Strain wars and the evolution of opportunistic pathogens

Bacteria live in complex communities with multiple species and strains competing with each other. Victories and defeats within these microbial wars are largely ignored unless they have a noticeable impact on the environment or the host, for example when a disease causing strain emerges as a winner. Evolutionary theory typically explains pathogen emergence as a trade-off between virulence and transmissibility. However, for opportunistic pathogens the secondary infection niche is often a dead end, as the host is either killed or cured, so a trade-off can not develop. In this context it is difficult to explain the maintenance of virulence genes in the population as they would be costly. Here, current literature is synthesized to address this apparent conundrum. The potential for adaptations to one niche to provide a benefit in another is described for some pathogenic species and this paradigm is extended to include genetic diversity and competition among individual strains. Finally, considering assemblages of strains in fluctuating immune environments with complex micro-niche structure, a scenario is presented in which commensal organisms can be primed for invasive disease should the opportunity arise.