S. Azimi, S. P. Diggle
In chronic pulmonary infection airways are colonized by genetically and phenotypically diverse Pseudomonas aeruginosa populations, yet almost everything known about P. aeruginosa pathogenesis has been learned from single clinical isolates or laboratory reference strains studied in isolation. Whether a diverse population behaves as the sum of its members remains unclear. We infected differentiated primary cystic fibrosis (CF) airway epithelial cells (CF-pAECs) at air-liquid interface with whole P. aeruginosa populations collected from the sputum of three adults with CF, and, in parallel, with genetic variants retrieved from the same populations. We found that host responses to whole populations were not predicted by responses to their derivative variants, and mixed populations did not exhibit the average expected response of the members. Variation in host response was dominated by the tissue-remodeling mediators vascular endothelial growth factor (VEGF) and matrix metalloproteinase-9 (MMP-9), which differed distinctly between individual variants, whereas core pro-inflammatory cytokines (IL-6, IL-8, TNF-, IFN-{gamma}) varied comparatively little. Bacterial transcriptomes recorded during infection showed the same asymmetry. Clinical populations shared a transcriptional state distinct from PAO1, and mixed populations maintained stable expression of core regulatory, secretion, and DNA-repair loci (including hfq, xcpT, and ssb), while the derivative variants grown alone exhibited differential transcriptional profiles. Interactions among co-existing lineages therefore shape both bacterial physiology and host response, suggesting that the diverse population, not the single clone, is the appropriate unit of study in chronic infection.