Rapid diversification of Pseudomonas aeruginosa in cystic fibrosis lung-like conditions

Abstract

Chronic infection of the cystic fibrosis (CF) airway by the opportunistic pathogen Pseudomonas aeruginosa is the leading cause of morbidity and mortality for adult CF patients. Prolonged infections are accompanied by adaptation of P. aeruginosa to the unique conditions of the CF lung environment, as well as marked diversification of the pathogen into phenotypically and genetically distinct strains that can coexist for years within a patient. Little is known, however, about the causes of this diversification and its impact on patient health. Here, we show experimentally that, consistent with ecological theory of diversification, the nutritional conditions of the CF airway can cause rapid and extensive diversification of P. aeruginosa Mucin, the substance responsible for the increased viscosity associated with the thick mucus layer in the CF airway, had little impact on within-population diversification but did promote divergence among populations. Furthermore, in vitro evolution recapitulated traits thought to be hallmarks of chronic infection, including reduced motility and increased biofilm formation, and the range of phenotypes observed in a collection of clinical isolates. Our results suggest that nutritional complexity and reduced dispersal can drive evolutionary diversification of P. aeruginosa independent of other features of the CF lung such as an active immune system or the presence of competing microbial species. We suggest that diversification, by generating extensive phenotypic and genetic variation on which selection can act, may be a key first step in the development of chronic infections.

Keywords: adaptation; chronic infection; diversity; experimental evolution; pathogen evolution.

Fig. 1.

Phenotypic adaptation of evolved isolates for pyoverdine production (A), biofilm formation (B), swim motility (C), and twitch motility (D). Within each treatment, each column/color represents a replicate evolved population (n = 12 populations for each treatment). Solid lines represent treatment means, and dashed lines represent the value of that phenotype in the ancestral strain (Pa14).

Fig. 2.

Euclidean distance within (A) and among (B) populations. (A) Each point represents a population with the distance within that population determined by the average of all pairwise comparisons of individuals within that population. (B) Each point represents the distance between the mean values of two populations. Solid lines represent treatment means. All distance calculations are Euclidean distance based on all 10 z-score-transformed phenotypic traits.

Fig. 3.

Comparison of clinical isolates to laboratory-evolved isolates. Each point represents the multivariate (Mahalanobis) distance between a clinical strain and the distribution of all individual isolates from replicate populations in each treatment. Mahalanobis distances are calculated for all traits (A) and excluding antibiotic resistance (AR) traits (B). Black circles are nonepidemic clinical isolates, and gray circles are epidemic isolates. Solid lines represent treatment means.