The role of eDNA in the formation of biofilm streamers

Across many different habitats, bacteria are often found as sessile communities embedded in a self-secreted matrix of extracellular polymeric substances (EPS)  [1]. The biofilm matrix enhances bacterial resistance to harsh environmental conditions and antimicrobial treatments, and thus hinders our ability to remove detrimental biofilms in medical and industrial applications. Depending on the environmental conditions, biofilms can be found as tethered filaments suspended in flow, known as streamers [2], or surface-attached communities. Despite the importance of the matrix to biofilm survival, little is known about how environmental features shape its microstructure and chemical composition.

Here, we show that a laminar flow of a diluted suspension of Pseudomonas aeruginosa PA14 around a pillar can trigger the formation of suspended filamentous biofilm structures known as streamers and that extracellular DNA (eDNA) plays a fundamental structural role in streamer formation  [3]. We have developed a microfluidic setup that allows real time visualization of the formation of biofilm streamers and the investigation of their biochemical composition by means of lectins staining. Our experiments confirmed that this phenomenon is dominated by the interplay between the viscoelastic nature of EPS, which is extruded by local flow shear, and the secondary flow around the pillar, which promotes the growth of the filaments due to a filtration mechanism. By varying the composition of the biofilm matrix using mutant strains of PA14 and by applying targeted treatment with the enzyme DNase I, we could shed light on the structural role of the different biochemical components: eDNA is essential for streamers formation, while Pel, a positively charged exopolysaccharide which binds to eDNA  [4], affects the filament morphology. In addition, since in this geometry we can study freestanding biofilm filaments  [5], we could probe the shear-induced deformation of streamers to investigate their material properties and reveal that eDNA affects the elastic behaviour of the biofilm matrix, while the viscous behaviour is determine by the quantity of Pel. Finally, thanks to our mechanistic understanding of the interplay between streamers composition and microstructure, we could surprisingly promote streamers formation by adding sublethal concentration of an antibiotic commonly used to treat P. aeruginosa infections. In summary, using the experimental toolbox from biophysics to characterize the biofilm matrix, we could elucidate the relation between chemical composition and microstructure, use our understanding to control streamers formation and gain an insight on this biological system that could make an impact in the medical sector.

 

[1]      H.-C. Flemming et al., Nat. Rev. Microbiol. 14, 563 (2016).

[2]      R. Rusconi, S. Lecuyer, L. Guglielmini, and H. A. Stone, J. R. Soc. Interface 7, 1293 (2010).

[3]      E. Secchi, G. Savorana, A. Vitale, L. Eberl, R. Rusconi, and R. Stocker, paper in preparation.

[4]      L. K. Jennings et al., Proc. Natl. Acad. Sci. 112, 11353 (2015).

[5]      G. Savorana, R. Rusconi, A. Sartori, L. Heltai, R. Stocker, and E. Secchi, paper in preparation.