Evolutionary dynamics of bacteria in a human host environment

Area of Science:

• Microbiology
• Evolutionary Biology
• Genomics

Background:

• Laboratory evolution experiments provide insights into organism adaptation and genomic evolution.
• Continuous monitoring of long-term evolution in natural systems has been limited, hindering in situ understanding.
• Pseudomonas aeruginosa is a clinically significant opportunistic pathogen often found in cystic fibrosis patients.

Purpose of the Study:

• To characterize the evolutionary dynamics of Pseudomonas aeruginosa adapting to the cystic fibrosis airways over an extended period (200,000 generations).
• To estimate bacterial mutation rates in a natural host environment.
• To compare in vivo evolution with in vitro evolution experiments.

Main Methods:

• Longitudinal monitoring of a Pseudomonas aeruginosa lineage within multiple cystic fibrosis patients.
• Genomic analysis to track mutations and evolutionary trajectories.
• Estimation of mutation rates in a natural setting.

Main Results:

• Limited diversification observed despite a complex host environment, contrasting with in vitro findings.
• An initial phase of rapid adaptation driven by pleiotropic mutations, followed by genetic drift and negative selection.
• Evidence suggests the bacteria reached a major adaptive peak, differing from continuous positive selection in vitro.
• The evolved bacteria exhibit a phenotype suggesting a transition from opportunistic to primary pathogen in cystic fibrosis.

Conclusions:

• Bacterial evolution in natural systems like cystic fibrosis airways can reach adaptive peaks, characterized by limited diversification and negative selection after initial rapid adaptation.
• The study provides in situ mutation rate estimates for bacteria.
• Pseudomonas aeruginosa may transition to a primary pathogen role in cystic fibrosis patients through adaptation.