Abstract
Many bacteria form long-lasting associations with plants and animals, including humans. Some are mutualistic and benefit both partners; others cause infections. These relationships have shaped the evolution of life: the mitochondria in our cells originated from the irreversible association between a cell related to the Archaea and a bacterium, while chloroplasts are derived from a cyanobacterium. Some microorganisms have developed highly specialized mechanisms to colonize their hosts. However, infections can also result from the proliferation of bacteria that are normally commensal—meaning they benefit from their host without harming it—when the host’s balance is disrupted. Mutualism, commensalism, and pathogenesis thus form a continuum whose outcome depends largely on the context.
