Evolutionary Transition from Pathogenicity to Commensalism: Global Regulator Mutations Mediate Fitness Gains through Virulence Attenuation

In this controlled evolution experiment, we studied insipient incipient symbiotic interactions between Pseudomonas bacteria and immunocompetent and immunocompromised metazoan C. elegans hosts. Initially pathogenic bacteria evolved novel commensalistic symbioses with all host types through functionally convergent mutations involving the global regulator of virulence and quorum sensing lasR and the polymerase gene rpoB. These mutations turned the pathogens into highly fit, avirulent commensalists that achieve high prevalence in host organisms while also supported supporting higher host fitness. Commensalism may thus evolve as a side-effect of selfish bacterial adaptation to novel hosts.