Marilyn J. Wells, Department of Physics, The University of Texas at Austin

"Biofilm viscoelasticity and microstructures impinge on immune clearance"

Abstract: Biofilms are communities of microbes that produce a matrix of extracellular polymeric substances (EPS) which provides protection against antibiotic treatment and immune system clearance. P. aeruginosa is an opportunistic human pathogen known for producing robust biofilms, notably in chronic wounds and in the airways of cystic fibrosis (CF) patients. An established biofilm infection is difficult to eradicate and is largely associated with poor patient prognosis in CF. The EPS matrix provides biofilms with viscoelastic properties that vary by the matrix components being produced. The primary components produced by P. aeruginosa are the polysaccharides Pel, Psl, and alginate, and extracellular DNA (eDNA). In addition to self-produced matrix components, biofilms can incorporate material from the host environment such as collagen, fibrin, and metals such as calcium and magnesium.

Biofilms are viscoelastic and possess characteristics of both solid- and fluid-like materials. On the bulk level, the mechanics of biofilms varies due to the presence of particular EPS components. Recent studies of hydrogel models for biofilms have shown that bulk rheological properties, namely elastic modulus and toughness, are predictors for phagocytic success by neutrophils. While associations have been found between these parameters and phagocytic success, micro-scale structure may also play a prominent role as biofilms are highly heterogeneous, highlighting the need for other methods to characterize the mechanics of biofilms.

Persistence of chronic P. aeruginosa infections in CF patients is largely due to alginate-overproducing (mucoid) biofilms. Calcium (Ca2+) plays an important role in the structural integrity of a biofilm by electrostatically binding with eDNA to form bacterial aggregates and unique biofilm structures. Calcium ions have also been reported to crosslink with alginate to form hydrogel-like biofilms. This work investigates the individual and combined effects of eDNA and alginate on biofilm structure, the impact of polymer-specific enzymes as mechanically compromising agents, and the role of viscoelasticity of calcium-gelled biofilms in the success of mechanical clearance by neutrophils.