Chronic pulmonary pseudomonal infection in patients with cystic fibrosis: A model for early phase symbiotic evolution.

Gain of "antimicrobial resistance" and "adaptive virulence" has been the dominant view of Pseudomonas aeruginosa (Pa) in cystic fibrosis (CF) in the progressively damaged host airway over the course of this chronic infection. However, the pathogenic effects of CF airway-adapted Pa strains are notably reduced. We propose that CF Pa and other bacterial cohabitants undergo host adaptation which resembles the changes found in bacterial symbionts in animal hosts. Development of clonally selected and intraspecific isogenic Pa strains which display divergent colony morphology, growth rate, auxotrophy, and antibiotic susceptibility in vitro suggests an adaptive sequence of infective exploitation-parasitism-symbiotic evolution driven by host defenses. Most importantly, the emergence of CF pseudomonal auxotrophy is frequently associated with a few specific amino acids. The selective retention or loss of specific amino acid biosynthesis in CF-adapted Pa reflects bacterium-host symbiosis and coevolution during chronic infection, not nutrient availability. This principle also argues against the long-standing concept of dietary availability leading to evolution of essential amino acid requirements in humans. A novel model of pseudomonal adaptation through multicellular bacterial syntrophy is proposed to explain early events in bacterial gene decay and decreased (not increased) virulence due to symbiotic response to host defense.