Titel: Competitive fitness experiments in sequential P. aeruginosa isolates
ID: 144/EKP
Art: Abstractautor
Session: P1
Eukaryotic Pathogens (FG EK)

Referent: Nina Cramer (Hannover)

Abstract - Text

The colonization of CF airways with the common gram-negative bacterium P. aeruginosa is one of the few opportunities to observe the microevolution of a pathogen during chronic infection in real life. We wanted to explore if and to what extent the microevolution in the CF lungs had influenced the fitness of P. aeruginosa to grow in an aquatic habitat.

Longitudinal clonal P. aeruginosa isolates that had been collected from 12 CF patients (six patients with a mild and six with a severe course of infection), since the onset of colonisation for up to 30 years, were subjected to within-clone competition experiments. Separate precultures of serial isolates of a patient course were grown in Luria Broth (LB; nutrient rich) and minimal medium (nutrient poor, sole carbon source succinate) until exponentially phase and were then mixed in equal amounts; samples were taken at 0 h, 48 h (with continuous culturing every 12 h) and 120 h (without continuous culturing). The composition of the bacterial communities at time points 48 h and 120 h was determined by sequencing of bacterial DNA-derived amplicons spanning strain-specific SNPs. By optimizing this method as a novel approach for microbial fitness studies, intraclonal comparisons within bacterial communities become possible.

Resulting from this competitive fitness experiments there are significantly less winner strains than expected. P. aeruginosa isolates retrieved during the first years of colonization won the competition in the presence of progeny for ten of the twelve examined patient courses suggesting that P. aeruginosa adaptation to the CF lung results in gaining fitness in this special habitat but otherwise with a loss of fitness in other environments. Three different modes of fitness could be verified: (I) gradual loss of fitness during the first years of colonization, (II) a gradual loss of fitness over the whole observation time and (III) no loss of fitness or even an increase of fitness over the years. Irrespective of clone, number of acquired mutations and horizontal transfer events, the early isolates were characterized by the highest fitness. These winner strains gained more biomass when growing in the community than in single culture. They did not suppress the growth of their clonal competitors completely. Instead, they grew faster in a community of genetically highly related strains than growing alone thereby benefitting from public goods.