Density-Dependent Modulation of the Oxidative Burst by Neutrophils in Response to Planktonic and Biofilm Growing Pseudomonas aeruginosa: When Less Is More.

The neutrophilic response to chronic Pseudomonas aeruginosa lung infection is strongly associated with collateral tissue damage impairing lung function in people with cystic fibrosis (pwCF). We recently demonstrated the concurrent presence of both biofilm bacteria and planktonic bacteria in chronically infected lungs in pwCF. However, the neutrophilic response to these distinct bacterial forms remains insufficiently characterized. To investigate this, we fractionated P. aeruginosa batch-cultures to enrich for biofilm or planktonic cells. Confocal microscopy showed that aggregates > 30 μm made up > 92.5% of biofilm biomass, while aggregates < 10 μm comprised > 95% of planktonic biomass. Viable cell numbers were validated by similar correlations between CFU/mL and intracellular DNA content. The intensity of the oxidative burst by neutrophils in response to the fractions was estimated by luminol-enhanced chemiluminescence. At low density, biofilm fractions triggered stronger responses than planktonic cells, whereas at high density, planktonic fractions induced the highest activation, indicating density-dependent modulation. These findings suggest that at low densities, biofilms may exacerbate tissue damage by amplifying neutrophil responses. Therefore, the reduced bacterial load in pwCF on ETI therapy could make the balance between biofilm and planktonic P. aeruginosa more critical for lung inflammation, though their distribution under ETI remains unknown.