mutualistic. These interactions are shaped by a complex set of selective forces, which include both host and microbially-derived selective pressures. Given the significant impact that both resident and pathogenic microbes can have on our health, there is now a move to develop a theoretical framework that may guide studies of human-microbe interactions. This should enable the deeper level of understanding required to model, predict and ultimately control human diseases related to antagonistic or aberrant host-microbe interactions. Here we explore the human-microbe coevolutionary continuum in the context of current and emerging theory, and with a focus on the opposite ends of the spectrum: mutualism and antagonism. In doing so we highlight areas in which theory is helping to enhance the understanding of this dynamic continuum and where current theory fails as well as suggesting future avenues of research.