Abstract
As we breathe, we bring bacterial, viral, and environmental particulates into our lungs. These particulates are deposited onto the airway surface of the human lungs where a complex viscoelastic hydrogel, mucus, traps the particles. The mucus and the inhaled particulates are cleared from the lungs via coordinated ciliary beating along bronchial epithelia in a process called mucus clearance. The impairment of this upward transport is a central feature of mucus-obstructive lung disease, including chronic obstructive pulmonary disease and cystic fibrosis. In these conditions, the lungs become highly susceptible to respiratory infections, including by the clinical pathogen Pseudomonas aeruginosa. P. aeruginosa is an opportunistic bacterium that is readily cleared by mucus clearance in healthy individuals but becomes infectious in patients with mucus obstructive lung diseases, which are characterized by the formation of bacterial colonies and biofilms. It is highly resistant to antibiotics, far more so as a biofilm than within its planktonic form, and is thus dangerous to those with mucus-obstructive lung diseases. Nearly 90% of cystic fibrosis related fatalities are the result of P. aeruginosa infections. It is known that the motility of P. aeruginosa is critical for biofilm formation, yet it remains poorly understood how the bacterium transports within native mucus. Existing studies on mucus-bacteria interactions and P. aeruginosa transport within mucus largely rely on reconstituted mucus or purified mucins, which have properties dramatically different from native mucus.
In the first part of this thesis, we report the transport of P. aeruginosa strain PA14, a human clinical isolate responsible for chronic lung infections, in normal and diseased native human airway mucus. We use well-differentiated human bronchial epithelial cells cultured at the air-liquid-interface to secrete and harvest native human airway mucus with concentrations matching health and disease states. Furthermore, we develop a microdevice for quantifying the transport of individual bacterium within bulk mucus. Remarkably, highly viscoelastic normal mucus promotes directional bacterial motility at a speed comparable to that in a low-viscosity physiological buffer, in which PA14 exhibited primarily circular motion. By contrast, concentrated, pathological mucus with elasticity dominating viscosity traps bacteria, reducing their motility more than 10-fold.
We then engineer mucus simulants with decoupled viscosity and elasticity to study the role of viscoelasticity in P. aeruginosa motility. We propose that the elasticity of complex fluids induces a qualitative change of bacterial motility from circular to directional motion and then to confined motion. Our discovery provides insights into the biophysical mechanisms of bacterial infection in the lung and reveals a previously unrecognized importance of elasticity in directional bacterial transport within complex fluids.
In the third part of this thesis, we explore the role and properties of mucus in efficient mucus clearance. We verify whether a key assumption of mucus clearance is true: without the presence of mucus, ciliary beating is unable to clear away inhaled particles. We use the air-liquid interface cell culture system to quantify the transport of P. aeruginosa and other probe particles within the mucus gel layer. We replace the mucus with complex fluids, including low viscosity buffers and non-Newtonian fluids, and quantity the transport of probe particles in complex fluids. We propose that efficient ciliary transport does not require mucus but instead only requires a viscoelastic solution. We also report that ciliary transport does not influence the motility of PA and other particles within a low viscosity buffer. Finally, we report a significantly reduced displacement of muco-inert microparticles compared to muco-adhesive particles, suggesting a critical role of muco-adhesion in successful mucus clearance. Collectively, our results reveal previously unrecognized roles of mucus rheology in mucus/bacteria interactions in the contexts of mucus clearance and mucosal lung defense, bacterial motility, and active matter in complex fluids.
