Lethality of Pseudomonas aeruginosa cystic fibrosis isolates in Drosophila melanogaster suggests cooperativity among isolates

Cystic fibrosis (CF) patients are initially infected by Pseudomonas aeruginosa strains possessing an array of virulence determinants required to establish the infections. Subsequently, loss-of-function mutations result in an overall loss of virulence. We hypothesized that CF isolates would vary in their virulence phenotypes and have evolved mechanisms of cooperation to enhance the fitness of the bacterial population. To explore this adaptation of P. aeruginosa to the CF lung, a total of 25 CF isolates from 13 chronically infected adult patients, were analyzed in both the D. melanogaster feeding and nicking infection models and surveyed for an array of virulence determinants. Our CF isolates produced a spectrum of lethality in both of the fly models as well as virulence determinant and quorum sensing signal production; with the variability in phenotypes extending down to the sub-population level. This suggested that the phenotypically distinct sub-populations may be present in CF lung infections and may play different roles in these infections. The importance of social behavior in P. aeruginosa infection was verified with isogenic mutants that showed that functional quorum sensing systems were required for lethality in the fly feeding model but not the fly nicking model. Evidence for cooperative behavior among P. aeruginosa isolates was detected in mixed infections between PAO1 and defined quorum sensing mutants that resulted in an overall increase in the lethality. Subsequently, mixed infections with phenotypically distinct CF isolates from the same sputum sample showed one of three outcomes; no apparent interaction, an increase in overall lethality or a decrease in overall lethality. Two CF isolates were found to be responsible for attenuating virulence in other isolates. These CF isolates could also attenuate the production of protease and elastase when grown in vitro. The negative interaction appears to be due a secreted factor and many but not all CF isolates can respond to the factor. Taken together, this data may suggest that some isolates in the CF lung have distinct roles in the maintenance of chronic infections or perhaps in exacerbation of infections.