Population diversity of Pseudomonas aeruginosa

Over the last 10 years, it has become apparent that Pseudomonas aeruginosa (Pa) populations in the cystic fibrosis (CF) lung are phenotypically and genotypically diverse. However, there have been no studies specifically addressing whether diversity matters for community function in Pa populations. In this paper, we investigate how phenotypic and genomic diversity specifically impacts on traits by evolving the strain PAO1 in biofilms grown in a synthetic sputum medium for 50 days. Our study is novel in comparison to previous long-term evolution studies as (i) we have evolved Pa in a chemical environment that recapitulates CF lungs chemical environment and (ii) instead of analysis of single evolved isolates, we have focused on changes in genomic and collective phenotypes of the evolved populations.We measured social trait production and used a metagenomic approach to analyze and assess phenotypic and genomic changes over the duration of the evolution experiment. We found that (i) evolutionary trajectories were reproducible in independently evolving populations; (ii) over 60% of genomic diversity occurred within the first 10 days of selection. We then focused more specifically on quorum sensing (QS), a well-studied Pa social trait that is commonly mutated in strains isolated from CF lungs. We found that at the population level (i) evolution in sputum medium selected for decreased production of QS and QS-dependent traits due to accumulation of an SNP in the lasR gene; (ii) there was a significant correlation between lasR mutant frequency and an increase in resistance to the clinically relevant beta-lactam antibiotics ceftazidime and piperacillin/tazobactam, despite no previous antibiotic exposure.