Viscoelastic properties of levan-DNA mixtures important in microbial biofilm formation as determined by micro- and macrorheology

Bayesian estimation of Pseudomonas aeruginosa viscoelastic properties based on creep responses of wild type, rugose, and mucoid variant biofilms

Pseudomonas aeruginosa biofilms are relevant for a variety of disease settings, including pulmonary infections in people with cystic fibrosis. Biofilms are initiated by individual bacteria that undergo a phenotypic switch and produce an extracellular polymeric slime (EPS). However, the viscoelastic characteristics of biofilms at different stages of formation and the contributions of different EPS constituents have not been fully explored. For this purpose, we develop and parameterize a mathematical model to study the rheological behavior of three biofilms - P. aeruginosa wild type PAO1, isogenic rugose small colony variant (RSCV), and mucoid variant biofilms against a range of experimental data. Using Bayesian inference to estimate these viscoelastic properties, we quantify the rheological characteristics of the biofilm EPS. We employ a Monte Carlo Markov Chain algorithm to estimate these properties of P. aeruginosa variant biofilms in comparison to those of wild type. This information helps us understand the rheological behavior of biofilms at different stages of their development. The mechanical properties of wild type biofilms change significantly over time and are more sensitive to small changes in their composition than the other two mutants.

Deep eutectic solvent assisted recovery of high molecular weight levan from an isolated Neobacillus pocheonensis BPSCM4

The present study demonstrates the usage of deep eutectic solvent to recover microbial levan from the clarified fermented broth. The classic ethanol precipitation method for levan recovery is expensive because ethanol can be utilized as a biofuel. Production of ethanol consumes more energy and is not easily recycled. As a result, the current work concentrates on using environmentally friendly solvents for levan recovery. Deep Eutectic Solvents (DES) are greener and can replace ethanol from the microbial polysaccharides precipitation. Thus the proposed approach is environment friendly, technically feasible, reliable and economically viable. The levan was produced from a microbial isolate of aged sugarcane molasses, recovered using traditional ethanol and proposed DES (Choline Chloride and Ethylene Glycol) assisted precipitation. The levan-producing strain was characterized and identified as Neobacillus pocheonensis BPSCM4. The DES-precipitated levan has a high molecular weight of levan, 1.54 × 106 KDa, compared with the ethanol-precipitated levan, 4.246 KDa. The high molecular weight of DES-precipitated levan is due to the low viscosity and hydrogen interaction of ChCl:EG with the levan present in the fermented broth. Further, the optimization enhanced the levan yield to 32.56 g/L when the sucrose concentration was 250 g/L.

Surface Energy and Viscoelastic Characteristics of Staphylococcus epidermidis and Cutibacterium acnes Biofilm on Commercial Skin Constructs versus agar

Biofilms are recalcitrant to both study and infectious disease treatment as it requires not only the study or management of single organism behavior, but also many dynamical interactions including but not limited to bacteria-bacteria, bacteria-host, bacteria-nutrients, and bacteria-material across multiple time scales. This study performs comparative and quantitative research of two materials used in biofilm research, TSA agar and skin epidermal, to reveal how adhesion effects viscoelastic properties of biofilms at long time scales. We show that the host surface stressors, such as wettability and surface energy, impact the biofilm's mechanical integrity and viscoelastic properties. While it is known that the bacteria-material interface influences initial biofilm formation and external stress influences mature biofilm function, this study examines the influence of the bacteria-material interface on mature biofilms. These mechanical viscoelastic properties have the potential to determine metabolite and pathogenesis pathways which means that the platform researchers use to study impacts the outcome.

eDNA Provides a Scaffold for Autoaggregation of B. subtilis in Bacterioplankton Suspension

The self-binding of bacterial cells, or autoaggregation, is, together with surface colonization, one of the first steps in the formation of a mature biofilm. In this work, the autoaggregation of B. subtilis in dilute bacterial suspensions was studied. The dynamics of cell lysis, eDNA release, and bacterial autoaggregate assembly were determined and related to the spatial autocorrelation of bacterial cells in dilute planktonic bacterial suspensions. The non-random distribution of cells was associated with an eDNA network, which stabilized the initial bacterial cell-cell aggregates. Upon the addition of DNase I, the aggregates were dispersed. The release of eDNA during cell lysis allows for the entrapment of bacterial drifters at a radius several times the size of the dying bacteria. The size of bacterial aggregates increased from 2 to about 100 μm in diameter in dilute bacterial suspensions. The results suggest that B. subtilis cells form previously unnoticed continuum of autoaggregate structures during planktonic growth.

Thermal and Rheological Properties of Gluten-Free, Starch-Based Model Systems Modified by Hydrocolloids

Obtaining good-quality gluten-free products represents a technological challenge; thus, it is important to understand how and why the addition of hydrocolloids influences the properties of starch-based products. To obtain insight into the physicochemical changes imparted by hydrocolloids on gluten-free dough, we prepared several suspensions with different corn starch/potato starch/hydroxpropyl methyl cellulose/xanthan gum/water ratios. Properties of the prepared samples were determined by differential scanning calorimetry and rheometry. Samples with different corn/potato starch ratios exhibited different thermal properties. Xanthan gum and HPMC (hydroxypropyl methyl cellulose) exhibited a strong influence on the rheological properties of the mixtures since they increased the viscosity and elasticity. HPMC and xanthan gum increased the temperature of starch gelatinization, as well as they increased the viscoelasticity of the starch model system. Although the two hydrocolloids affected the properties of starch mixtures in the same direction, the magnitude of their effects was different. Our results indicate that water availability, which plays a crucial role in the starch gelatinization process, could be modified by adding hydrocolloids such as, hydroxypropyl methyl cellulose and xanthan gum. By adding comparatively small amounts of the studied hydrocolloids to starch, one can achieve similar thermo-mechanical effects by the addition of gluten. Understanding these effects of hydrocolloids could contribute to the development of better quality gluten-free bread with optimized ingredient content.

Fructanogenic traits in halotolerant Bacillus licheniformis OK12 and their predicted functional significance

AIMS

Isolating a novel bacterial source of fructan from a saltern and analysis of its genome to better understand the possible roles of fructans in hypersaline environments.

METHODS AND RESULTS

Bacteria were isolated from crude salt samples originating from Çamaltı Saltern in Western Turkey and screened for fructanogenic traits in high-salt and sucrose-rich selective medium. Exopolysaccharide accumulated in the presence of sucrose by isolate OK12 was purified and chemically characterized via HPLC, FT-IR and NMR, which revealed that it was a levan-type fructan (β-2,6 linked homopolymer of fructose). The isolate was taxonomically classified as Bacillus licheniformis OK12 through 16S rRNA gene and whole-genome sequencing methods. Strain OK12 harbours one levansucrase and two different levanase genes, which altogether were predicted to significantly contribute to intracellular glucose and fructose pools. The isolate could withstand 15% NaCl, and thus classified as a halotolerant.

CONCLUSIONS

Fructanogenic traits in halotolerant B. licheniformis OK12 are significant due to predicted influx of glucose and fructose as a result of levan biosynthesis and levan hydrolysis, respectively.

SIGNIFICANCE AND IMPACT OF THE STUDY

Fructans from the residents of hypersaline habitats are underexplored compounds and are expected to demonstrate physicochemical properties different from their non-halophilic counterparts. Revealing fructanogenic traits in the genome of a halotolerant bacterium brings up a new perspective in physiological roles of fructans.

Apoplastic maize fructan exohydrolase Zm-6-FEH displays substrate specificity for levan and is induced by exposure to levan-producing bacteria

Fructan exohydrolases (FEHs) are structurally related to cell wall invertases. While the latter are ubiquitous in higher plants, the role of FEHs in non-fructan species has remained enigmatic. To explore possible roles of FEHs in maize, a full length putative Zm-6-FEH-encoding cDNA was cloned displaying high sequence similarity with cell wall invertases. For functional characterization, Zm-6-FEH protein was expressed in Picha pastoris and in Nicotiana benthamiana leaves. Enzyme activity of recombinant Zm-6-FEH protein showed a strong preference for levan as substrate. Expression profiling in maize seedlings revealed higher transcript amounts in the more mature leaf parts as compared to the growth zone at the base of the leaf, in good correlation with FEH enzyme activities. Subcellular localization analysis indicated Zm-6-FEH location in the apoplast. Noteworthy, incubation of leaf discs with levan and co-incubation with high levan-producing bacteria selectively up-regulated transcript levels of Zm-6-FEH, accompanied by an increase of 6-FEH enzyme activity. In summary, the results indicate that Zm-6-FEH, a novel fructan exohydrolase of a non-fructan species, may have a role in plant defense against levan-producing bacteria.

The Antimicrobial Peptide Human Beta-Defensin 2 Inhibits Biofilm Production of Pseudomonas aeruginosa Without Compromising Metabolic Activity

Biofilm production is a key virulence factor that facilitates bacterial colonization on host surfaces and is regulated by complex pathways, including quorum sensing, that also control pigment production, among others. To limit colonization, epithelial cells, as part of the first line of defense, utilize a variety of antimicrobial peptides (AMPs) including defensins. Pore formation is the best investigated mechanism for the bactericidal activity of AMPs. Considering the induction of human beta-defensin 2 (HBD2) secretion to the epithelial surface in response to bacteria and the importance of biofilm in microbial infection, we hypothesized that HBD2 has biofilm inhibitory activity. We assessed the viability and biofilm formation of a pyorubin-producing Pseudomonas aeruginosa strain in the presence and absence of HBD2 in comparison to the highly bactericidal HBD3. At nanomolar concentrations, HBD2 - independent of its chiral state - significantly reduced biofilm formation but not metabolic activity, unlike HBD3, which reduced biofilm and metabolic activity to the same degree. A similar discrepancy between biofilm inhibition and maintenance of metabolic activity was also observed in HBD2 treated Acinetobacter baumannii, another Gram-negative bacterium. There was no evidence for HBD2 interference with the regulation of biofilm production. The expression of biofilm-related genes and the extracellular accumulation of pyorubin pigment, another quorum sensing controlled product, did not differ significantly between HBD2 treated and control bacteria, and in silico modeling did not support direct binding of HBD2 to quorum sensing molecules. However, alterations in the outer membrane protein profile accompanied by surface topology changes, documented by atomic force microscopy, was observed after HBD2 treatment. This suggests that HBD2 induces structural changes that interfere with the transport of biofilm precursors into the extracellular space. Taken together, these data support a novel mechanism of biofilm inhibition by nanomolar concentrations of HBD2 that is independent of biofilm regulatory pathways.

Nonlinear rheological characteristics of single species bacterial biofilms

Bacterial biofilms in natural and artificial environments perform a wide array of beneficial or detrimental functions and exhibit resistance to physical as well as chemical perturbations. In dynamic environments, where periodic or aperiodic flows over surfaces are involved, biofilms can be subjected to large shear forces. The ability to withstand these forces, which is often attributed to the resilience of the extracellular matrix. This attribute of the extracellular matrix is referred to as viscoelasticity and is a result of self-assembly and cross-linking of multiple polymeric components that are secreted by the microbes. We aim to understand the viscoelastic characteristic of biofilms subjected to large shear forces by performing Large Amplitude Oscillatory Shear (LAOS) experiments on four species of bacterial biofilms: Bacillus subtilis, Comamonas denitrificans, Pseudomonas fluorescens and Pseudomonas aeruginosa. We find that nonlinear viscoelastic measures such as intracycle strain stiffening and intracycle shear thickening for each of the tested species, exhibit subtle or distinct differences in the plot of strain amplitude versus frequency (Pipkin diagram). The biofilms also exhibit variability in the onset of nonlinear behaviour and energy dissipation characteristics, which could be a result of heterogeneity of the extracellular matrix constituents of the different biofilms. The results provide insight into the nonlinear rheological behaviour of biofilms as they are subjected to large strains or strain rates; a situation that is commonly encountered in nature, but rarely investigated.

Size-Dependent Variability in Flow and Viscoelastic Behavior of Levan Produced by Gluconobacter albidus TMW 2.1191

Levan is a fructan-type exopolysaccharide which is produced by many microbes from sucrose via extracellular levansucrases. The hydrocolloid properties of levan depend on its molecular weight, while it is unknown why and to what extent levan is functionally diverse depending on its size. The aim of our study was to gain deeper insight into the size-dependent functional variability of levan. For this purpose, levans of different sizes were produced using the water kefir isolate Gluconobacter albidus TMW 2.1191 and subsequently rheologically characterized. Three levan types could be identified, which are similarly branched, but differ significantly in their molecular size and rheological properties. The smallest levan (<107 Da), produced without adjustment of the pH, exhibited Newton-like flow behavior up to a specific concentration of 25% (w/v). By contrast, larger levans (>108 Da) produced at pH ≥ 4.5 were shear-thinning, and the levan produced at pH 5.0 showed a gel-like behavior at 5% (w/v). A third (intermediate) levan variant was obtained through production in buffers at pH 4.0 and exhibited the properties of a viscoelastic fluid up to concentrations of 15% (w/v). Our study reveals that the rheological properties of levan are determined by its size and polydispersity, rather than by the amount of levan used or the structural composition.

Regulating, Measuring, and Modeling the Viscoelasticity of Bacterial Biofilms

Biofilms occur in a broad range of environments under heterogeneous physicochemical conditions, such as in bioremediation plants, on surfaces of biomedical implants, and in the lungs of cystic fibrosis patients. In these scenarios, biofilms are subjected to shear forces, but the mechanical integrity of these aggregates often prevents their disruption or dispersal. Biofilms' physical robustness is the result of the multiple biopolymers secreted by constituent microbial cells which are also responsible for numerous biological functions. A better understanding of the role of these biopolymers and their response to dynamic forces is therefore crucial for understanding the interplay between biofilm structure and function. In this paper, we review experimental techniques in rheology, which help quantify the viscoelasticity of biofilms, and modeling approaches from soft matter physics that can assist our understanding of the rheological properties. We describe how these methods could be combined with synthetic biology approaches to control and investigate the effects of secreted polymers on the physical properties of biofilms. We argue that without an integrated approach of the three disciplines, the links between genetics, composition, and interaction of matrix biopolymers and the viscoelastic properties of biofilms will be much harder to uncover.

Testing Anti-Biofilm Polymeric Surfaces: Where to Start?

Present day awareness of biofilm colonization on polymeric surfaces has prompted the scientific community to develop an ever-increasing number of new materials with anti-biofilm features. However, compared to the large amount of work put into discovering potent biofilm inhibitors, only a small number of papers deal with their validation, a critical step in the translation of research into practical applications. This is due to the lack of standardized testing methods and/or of well-controlled in vivo studies that show biofilm prevention on polymeric surfaces; furthermore, there has been little correlation with the reduced incidence of material deterioration. Here an overview of the most common methods for studying biofilms and for testing the anti-biofilm properties of new surfaces is provided.

Viscoelastic response of Escherichia coli biofilms to genetically altered expression of extracellular matrix components

How the viscoelastic properties of the extracellular matrix affect the various biological functions conferred by biofilms is an important question in microbiology. In this study, the viscoelastic response of Escherichia coli biofilms to the genetically altered expression of extracellular matrix components was studied. Biofilms of the wild type E. coli MG1655 and its mutant strains producing different amounts of extracellular matrix components (curli, colanic acid, and poly-β-1,6-N-acetyl-d-glucosamine) were used to examine the viscoelastic behavior of biofilms grown at the solid-atmosphere interface. The results suggest that the presence of curli proteins dominates biofilm mechanical behavior. The rheological data indicate that the cohesive energy of the biofilm was the highest in the wild type strain. The results demonstrate the importance of extracellular matrix composition for biofilm mechanical properties. We propose that by genetically altering the expression of extracellular matrix polymers, bacteria are able to modulate the mechanical properties of their local environment in accordance with bulk environmental conditions.

Mechanism of bacterial adhesion on ultrafiltration membrane modified by natural antimicrobial polymers (chitosan) and combination with activated carbon (PAC)

Bacterial adhesion to surfaces is related to several factors, such as surface charge, surface energy, and substrate characteristics (leading to the formation of biofilms). Organisms are dominant in most environmental, industrial, and medical problems and processes that are of interest to microbiologists. Biofilm cells are at least 500 times more resistant to antibacterial agents compared to planktonic cells. The usage of ultrafiltration membranes is fast becoming popular for water treatment. Membrane lifetime and permeate flux are primarily affected by the phenomena of microbial accumulation and fouling at the membrane’s surface. This review intends to understand the mechanism of membrane fouling by bacterial attachment on polymeric ultrafiltration membrane modified by natural antimicrobial polymers (chitosan) combined with powder activated carbon. Also, to guide future research on membrane water treatment processes, adhesion prediction using the extended Derjaguin-Landau-Verwey-Overbeek theory is discussed.

Biofilm Surface Density Determines Biocide Effectiveness

High resistance of biofilms for chemical challenges is a serious industrial and medical problem. In this work a gradient of surface covered with biofilm has been produced and correlated to the effectiveness of different commercially available oxidative biocides. The results for thin Escherichia coli biofilms grown in rich media supplemented with glucose or lactose on glass or poly methyl methacrylate surfaces indicate that the effectiveness of hydrogen peroxide or chlorine dioxide and quaternary ammonium compounds is inversely proportional to the fraction of the surface covered with the biofilm. In areas where biofilm covered more than 90% of the available surface the biocide treatment was inefficient after 60 min of incubation. The combined effect of oxidant and surfactant increased the effectiveness of the biocide. On the other hand, the increased biofilm viscoelasticity reduced biocide effectiveness. The results emphasize differential biocide effectiveness depending on the fraction of the attached bacterial cells. The results suggest that biofilm biocide resistance is an acquired property that increases with biofilm maturation. The more dense sessile structures present lower log reductions compared to less dense ones.

An early mechanical coupling of planktonic bacteria in dilute suspensions

It is generally accepted that planktonic bacteria in dilute suspensions are not mechanically coupled and do not show correlated motion. The mechanical coupling of cells is a trait that develops upon transition into a biofilm, a microbial community of self-aggregated bacterial cells. Here we employ optical tweezers to show that bacteria in dilute suspensions are mechanically coupled and show long-range correlated motion. The strength of the coupling increases with the growth of liquid bacterial culture. The matrix responsible for the mechanical coupling is composed of cell debris and extracellular polymer material. The fragile network connecting cells behaves as viscoelastic liquid of entangled extracellular polymers. Our findings point to physical connections between bacteria in dilute bacterial suspensions that may provide a mechanistic framework for understanding of biofilm formation, osmotic flow of nutrients, diffusion of signal molecules in quorum sensing, or different efficacy of antibiotic treatments at low and high bacterial densities.Planktonic bacteria are untethered to surfaces or to each other, and thus are expected to move independently when at low cell densities. Here Sretenovic et al. show, using optical tweezers, that bacteria in dilute suspensions are mechanically coupled and show long-range correlated motion.

Revealing region-specific biofilm viscoelastic properties by means of a micro-rheological approach

Particle-tracking microrheology is an in situ technique that allows quantification of biofilm material properties. It overcomes the limitations of alternative techniques such as bulk rheology or force spectroscopy by providing data on region specific material properties at any required biofilm location and can be combined with confocal microscopy and associated structural analysis. This article describes single particle tracking microrheology combined with confocal laser scanning microscopy to resolve the biofilm structure in 3 dimensions and calculate the creep compliances locally. Samples were analysed from Pseudomonas fluorescens biofilms that were cultivated over two timescales (24 h and 48 h) and alternate ionic conditions (with and without calcium chloride supplementation). The region-based creep compliance analysis showed that the creep compliance of biofilm void zones is the primary contributor to biofilm mechanical properties, contributing to the overall viscoelastic character.

Structure and Dynamics of a Model Polymer Mixture Mimicking a Levan-Based Bacterial Biofilm of Bacillus subtilis

In this paper, we report on the structure and dynamics of biologically important model polymer mixtures that mimic the extracellular polymeric matrix in native biofilm of Bacillus subtilis. This biofilm is rich in nonionic polysaccharide levan, but also contains other biopolymers such as DNA and proteins in small concentrations. Aiming to identify the contribution of each component to the formation of the biofilm, our investigations encompassed dynamic rheology, small-angle X-ray scattering, dynamic light scattering, microscopy, densitometry, and sound velocity measurements. As it turned out, this very powerful combination of techniques is able to provide solid results on the dynamical and structural aspects of the microbiologically and chemically complex biofilm formations. Macroscopic rheological measurements revealed that the addition of DNA to levan solution increased the viscosity, pseudoplasticity, and elasticity of the system. The addition of protein contributed similarly, but also increased the rigidity of the system. This confirms that the presence of minor biofilm components is essential for biofilm formation. DNA and proteins appear to confine levan molecules within their supramolecular structure and, in this way, restrict the role of levan to merely a filling agent. These findings were complemented by small-angle X-ray scattering data, which provided insight into the structure on a molecular scale. One of the essential goals of this work was to compare the structural properties of the native biofilm and synthetic biofilm mixture.