Calcium promotes Pseudomonas aeruginosa adherence to airway epithelial cells during infection.

Pseudomonas aeruginosa is a key bacterial pathogen causing lethal infections in the lungs of cystic fibrosis (CF) patients. These infections involve complex interactions between P. aeruginosa and the airway mucosal epithelium, which are regulated by multiple factors including bacterial adhesion. Calcium (Ca2+), a potent cellular signal, the levels of which increase in the airway fluids during CF, was shown to alter the expression of multiple virulence factors in P. aeruginosa. However, its role in regulating the host-pathogen interactions is not well understood. This study investigates the impact of Ca2+ on the initial adherence of P. aeruginosa to lung epithelial cells and its role in regulating the expression of several adhesins, including flagellin (fliC), type IV fimbrial precursor (pilA), and galactophilic lectin (lecA). Two epithelial cell lines, A549 and CuFi-5 cells, were infected with two strains of P. aeruginosa, PAO1 and FRD1. According to quantitative adherence assays, immunofluorescence microscopy, and scanning electron microscopy, adherence of P. aeruginosa to both cell lines was enhanced at elevated Ca2+. RNA-seq and RT-qPCR showed the transcription of adhesins (fliC, pilA, and lecA) was differentially regulated by Ca2+ across bacterial strains and host cell models. Deleting lecA either completely or partially abolished Ca2+-dependent increase in adherence of FRD1. This impact was reversed by gene complementation. In agreement with the Ca2+-induced expression of fliC, the proportion of flagellated PAO1 cells increased within the bacterial populations grown in the presence of Ca2+. The observed Ca2+-dependent increase in P. aeruginosa adherence to lung epithelial cells may promote the initiation of the pathogen's infections in CF airways.