Pseudomonas aeruginosa contracts mucus to rapidly form biofilms in tissue-engineered human airways

Bacteria predominantly live as biofilms, a lifestyle conferring protection against threats such as antibiotics and a common cause of chronic infections. Studies of biofilms are mostly performed on stiff abiotic substrates, thereby lacking physical aspects of soft human tissues. To understand how mechanical tissue properties influence biofilm biogenesis, we developed a tissue-engineered airway called AirGel, composed of a tube-shaped epithelium in an optically-clear matrix. Cells in AirGels secrete a viscoelastic mucus gel, which is the first line of defense against inhaled pathogens. By infecting AirGels with Pseudomonas aeruginosa we could visualize biofilm formation in real time. We found that biofilms form unexpectedly rapidly, within hours. A contraction of the mucus gel substrate speeds up biofilm formation. We found that P. aeruginosa caused this contraction using retractile filaments called type IV pili. As mucus contracts, bacteria are brought closer to each other, nucleating aggregation and fusion of existing clusters. Overall, we uncovered a novel mechanism of mucus-pathogen interaction.