Evolutionary adaptations of biofilms infecting cystic fibrosis lungs promote mechanical toughness by adjusting polysaccharide production

Abstract

Biofilms are communities of microbes embedded in a matrix of extracellular polymeric substances, largely polysaccharides. Multiple types of extracellular polymeric substances can be produced by a single bacterial strain. The distinct polymer components of biofilms are known to provide chemical protection, but little is known about how distinct extracellular polysaccharides may also protect biofilms against mechanical stresses such as shear or phagocytic engulfment. Decades-long infections of Pseudomonas. aeruginosa biofilms in the lungs of cystic fibrosis patients are natural models for studies of biofilm fitness under pressure from antibiotics and the immune system. In cystic fibrosis infections, production of the extracellular polysaccharide alginate has long been known to increase with time and to chemically protect biofilms. More recently, it is being recognized that chronic cystic fibrosis infections also evolve to increase production of another extracellular polysaccharide, Psl; much less is known about Psl’s protective benefits to biofilms. We use oscillatory bulk rheology, on biofilms grown from longitudinal clinical isolates and from genetically-manipulated lab strains, to show that increased Psl stiffens biofilms and increases biofilm toughness, which is the energy cost to cause the biofilm to yield mechanically. Further, atomic force microscopy measurements reveal greater intercellular cohesion for higher Psl expression. Of the three types of extracellular polysaccharides produced by P. aeruginosa, only Psl increases the stiffness. Stiffening by Psl requires CdrA, a protein that binds to mannose groups on Psl and is a likely cross-linker for the Psl components of the biofilm matrix. We compare the elastic moduli of biofilms to the estimated stresses exerted by neutrophils during phagocytosis, and infer that increased Psl could confer a mechanical protection against phagocytic clearance.

The social network of microorganisms — how auxotrophies shape complex communities

“…indicates that these bacteria may support the B vitamin requirements of the entire community 61 . A host-associated study evaluated the long-term persistence of the microbiota after faecal transplantation in patients infected with C. difficile 62 and identified Bacteroides spp. as the most common donor microorganism in recipients after transplantation.…”

Layered structure and complex mechanochemistry of a strong bacterial adhesive

“…Since the tip/sample area increases in principle while indenting, the dependence of force on indentation is nonlinear and described by the Hertz model for small indentations (Supplementary information). Application of this model to the stiff response portion of F vs. Ind curves, corresponding to the second regime, allow estimating the average Young's modulus, E, of the core material of 0.37 ± 0.18 GPa (Table 1), comparable to that of amyloid fibers 35 , which are involved in adhesion of different bacteria 36 or of mussels byssal threads 37 ), but one and two orders of magnitude higher than the EPS from Staphylococcus epidermidis 14 and Pseudomonas aeruginosa 38 , respectively.…”

Assaying how the success of phagocytosis depends on the mechanics of a large viscoelastic target

“…It is well-known that increasing the concentration of polymer in a gel will increase the gel's elastic modulus 31 . We varied agarose concentrations from 0.3% -2% and the resulting range of elastic moduli, ~0.1 to ~10 kPa, roughly covered the range of elastic moduli that we measured previously for biofilms grown from clinical isolates of Pseudomonas aeruginosa 18 . The value of the elastic modulus at the midpoint of the plateau region in strain sweeps is the value we will use to characterize these gels' mechanics, unless the material was already yielding for the lowest strain usedin that case, the value of the elastic modulus that was measured at the lowest strain will be used ( Figure 1B).…”

“…Imaging studies of chronic biofilm infections show bacterial aggregates that are densely surrounded by neutrophils that do not enter the aggregates and are unable to clear the infection 12,13 However, biofilms are not rigid solids but are composite viscoelastic materials. We have shown that biofilms re-grown from clinical isolates have elastic moduli in the range ~0.05-10 kPa 18 . We also found that in vivo evolution in chronic Cystic Fibrosis (CF) infections changed the production of matrix polymers in a way that promoted mechanical toughness 18 .…”

“…We have shown that biofilms re-grown from clinical isolates have elastic moduli in the range ~0.05-10 kPa 18 . We also found that in vivo evolution in chronic Cystic Fibrosis (CF) infections changed the production of matrix polymers in a way that promoted mechanical toughness 18 . Others have found that neutrophils can exert an attractive stress of about 1kPa during phagocytosis 7 ; this is consistent with the forces exerted by macrophages and blood granulocytes during phagocytosis 8,[19][20][21] .…”