Origins of symbiosis: shared mechanisms underlying microbial pathogenesis, commensalism and mutualism of plants and animals.

Regardless of the outcome of symbiosis, whether it is pathogenic, mutualistic or commensal, bacteria must first colonize their hosts. Intriguingly, closely-related bacteria that colonize diverse hosts with diverse outcomes of symbiosis have conserved host-association or virulence factors. This review describes commonalities in the process of becoming host associated amongst bacteria with diverse lifestyles. Whether a pathogen, commensal or mutualist, bacteria must sense the presence of and migrate towards a host, compete for space and nutrients with other microbes, evade the host immune system, and change their physiology to enable long-term host association. We primarily focus on well-studied taxa, such as Pseudomonas, that associate with diverse model plant and animal hosts, with far-ranging symbiotic outcomes. Given the importance of opportunistic pathogens and chronic infections in both human health and agriculture, understanding the mechanisms that facilitate symbiotic relationships between bacteria and their hosts will help inform the development of disease treatments for both humans, and the plants we eat.