Strong Shear Flow Persister Bacteria Resist Mechanical Washings on the Surfaces of Various Polymer Materials

Early biofilm and streamer formation is mediated by wall shear stress and surface wettability: A multifactorial microfluidic study

Biofilms are intricate communities of microorganisms encapsulated within a self‐produced matrix of extra‐polymeric substances (EPS), creating complex three‐dimensional structures allowing for liquid and nutrient transport through them. These aggregations offer constituent microorganisms enhanced protection from environmental stimuli—like fluid flow—and are also associated with higher resistance to antimicrobial compounds, providing a persistent cause of concern in numerous sectors like the marine (biofouling and aquaculture), medical (infections and antimicrobial resistance), dentistry (plaque on teeth), food safety, as well as causing energy loss and corrosion. Recent studies have demonstrated that biofilms interact with microplastics, often influencing their pathway to higher trophic levels. Previous research has shown that initial bacterial attachment is affected by surface properties. Using a microfluidic flow cell, we have investigated the relationship between both wall shear stress (τ_w) and surface properties (surface wettability) upon biofilm formation of two species (Cobetia marina and Pseudomonas aeruginosa). We investigated biofilm development on low‐density polyethylene (LDPE) membranes, Permanox® slides, and glass slides, using nucleic acid staining and end‐point confocal laser scanning microscopy. The results show that flow conditions affect biomass, maximum thickness, and surface area of biofilms, with higher τ_w (5.6 Pa) resulting in thinner biofilms than lower τ_w (0.2 Pa). In addition, we observed differences in biofilm development across the surfaces tested, with LDPE typically demonstrating more overall biofilm in comparison to Permanox® and glass. Moreover, we demonstrate the formation of biofilm streamers under laminar flow conditions within straight micro‐channels.

Regulation of Biofilm Exopolysaccharide Biosynthesis and Degradation in Pseudomonas aeruginosa

Microbial communities enmeshed in a matrix of macromolecules, termed as biofilms, are the natural setting of bacteria. Exopolysaccharide is a critical matrix component of biofilms. Here, we focus on biofilm matrix exopolysaccharides in Pseudomonas aeruginosa. This opportunistic pathogen can adapt to a wide range of environments and can form biofilms or aggregates in a variety of surfaces or environments, such as the lungs of people with cystic fibrosis, catheters, wounds, and contact lenses. The ability to synthesize multiple exopolysaccharides is one of the advantages that facilitate bacterial survival in different environments. P. aeruginosa can produce several exopolysaccharides, including alginate, Psl, Pel, and lipopolysaccharide. In this review, we highlight the roles of each exopolysaccharide in P. aeruginosa biofilm development and how bacteria coordinate the biosynthesis of multiple exopolysaccharides and bacterial motility. In addition, we present advances in antibiofilm strategies targeting matrix exopolysaccharides, with a focus on glycoside hydrolases.

Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Carbon Starvation Induces the Expression of PprB-Regulated Genes in Pseudomonas aeruginosa

Pseudomonas aeruginosa can cause severe infections in humans. This bacterium often adopts a biofilm lifestyle that is hard to treat. In several previous studies, the PprA-PprB two-component system (TCS), which controls the expression of type IVb pili, BapA adhesin, and CupE fimbriae, was shown to be involved in biofilm formation (M. Romero, H. Silistre, L. Lovelock, V. J. Wright, K.-G. Chan, et al., Nucleic Acids Res 46:6823-6840, 2018, https://doi.org/10.1093/nar/gky324; S. de Bentzmann, C. Giraud, C. S. Bernard, V. Calderon, F. Ewald F, et al., PLoS Pathog 8:e1003052, 2012, https://doi.org/10.1371/journal.ppat.1003052). However, signals or environmental conditions that can trigger the PprA-PprB TCS are still unknown, and the molecular mechanisms of PprB-mediated biofilm formation are poorly characterized. Here, we report that carbon starvation stress (CSS) can induce the expression of pprB and genes in the PprB regulon. CSS-induced pprB transcription is mediated by the stress response sigma factor RpoS rather than the two-component sensor PprA. We also observed a strong negative regulation of PprB on the transcription of itself. Further experiments showed that PprB overexpression greatly enhanced cell-cell adhesion (CCA) and cell-surface adhesion (CSA) in P. aeruginosa Specifically, under the background of PprB overexpression, both the BapA adhesin and CupE fimbriae displayed positive effects on CCA and CSA, while the type IVb pili showed an unexpected negative effect on CCA and no effect on CSA. In addition, expression of the PprB regulon genes were significantly increased in 3-day colony biofilms, indicating a possible carbon limitation state. The CSS-RpoS-PprB-Bap/Flp/CupE pathway identified in this study provides a new perspective on the process of biofilm formation in carbon-limited environments.IMPORTANCE Typically, the determination of the external signals that can trigger a regulatory system is crucial to understand the regulatory logic and inward function of that system. The PprA-PprB two-component system was reported to be involved in biofilm formation in Pseudomonas aeruginosa, but the signals triggering this system are unknown. In this study, we found that carbon starvation stress (CSS) induces transcription of pprB and genes in the PprB regulon through an RpoS-dependent pathway. Increased PprB expression leads to enhanced cell-cell adhesion (CCA) and cell-surface adhesion (CSA) in P. aeruginosa Both CCA and CSA are largely dependent on the Bap secretion system and are moderately dependent on the CupE fimbriae. Our findings suggest that PprB reinforces the structure of biofilms under carbon-limited conditions, and the Bap secretion system and CupE fimbriae are two potential targets for biofilm treatment.

CdrA Interactions within the Pseudomonas aeruginosa Biofilm Matrix Safeguard It from Proteolysis and Promote Cellular Packing

Biofilms are robust multicellular aggregates of bacteria that are encased in an extracellular matrix. Different bacterial species have been shown to use a range of biopolymers to build their matrices. Pseudomonas aeruginosa is a model organism for the laboratory study of biofilms, and past work has suggested that exopolysaccharides are a required matrix component. However, we found that expression of the matrix protein CdrA, in the absence of biofilm exopolysaccharides, allowed biofilm formation through the production of a CdrA-rich proteinaceous matrix. This represents a novel function for CdrA. Similar observations have been made for other species such as Escherichia coli and Staphylococcus aureus, which can utilize protein-dominant biofilm matrices. However, we found that these CdrA-containing matrices were susceptible to both exogenous and self-produced proteases. We previously reported that CdrA directly binds the biofilm matrix exopolysaccharide Psl. Now we have found that when CdrA bound to Psl, it was protected from proteolysis. Together, these results support the idea of the importance of multibiomolecular components in matrix stability and led us to propose a model in which CdrA-CdrA interactions can enhance cell-cell packing in an aggregate that is resistant to physical shear, while Psl-CdrA interactions enhance aggregate integrity in the presence of self-produced and exogenous proteases.IMPORTANCEPseudomonas aeruginosa forms multicellular aggregates or biofilms using both exopolysaccharides and the CdrA matrix adhesin. We showed for the first time that P. aeruginosa can use CdrA to build biofilms that do not require known matrix exopolysaccharides. It is appreciated that biofilm growth is protective against environmental assaults. However, little is known about how the interactions between individual matrix components aid in this protection. We found that interactions between CdrA and the exopolysaccharide Psl fortify the matrix by preventing CdrA proteolysis. When both components-CdrA and Psl-are part of the matrix, robust aggregates form that are tightly packed and protease resistant. These findings provide insight into how biofilms persist in protease-rich host environments.