Flagella but not type IV pili are involved in the initial adhesion of Pseudomonas aeruginosa PAO1 to hydrophobic or superhydrophobic surfaces

Distribution of extracellular adhesins in environmental biofilms and flocs: Reimagining the microbial structure

Extracellular cellular adhesins facilitate microbial aggregation; however, most of the information about extracellular adhesins is based on pure culture studies. In this study, we characterized the hydrophobic characteristics and distribution of the extracellular adhesins in environmental biofilms and flocs. The hydrophobic characteristics of the extracellular adhesins were studied by sonicating the microbial aggregates to disperse the cells and by fractionating them using the microbial adhesion to the hydrocarbon method. Furthermore, we probed environmental biofilms and flocs using immunohistochemistry coupled with confocal laser scanning microscopy for reimaging the microbial aggregates based on extracellular adhesins. Small flocs have a relatively dispersed distribution of extracellular adhesins (flagella, fimbriae, pili, and amyloid adhesins). The stratified distribution of extracellular adhesins was observed in environmental biofilms. It was observed that the pili and amyloid adhesins were predominantly present in the core of biofilms, whereas flagella and fimbriae were present in the outer layer of the microbial aggregates. The dispersion of microbial aggregates is one of the limiting factors that challenge the sustainable application of wastewater treatment processes. Greater attention to the components of extracellular protein (such as the adhesins) is required to understand the aggregation of dispersible environmental microbial aggregates.

Meningitis caused by Aeromonas hydrophila in Oreochromis niloticus: Proteomics and druggability of virulence factors

Meningitis caused by Gram-negative bacteria is a serious public health problem, causing morbidity and mortality in both children and adults. Here, we propose a novel experimental model using Nile tilapia (Oreochromis niloticus) to study neuroinflammation. The fish were infected with Aeromonas hydrophila, and the course of infection was monitored in the peripheral blood. Septicemia was obvious in the blood, while in the brain tissue, infection of the meninges was present. The histopathological examination showed suppurative meningitis, and the cellular immune response in the brain tissue during infection was mediated by microglia. These cells were morphologically characterized and phenotyped by MHC class II markers and CD68. The increased production of TNF-α, IL-1β and iNOS supported the infiltration of these cells during the neuroinflammatory process. In the proteomic analysis of A. hydrophila isolated from brain tissue, we found chemotactic and transport proteins, proteolytic enzymes and enzymes associated with the dismutation of nitric oxide (NO), as well as motor proteins and those responsible for cell division. After characterizing the most abundant proteins during the course of infection, we investigated the druggability index of these proteins and identified promising peptide sequences as molecular targets that are similar among bacteria. Thus, these findings deepened the understanding of the pathophysiology of meningitis caused by A. hydrophila. Moreover, through the proteomics analysis, important mechanisms and pathways used by the pathogen to subvert the host response were revealed, providing insights for the development of novel antibiotics and vaccines.

Co-Expression of type 1 fimbriae and flagella in Escherichia coli: consequences for adhesion at interfaces

Escherichia coli expresses surface appendages including fimbriae, flagella, and curli, at various levels in response to environmental conditions and external stimuli. Previous studies have revealed an interplay between expression of fimbriae and flagella in several E. coli strains, but how this regulation between fimbrial and flagellar expression affects adhesion to interfaces is incompletely understood. Here, we investigate how the concurrent expression of fimbriae and flagella by engineered strains of E. coli MG1655 affects their adhesion at liquid-solid and liquid-liquid interfaces. We tune fimbrial and flagellar expression on the cell surface through plasmid-based inducible expression of the fim operon and fliC-flhDC genes. We show that increased fimbrial expression increases interfacial adhesion as well as bacteria-driven actuation of micron-sized objects. Co-expression of flagella in fimbriated bacteria, however, does not greatly affect either of these properties. Together, these results suggest that interfacial adhesion as well as motion actuated by adherent bacteria can be altered by controlling the expression of surface appendages.

Plant-derived bioactive compounds for the inhibition of biofilm formation: a comprehensive review

Biofilm formation is a widespread phenomenon that impacts different fields, including the food industry, agriculture, health care and the environment. Accordingly, there is a serious need for new methods of managing the problem of biofilm formation. Natural products have historically been a rich source of varied compounds with a wide variety of biological functions, including antibiofilm agents. In this review, we critically highlight and discuss the recent progress in understanding the antibiofilm effects of several bioactive compounds isolated from different plants, and in elucidating the underlying mechanisms of action and the factors influencing their adhesion. The literature shows that bioactive compounds have promising antibiofilm potential against both Gram-negative and Gram-positive bacterial and fungal strains, via several mechanisms of action, such as suppressing the formation of the polymer matrix, limiting O2 consumption, inhibiting microbial DNA replication, decreasing hydrophobicity of cell surfaces and blocking the quorum sensing network. This antibiofilm activity is influenced by several environmental factors, such as nutritional cues, pH values, O2 availability and temperature. This review demonstrates that several bioactive compounds could mitigate the problem of biofilm production. However, toxicological assessment and pharmacokinetic investigations of these molecules are strongly required to validate their safety.

Flagella-mediated adhesion of Escherichia coli O157:H7 to surface of stainless steel, glass and fresh produces during sublethal injury and recovery

E. coli O157:H7 can be induced into sublethally injured (SI) state by lactic acid (LA) and regain activity in nutrient environments. This research clarified the role of flagella-related genes (fliD, fliS, cheA and motA) in adhesion of E. coli O157:H7 onto stainless steel, glass, lettuce, spinach, red cabbage and cucumber during LA-induced SI and recovery by plate counting. Results of adhesion showed improper flagellar rotation caused by the deletion of motA resulting in the decreased adhesion. Motility of wildtype determined by diameter of motility halo decreased in SI state and repaired with recovery time increasing, lagging behind changes in expression of flagella-related genes. Flagellar function-impaired strains all exhibited non-motile property. Thus, we speculated that flagella-mediated motility is critical in early stage of adhesion. We also found the effects of Fe2+, Ca2+ and Mn2+ on adhesion or motility of wildtype was independent of bacterial states. However, the addition of Ca2+ and Mn2+ did not affect motility of flagellar function-impaired strains as they did on wildtype. This research provides new insights to understand the role of flagella and cations in bacterial adhesion, which will aid in development of anti-adhesion agents to reduce bio-contamination in food processing.

Bacteria in Fluid Flow

Bacteria thrive in environments rich in fluid flow, such as the gastrointestinal tract, bloodstream, aquatic systems, and the urinary tract. Despite the importance of flow, how flow affects bacterial life is underappreciated. In recent years, the combination of approaches from biology, physics, and engineering has led to a deeper understanding of how bacteria interact with flow. Here, we highlight the wide range of bacterial responses to flow, including changes in surface adhesion, motility, surface colonization, quorum sensing, virulence factor production, and gene expression. To emphasize the diversity of flow responses, we focus our review on how flow affects four ecologically distinct bacterial species: Escherichia coli, Staphylococcus aureus, Caulobacter crescentus, and Pseudomonas aeruginosa. Additionally, we present experimental approaches to precisely study bacteria in flow, discuss how only some flow responses are triggered by shear force, and provide perspective on flow-sensitive bacterial signaling.

Material Substrate Physical Properties Control Pseudomonas aeruginosa Biofilm Architecture

In the wild, bacteria are most frequently found in the form of multicellular structures called biofilms. Biofilms grow at the surface of abiotic and living materials with wide-ranging mechanical properties. The opportunistic pathogen Pseudomonas aeruginosa forms biofilms on indwelling medical devices and on soft tissues, including burn wounds and the airway mucosa. Despite the critical role of substrates in the foundation of biofilms, we still lack a clear understanding of how material mechanics regulate their architecture and the physiology of resident bacteria. Here, we demonstrate that physical properties of hydrogel material substrates define P. aeruginosa biofilm architecture. We show that hydrogel mesh size regulates twitching motility, a surface exploration mechanism priming biofilms, ultimately controlling the organization of single cells in the multicellular community. The resulting architectural transitions increase P. aeruginosa's tolerance to colistin, a last-resort antibiotic. In addition, mechanical regulation of twitching motility affects P. aeruginosa clonal lineages, so that biofilms are more mixed on relatively denser materials. Our results thereby establish material properties as a factor that dramatically affects biofilm architecture, antibiotic efficacy, and evolution of the resident population. IMPORTANCE The biofilm lifestyle is the most widespread survival strategy in the bacterial world. Pseudomonas aeruginosa biofilms cause chronic infections and are highly recalcitrant to antimicrobials. The genetic requirements allowing P. aeruginosa to grow into biofilms are known, but not the physical stimuli that regulate their formation. Despite colonizing biological tissues, investigations of biofilms on soft materials are limited. In this work, we show that biofilms take unexpected forms when growing on soft substrates. The physical properties of the material shape P. aeruginosa biofilms by regulating surface-specific twitching motility. Physical control of biofilm morphogenesis ultimately influences the resilience of biofilms to antimicrobials, linking physical environment with tolerance to treatment. Altogether, our work established that the physical properties of a surface are a critical environmental regulator of biofilm biogenesis and evolution.

Interplay of physico-chemical and mechanical bacteria-surface interactions with transport processes controls early biofilm growth: A review

Biofilms initiate when bacteria encounter and are retained on surfaces. The surface orchestrates biofilm growth through direct physico-chemical and mechanical interactions with different structures on bacterial cells and, in turn, through its influence on cell-cell interactions. Individual cells respond directly to a surface through mechanical or chemical means, initiating "surface sensing" pathways that regulate gene expression, for instance producing extra cellular matrix or altering phenotypes. The surface can also physically direct the evolving colony morphology as cells divide and grow. In either case, the physico-chemistry of the surface influences cells and cell communities through mechanisms that involve additional factors. For instance the numbers of cells arriving on a surface from solution relative to the generation of new cells by division depends on adhesion and transport kinetics, affecting early colony density and composition. Separately, the forces experienced by adhering cells depend on hydrodynamics, gravity, and the relative stiffnesses and viscoelasticity of the cells and substrate materials, affecting mechanosensing pathways. Physical chemistry and surface functionality, along with interfacial mechanics also influence cell-surface friction and control colony morphology, in particular 2D and 3D shape. This review focuses on the current understanding of the mechanisms in which physico-chemical interactions, deriving from surface functionality, impact individual cells and cell community behavior through their coupling with other interfacial processes.

Nutrient starvation intensifies chlorine disinfection-stressed biofilm formation

Bacterial surface attachment and subsequent biofilm expansion represent an essential adaptation to environmental signals and stresses, which are of great concern for many natural and engineered ecosystems. Yet the underlying mechanisms and driving forces of biofilm formation in a chlorinated and nutrient-restricted system remain sketchy. In this study, we coupled an experimental investigation and modeling simulation to understand how chlorination and nutrient limitation conspire to form biofilm using Pseudomonas aeruginosa as a model bacterium. Experimental results showed that moderate chlorination at 1.0 mg/L led to biofilm development amplified to 2.6 times of those without chlorine, while additional nutrient limitation (of 1/50-diluted or 0.4 g/L LB broth culture) achieved 4.6 times increment as compared to those of undiluted scenarios (of 20 g/L LB broth culture) with absence of chlorination after 24 h exposure. Meanwhile, intermediate chlorination stimulated instant flagellar motility and subsequently extracellular polymeric substances (EPS) secretion, particularly under limited nutrient condition (of 1/50-diluted or 0.4 g/L LB broth culture) that retarded chlorine consumption and provoked bacterial nutrient-limitation response. From our simulations, chlorine and resource levels along with associated spatio-temporal variations collectively drove bacterial cell movement and EPS excretion. Our results demonstrated that restraining nutrient intensified chlorination-excited cell movement and EPS production that reinforced biological and cell-surface interactions, thereby encouraging bacterial surface attachment and subsequent biofilm development. The findings provide the insights into the linkage of disinfectant and nutrient-regulated bacterial functional responses with consequent micro-habitats and biofilm dynamics.

Intracellular glycosyl hydrolase PslG shapes bacterial cell fate, signaling, and the biofilm development of Pseudomonas aeruginosa

Biofilm formation is one of most important causes leading to persistent infections. Exopolysaccharides are usually a main component of biofilm matrix. Genes encoding glycosyl hydrolases are often found in gene clusters that are involved in the exopolysaccharide synthesis. It remains elusive about the functions of intracellular glycosyl hydrolase and why a polysaccharide synthesis gene cluster requires a glycosyl hydrolase-encoding gene. Here, we systematically studied the physiologically relevant role of intracellular PslG, a glycosyl hydrolase whose encoding gene is co-transcribed with 15 psl genes, which is responsible for the synthesis of exopolysaccharide PSL, a key biofilm matrix polysaccharide in opportunistic pathogen Pseudomonas aeruginosa. We showed that lack of PslG or its hydrolytic activity in this opportunistic pathogen enhances the signaling function of PSL, changes the relative level of cyclic-di-GMP within daughter cells during cell division and shapes the localization of PSL on bacterial periphery, thus results in long chains of bacterial cells, fast-forming biofilm microcolonies. Our results reveal the important roles of intracellular PslG on the cell fate and biofilm development.

Spoilage of tilapia by Pseudomonas putida with different adhesion abilities

Four Pseudomonas putida strains isolated from spoiled tilapia were divided into three adhesion abilities—high, medium, and low—by an in vitro mucus model. Four strains had no significant difference in spoilage ability to the inoculated fish fillets. However, according to the in vivo experiment, the spoilage caused by the four P.putida was positively correlated with their adhesion abilities. High adhesion strains not only caused more TVB-N in chilled fish, but also activated the spoilage activity of intestinal flora. The diversity of intestinal flora and the changes in volatile components in fish were detected by high-throughput sequencing and SPME-GC/MS. The strains with high adhesion abilities significantly changed the intestinal flora, which led to a significant increase in low-grade aldehydes, indole, and esters in flesh of fish, as well as the production of a fishy and pungent odor. The intestinal adhesion ability of spoilage bacteria was considered the key factor in spoilage of chilled fish.

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A positive correlation between the intestinal adhesion ability of P.putida and the spoilage ability in vivo.

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P.putida affected the intestinal microflora and led to increase in fishy and pungent odor.

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The intestinal adhesion ability of P.putida was considered as a key factor in spoilage.

Flagellar Hook Protein FlgE Induces Microvascular Hyperpermeability via Ectopic ATP Synthase β on Endothelial Surface

We previously demonstrated the immunostimulatory efficacy of Pseudomonas aeruginosa flagellar hook protein FlgE on epithelial cells, presumably via ectopic ATP synthases or subunits ATP5B on cell membranes. Here, by using recombinant wild-type FlgE, mutant FlgE (FlgEM; bearing mutations on two postulated critical epitopes B and F), and a FlgE analog in pull-down assay, Western blotting, flow cytometry, and ELISA, actual bindings of FlgE proteins or epitope B/F peptides with ATP5B were all confirmed. Upon treatment with FlgE proteins, human umbilical vein endothelial cells (HUVECs) and SV40-immortalized murine vascular endothelial cells manifested decreased proliferation, migration, tube formation, and surface ATP production and increased apoptosis. FlgE proteins increased the permeability of HUVEC monolayers to soluble large molecules like dextran as well as to neutrophils. Immunofluorescence showed that FlgE induced clustering and conjugation of F-actin in HUVECs. In Balb/c-nude mice bearing transplanted solid tumors, FlgE proteins induced a microvascular hyperpermeability in pinna, lungs, tumor mass, and abdominal cavity. All effects observed in FlgE proteins were partially or completely impaired in FlgEM proteins or blocked by pretreatment with anti-ATP5B antibodies. Upon coculture of bacteria with HUVECs, FlgE was detectable in the membrane and cytosol of HUVECs. It was concluded that FlgE posed a pathogenic ligand of ectopic ATP5B that, upon FlgE-ATP5B coupling on endothelial cells, modulated properties and increased permeability of endothelial layers both in vitro and in vivo. The FlgE-ectopic ATP5B duo might contribute to the pathogenesis of disorders associated with bacterial infection or ectopic ATP5B-positive cells.

Antimicrobial coatings for environmental surfaces in hospitals: a potential new pillar for prevention strategies in hygiene

Recent reports provide evidence that contaminated healthcare environments represent major sources for the acquisition and transmission of pathogens. Antimicrobial coatings (AMC) may permanently and autonomously reduce the contamination of such environmental surfaces complementing standard hygiene procedures. This review provides an overview of the current status of AMC and the demands to enable a rational application of AMC in health care settings. Firstly, a suitable laboratory test norm is required that adequately quantifies the efficacy of AMC. In particular, the frequently used wet testing (e.g. ISO 22196) must be replaced by testing under realistic, dry surface conditions. Secondly, field studies should be mandatory to provide evidence for antimicrobial efficacy under real-life conditions. The antimicrobial efficacy should be correlated to the rate of nosocomial transmission at least. Thirdly, the respective AMC technology should not add additional bacterial resistance development induced by the biocidal agents and co- or cross-resistance with antibiotic substances. Lastly, the biocidal substances used in AMC should be safe for humans and the environment. These measures should help to achieve a broader acceptance for AMC in healthcare settings and beyond. Technologies like the photodynamic approach already fulfil most of these AMC requirements.

Surface Sensing and Adaptation in Bacteria

Bacteria thrive both in liquids and attached to surfaces. The concentration of bacteria on surfaces is generally much higher than in the surrounding environment, offering bacteria ample opportunity for mutualistic, symbiotic, and pathogenic interactions. To efficiently populate surfaces, they have evolved mechanisms to sense mechanical or chemical cues upon contact with solid substrata. This is of particular importance for pathogens that interact with host tissue surfaces. In this review we discuss how bacteria are able to sense surfaces and how they use this information to adapt their physiology and behavior to this new environment. We first survey mechanosensing and chemosensing mechanisms and outline how specific macromolecular structures can inform bacteria about surfaces. We then discuss how mechanical cues are converted to biochemical signals to activate specific cellular processes in a defined chronological order and describe the role of two key second messengers, c-di-GMP and cAMP, in this process.

Comparative Genomic Analyses Reveal Functional Insights Into Key Determinants of the Pathogenesis of Pectobacterium actinidiae in Kiwifruit

The Gram-negative bacterial species Pectobacterium actinidiae causes summer canker in kiwifruit plants. However, little is known about its virulence factors and mechanisms of genetic adaptation. We aimed to identify the key determinants that control the virulence of P. actinidiae in kiwifruit by genomic and functional analyses. Analysis of four P. actinidiae isolates indicated low genetic variability with an average of 98.7% genome-level sequence similarity and 82% shared protein-coding gene content. Phylogenetic analysis, based on both bulk single nucleotide polymorphisms (SNPs) and single-copy genes, revealed that P. actinidiae strains cluster into a single clade, which is closely related to the clades of P. odoriferum (species with a completely different host range). Through comparison between these two clades of strains, 746 unique core orthologs/genes were clustered in the clades of P. actinidiae, especially key virulence determinants involved in the biosynthesis of secretion systems (type III, IV, and VI), iron, flagellar structure, and the quorum-sensing system. Our results provide insights into the pathogenomics underlying the genetic diversification and evolution of pathogenicity in P. actinidiae species.

Genetic Cell-Surface Modification for Optimized Foam Fractionation

Rhamnolipids are among the glycolipids that have been investigated intensively in the last decades, mostly produced by the facultative pathogen Pseudomonas aeruginosa using plant oils as carbon source and antifoam agent. Simplification of downstream processing is envisaged using hydrophilic carbon sources, such as glucose, employing recombinant non-pathogenic Pseudomonas putida KT2440 for rhamnolipid or 3-(3-hydroxyalkanoyloxy)alkanoic acid (HAA, i.e., rhamnolipid precursors) production. However, during scale-up of the cultivation from shake flask to bioreactor, excessive foam formation hinders the use of standard fermentation protocols. In this study, the foam was guided from the reactor to a foam fractionation column to separate biosurfactants from medium and bacterial cells. Applying this integrated unit operation, the space-time yield (STY) for rhamnolipid synthesis could be increased by a factor of 2.8 (STY = 0.17 gRL/L·h) compared to the production in shake flasks. The accumulation of bacteria at the gas-liquid interface of the foam resulted in removal of whole-cell biocatalyst from the reactor with the strong consequence of reduced rhamnolipid production. To diminish the accumulation of bacteria at the gas-liquid interface, we deleted genes encoding cell-surface structures, focusing on hydrophobic proteins present on P. putida KT2440. Strains lacking, e.g., the flagellum, fimbriae, exopolysaccharides, and specific surface proteins, were tested for cell surface hydrophobicity and foam adsorption. Without flagellum or the large adhesion protein F (LapF), foam enrichment of these modified P. putida KT2440 was reduced by 23 and 51%, respectively. In a bioreactor cultivation of the non-motile strain with integrated rhamnolipid production genes, biomass enrichment in the foam was reduced by 46% compared to the reference strain. The intensification of rhamnolipid production from hydrophilic carbon sources presented here is an example for integrated strain and process engineering. This approach will become routine in the development of whole-cell catalysts for the envisaged bioeconomy. The results are discussed in the context of the importance of interacting strain and process engineering early in the development of bioprocesses.

How bacteria recognise and respond to surface contact

Bacterial biofilms can cause medical problems and issues in technical systems. While a large body of knowledge exists on the phenotypes of planktonic and of sessile cells in mature biofilms, our understanding of what happens when bacteria change from the planktonic to the sessile state is still very incomplete. Fundamental questions are unanswered: for instance, how do bacteria sense that they are in contact with a surface, and what are the very initial cellular responses to surface contact. Here, we review the current knowledge on the signals that bacteria could perceive once they attach to a surface, the signal transduction systems that could be involved in sensing the surface contact and the cellular responses that are triggered as a consequence to surface contact ultimately leading to biofilm formation. Finally, as the main obstacle in investigating the initial responses to surface contact has been the difficulty to experimentally study the dynamic response of single cells upon surface attachment, we also review recent experimental approaches that could be employed to study bacterial surface sensing, which ultimately could lead to an improved understanding of how biofilm formation could be prevented.

Bacterial nanotubes mediate bacterial growth on periodic nano-pillars

Surface topography designed to achieve spatial segregation has shown promise in delaying bacterial attachment and biofilm growth. However, the underlying mechanisms linking surface topography to the inhibition of microbial attachment and growth still remain unclear. Here, we investigated bacterial attachment, cell alignment and biofilm formation of Pseudomonas aeruginosa on periodic nano-pillar surfaces with different pillar spacing. Using fluorescence and scanning electron microscopy, bacteria were shown to align between the nanopillars. Threadlike structures ("bacterial nanotubes") protruded from the majority of bacterial cells and appeared to link cells directly with the nanopillars. Using ΔfliM and ΔpilA mutants lacking flagella or pili, respectively, we further demonstrated that cell alignment behavior within nano-pillars is independent of the flagella or pili. The presence of bacteria nanotubes was found in all cases, and is not linked to the expression of flagella or pili. We propose that bacterial nanotubes are produced to aid in cell-surface or cell-cell connections. Nano-pillars with smaller spacing appeared to enhance the extension and elongation of bacterial nanotube networks. Therefore, nano-pillars with narrow spacing can be easily overcome by nanotubes that connect isolated bacterial aggregates. Such nanotube networks may aid cell-cell communication, thereby promoting biofilm development.

Methylation of Salmonella Typhimurium flagella promotes bacterial adhesion and host cell invasion

The long external filament of bacterial flagella is composed of several thousand copies of a single protein, flagellin. Here, we explore the role played by lysine methylation of flagellin in Salmonella, which requires the methylase FliB. We show that both flagellins of Salmonella enterica serovar Typhimurium, FliC and FljB, are methylated at surface-exposed lysine residues by FliB. A Salmonella Typhimurium mutant deficient in flagellin methylation is outcompeted for gut colonization in a gastroenteritis mouse model, and methylation of flagellin promotes bacterial invasion of epithelial cells in vitro. Lysine methylation increases the surface hydrophobicity of flagellin, and enhances flagella-dependent adhesion of Salmonella to phosphatidylcholine vesicles and epithelial cells. Therefore, posttranslational methylation of flagellin facilitates adhesion of Salmonella Typhimurium to hydrophobic host cell surfaces, and contributes to efficient gut colonization and host infection.

Flagellin proteins of Salmonella flagella are methylated. Here, the authors show that flagellin methylation facilitates adhesion of Salmonella to hydrophobic host-cell surfaces, and contributes to efficient gut colonization and host infection.

Shear Stress as a Major Driver of Marine Biofilm Communities in the NW Mediterranean Sea

While marine biofilms depend on environmental conditions and substrate, little is known about the influence of hydrodynamic forces. We tested different immersion modes (dynamic, cyclic and static) in Toulon Bay (north-western Mediterranean Sea; NWMS). The static mode was also compared between Toulon and Banyuls Bays. In addition, different artificial surfaces designed to hamper cell attachment (self-polishing coating: SPC; and fouling-release coating: FRC) were compared to inert plastic. Prokaryotic community composition was affected by immersion mode, surface characteristics and site. Rhodobacteriaceae and Flavobacteriaceae dominated the biofilm community structure, with distinct genera according to surface type or immersion mode. Cell density increased with time, greatly limited by hydrodynamic forces, and supposed to delay biofilm maturation. After 1 year, a significant impact of shear stress on the taxonomic structure of the prokaryotic community developed on each surface type was observed. When surfaces contained no biocides, roughness and wettability shaped prokaryotic community structure, which was not enhanced by shear stress. Conversely, the biocidal effect of SPC surfaces, already major in static immersion mode, was amplified by the 15 knots speed. The biofilm community on SPC was 60% dissimilar to the biofilm on the other surfaces and was distinctly colonized by Sphingomonadaceae ((Alter)Erythrobacter). At Banyuls, prokaryotic community structures were more similar between the four surfaces tested than at Toulon, due possibly to a masking effect of environmental constraints, especially hydrodynamic, which was greater than in Toulon. Finally, predicted functions such as cell adhesion confirmed some of the hypotheses drawn regarding biofilm formation over the artificial surfaces tested here.

Curli production enhances clay-E. coli aggregation and sedimentation

Curli are amyloid fibrils that polymerize extracellularly from curlin, a protein that is secreted by many enteric bacteria and is important for biofilm formation. Presented here is a systematic study of the effects of curli on bacteria-clay interactions. The aggregation trends of curli-producing and curli-deficient bacteria with clay minerals were followed using gradient-sedimentation experiments, Lumisizer measurements, bright-field and electron microscopy. The results revealed that curli-producing bacteria auto-aggregated into high-density flocs (1.23 g/cm³), ranging in size from 10 to 50 μm, that settle spontaneously. In contrast, curli-deficient bacteria remained relatively stable in solution as individual cells (1-2 μm, 1.18 g/cm³), even at high ionic strength (350 mM). The stability of clay suspensions mixed with curli-deficient bacteria depended on clay type and ionic strength, the general trends being consistent with the classic DLVO theory. However, suspensions of curli-producing bacteria mixed with clays were highly unstable regardless of clay type and solution chemistry, suggesting extensive interactions between the clays and the bacteria-curli aggregates. SEM measurements revealed interesting differences in morphologies of the aggregates; montmorillonite particles coated the bacterial auto-aggregates whereas the kaolinite platelets were embedded within the larger curli-bacteria aggregates. These new observations regarding the densities, aggregation trends, and morphologies of bacteria-curli and bacteria-curli-clay complexes make it clear that production of surface appendages, such as curli, need to be considered when addressing the fate, activity and transport of bacteria - particularly in aquatic environments.

Physico-chemistry from initial bacterial adhesion to surface-programmed biofilm growth

Biofilm formation is initiated by adhesion of individual bacteria to a surface. However, surface adhesion alone is not sufficient to form the complex community architecture of a biofilm. Surface-sensing creates bacterial awareness of their adhering state on the surface and is essential to initiate the phenotypic and genotypic changes that characterize the transition from initial bacterial adhesion to a biofilm. Physico-chemistry has been frequently applied to explain initial bacterial adhesion phenomena, including bacterial mass transport, role of substratum surface properties in initial adhesion and the transition from reversible to irreversible adhesion. However, also emergent biofilm properties, such as production of extracellular-polymeric-substances (EPS), can be surface-programmed. This review presents a four-step, comprehensive description of the role of physico-chemistry from initial bacterial adhesion to surface-programmed biofilm growth: (1) bacterial mass transport towards a surface, (2) reversible bacterial adhesion and (3) transition to irreversible adhesion and (4) cell wall deformation and associated emergent properties. Bacterial transport mostly occurs from sedimentation or convective-diffusion, while initial bacterial adhesion can be described by surface thermodynamic and Derjaguin-Landau-Verwey-Overbeek (DLVO)-analyses, considering bacteria as smooth, inert colloidal particles. DLVO-analyses however, require precise indication of the bacterial cell surface, which is impossible due to the presence of bacterial surface tethers, creating a multi-scale roughness that impedes proper definition of the interaction distance in DLVO-analyses. Application of surface thermodynamics is also difficult, because initial bacterial adhesion is only an equilibrium phenomenon for a short period of time, when bacteria are attached to a substratum surface through few surface tethers. Physico-chemical bond-strengthening occurs in several minutes leading to irreversible adhesion due to progressive removal of interfacial water, conformational changes in cell surface proteins, re-orientation of bacteria on a surface and the progressive involvement of more tethers in adhesion. After initial bond-strengthening, adhesion forces arising from a substratum surface cause nanoscopic deformation of the bacterial cell wall against the elasticity of the rigid peptidoglycan layer positioned in the cell wall and the intracellular pressure of the cytoplasm. Cell wall deformation not only increases the contact area with a substratum surface, presenting another physico-chemical bond-strengthening mechanism, but is also accompanied by membrane surface tension changes. Membrane-located sensor molecules subsequently react to control emergent phenotypic and genotypic properties in biofilms, most notably adhesion-associated ones like EPS production. Moreover, also bacterial efflux pump systems may be activated or mechano-sensitive channels may be opened upon adhesion-induced cell wall deformation. The physico-chemical properties of the substratum surface thus control the response of initially adhering bacteria and through excretion of autoinducer molecules extend the awareness of their adhering state to other biofilm inhabitants who subsequently respond with similar emergent properties. Herewith, physico-chemistry is not only involved in initial bacterial adhesion to surfaces but also in what we here propose to call "surface-programmed" biofilm growth. This conclusion is pivotal for the development of new strategies to control biofilm formation on substratum surfaces, that have hitherto been largely confined to the initial bacterial adhesion phenomena.

Surface Functionalization of an Aluminum Alloy to Generate an Antibiofilm Coating Based on Poly(Methyl Methacrylate) and Silver Nanoparticles

An experimental protocol was studied to improve the adhesion of a polymeric poly(methyl methacrylate) coating that was modified with silver nanoparticles to an aluminum alloy, AA2024. The nanoparticles were incorporated into the polymeric matrix to add the property of inhibiting biofilm formation to the anticorrosive characteristics of the film, thus also making the coating antibiocorrosive. The protocol consists of functionalizing the surface through a pseudotransesterification treatment using a methyl methacrylate monomer that bonds covalently to the surface and leaves a terminal double bond that promotes and directs the polymerization reaction that takes place in the process that follows immediately after. This results in more compact and thicker poly(methyl methacrylate) (PMMA) coatings than those obtained without pseudotransesterification. The poly(methyl methacrylate) matrix modified with nanoparticles was obtained by incorporating both the nanoparticles and the methyl methacrylate in the reactor. The in situ polymerization involved combining the pretreated AA2024 specimens combined with the methyl methacrylate monomer and AgNps. The antibiofilm capacity of the coating was evaluated against P. aeruginosa, with an excellent response. Not only did the presence of bacteria decrease, but the formation of the exopolymer subunits was 99.99% lower than on the uncoated aluminum alloy or the alloy coated with unmodified poly(methyl methacrylate). As well and significantly, the potentiodynamic polarization measurements indicate that the PMMA-Ag coating has a good anticorrosive property in a 0.1-M NaCl medium.

Biofilm and motility in response to environmental and host-related signals in Gram negative opportunistic pathogens

Most bacteria can switch between a planktonic, sometimes motile, form and a biofilm mode, in which bacterial cells can aggregate and attach to a solid surface. The transition between these two forms represents an example of bacterial adaptation to environmental signals and stresses. In 'environmental pathogens', namely, environmental bacteria that are also able to cause disease in animals and humans, signals associated either with the host or with the external environment, such as temperature, oxygen availability, nutrient concentrations etc., play a major role in triggering the switch between the motile and the biofilm mode, via complex regulatory mechanisms that control flagellar synthesis and motility, and production of adhesion factors. In this review article, we present examples of how environmental signals can impact biofilm formation and cell motility in the Gram negative bacteria Pseudomonas aeruginosa, Escherichia coli and in the Burkholderia genus, and how the switch between motile and biofilm mode can be an essential part of a more general process of adaptation either to the host or to the external environment.

Biosurfactant from a marine bacterium disrupts biofilms of pathogenic bacteria in a tropical aquaculture system

Bacterial infections are major constraints in aquaculture farming. These pathogens often adapt to the biofilm mode of growth and resist antibiotic treatments. We have used a non-toxic glycolipid biosurfactant (BS-SLSZ2) derived from a marine epizootic bacterium Staphylococcus lentus to treat aquaculture associated infections in an eco-friendly manner. We found that BS-SLSZ2 contained threose, a four-carbon sugar as the glycone component, and hexadecanoic and octadecanoic acids as the aglycone components. The critical micelle concentration of the purified glycolipid was 18 mg mL-1. This biosurfactant displayed anti-adhesive activity and inhibited biofilm formation by preventing initial attachment of cells onto surfaces. The biosurfactant (at a concentration of 20 μg) was able to inhibit Vibrio harveyi and Pseudomonas aeruginosa biofilms by 80.33 ± 2.16 and 82 ± 2.03%, respectively. At this concentration, it was also able to disrupt mature biofilms of V. harveyi (78.7 ± 1.93%) and P. aeruginosa (81.7 ± 0.59%). The biosurfactant was non-toxic towards Artemia salina. In vivo challenge experiments showed that the glycolipid was effective in protecting A. salina nauplii against V. harveyi and P. aeruginosa infections. This study highlights the significance of marine natural products in providing alternative biofilm controlling agents and decreasing the usage of antibiotics in aquaculture settings.

Efficacy of cell free supernatant from Bacillus licheniformis in protecting Artemia salina against Vibrio alginolyticus and Pseudomonas gessardii

Bacterial diseases are widespread in aquaculture farms and causative agents often adapt to biofilm mode of growth. These biofilms are detrimental to aquaculture species as they resist antibiotics and other agents that are used to control them. Two bacterial pathogens isolated from infected prawn samples were identified as Vibrio alginolyticus and Pseudomonas gessardii on the basis of morphological features, biochemical characteristics, 16S r RNA gene sequencing and phylogenetic analysis. Their pathogenic nature was confirmed by performing in vivo challenge experiments using Artemia salina as a model system. Seven days post infection, the mortality observed with V. alginolyticus and P. gessardii was 97 ± 4.08% and 77.5 ± 5.24%, respectively. The isolates formed extensive biofilms on polystyrene and glass surfaces. These infections could be controlled in an effective manner by using the cell free supernatant (CFS) of a tropical marine epizoic strain of Bacillus licheniformis D1 that is earlier reported to contain an antimicrobial protein (BLDZ1). The CFS inhibited biofilms in an efficient manner (82.35 ± 1.69 and 82.52 ± 1.11% for V. alginolyticus and P. gessardii, respectively) on co-incubation. In addition, pre-formed biofilms of V. alginolyticus and P. gessardii were also removed (84.53 ± 1.26 and 67.08 ± 1.43%, respectively). Fluorescence and scanning electron microscopic studies confirmed the antibiofilm potential of this protein on glass surfaces. The antibiofilm nature was due to the anti-adhesion and antimicrobial properties exhibited by the CFS. Treatment of A. salina with CFS (6 h prior to infections) was effective in protecting larvae against infections by field isolates. This study highlights the significance of marine natural products in providing alternative biofilm controlling agents to tackle infections and decreasing the usage of antibiotics in aquaculture settings.

The effective migration of Massilia sp. WF1 by Phanerochaete chrysosporium and its phenanthrene biodegradation in soil

Pollutant-degrading bacteria migrated by fungi may enhance the contacts between microorganisms and pollutants and improve the bioremediation efficiency of persistent organic pollutants in soil. Here, the migration of phenanthrene (PHE)-degrading bacteria Massilia sp. WF1 and Mycobacterium sp. WY10 by the hydrophobic fungi Phanerochaete chrysosporium (P. chrysosporium) and its effects on the PHE biodegradation in soil were investigated. Migration of the hydrophilic bacterium WF1 was better than that of the hydrophobic bacterium WY10 by P. chrysosporium mycelia since strain WF1 possesses flagellum and the type III secretion system. The interaction energy change of P. chrysosporium-WF1 was lower, but the interaction forces (van der Waals attractions, capillary forces, and cross-linking effects) were stronger than those of P. chrysosporium-WY10. Thus, the adhesive attraction between strain WF1 and P. chrysosporium was stronger, and consequently, strain WF1 was migrated by P. chrysosporium to a greater extent than WY10. The corresponding migration mechanism was inferred to be a bacterial 'passive' method: bacteria adhered to mycelia before they migrated with the growing mycelia. Moreover, migrated strain WF1 via P. chrysosporium showed effective PHE biodegradation in soil. Fungus-mediated migration of pollutant-degrading bacteria may play an important role in the bioremediation of pollutants in soil.

Bactericidal activity of the Ti-13Nb-13Zr alloy against different species of bacteria related with implant infection

The Ti-6Al-4V alloy is one of the most commonly used in orthopedic surgery. Despite its advantages, there is an increasing need to use new titanium alloys with no toxic elements and improved biomechanical properties, such as Ti-13Nb-13Zr. Prosthetic joint infections (PJI) are mainly caused by Gram-positive bacteria; however, Gram-negative bacteria are a growing problem due to associated multidrug resistance. In this study, the bacterial adherence and viability on the Ti-13Nb-13Zr alloy have been compared to that of the Ti-6Al-4V alloy using 16 collection and clinical strains of bacterial species related to PJI: Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, and Pseudomonas aeruginosa. When compared with the Ti-6Al-4V alloy, bacterial adherence on the Ti-13Nb-13Zr alloy was significantly higher in most staphylococcal and P. aeruginosa strains and lower for E. coli strains. The proportion of live bacteria was significantly lower for both Gram-negative species on the Ti-13Nb-13Zr alloy than on the Ti-6Al-4V alloy pointing to some bactericidal effect of the Ti-13Nb-13Zr alloy. This bactericidal effect appears to be a consequence of the formation of hydroxyl radicals, since this effect is neutralized when dimethylsulfoxide was added to both the saline solution and water used to wash the stain. The antibacterial effect of the Ti-13Nb-13Zr alloy against Gram-negative bacteria is an interesting property useful for the prevention of PJI caused by these bacteria on this potential alternative to the Ti-6Al-4V alloy for orthopedic surgery.

Proteomic Response of Pseudomonas aeruginosa PAO1 Adhering to Solid Surfaces

Pseudomonas aeruginosa is a pathogenic micro-organism responsible for many hospital-acquired infections. It is able to adhere to solid surfaces and develop an immobilized community or so-called biofilm. Many studies have been focusing on the use of specific materials to prevent the formation of these biofilms, but the reactivity of the bacteria in contact to surfaces remains unknown. The aim of this study was to evaluate the impact of the abiotic surface on the physiology of adherent bacteria. Three different materials, stainless steel (SS), glass (G), and polystyrene (PS) that were relevant to industrial or medical environments were characterized at the physicochemical level in terms of their hydrophobicity and roughness. We showed that SS was moderately hydrophilic and rough, potentially containing crevices, G was hydrophilic and smooth while PS was hydrophobic and smooth. We further showed that P. aeruginosa cells were more likely able to adhere to SS and G rather than PS surfaces under our experimental conditions. The physiological response of P. aeruginosa when adhering to each of these materials was then evaluated by global proteomic analysis. The abundance of 70 proteins was shown to differ between the materials suggesting that their abundance was modified as a function of the material to which bacteria adhered. Our data lead to enabling the identification of abundance patterns that appeared to be specific to a given surface. Taken together, our data showed that P. aeruginosa is capable of sensing and responding to a surface probably via specific programmes to adapt its physiological response accordingly.

Foam adsorption as an ex situ capture step for surfactants produced by fermentation

In this report, a method for a simultaneous production and separation of a microbially synthesized rhamnolipid biosurfactant is presented. During the aerobic cultivation of flagella-free Pseudomonas putida EM383 in a 3.1L stirred tank reactor on glucose as a sole carbon source, rhamnolipids are produced and excreted into the fermentation liquid. Here, a strategy for biosurfactant capture from rhamnolipid enriched fermentation foam using hydrophobic-hydrophobic interaction was investigated. Five adsorbents were tested independently for the application of this capture technique and the best performing adsorbent was tested in a fermentation process. Cell-containing foam was allowed to flow out of the fermentor through the off-gas line and an adsorption packed bed. Foam was observed to collapse instantly, while the resultant liquid flow-through, which was largely devoid of the target biosurfactant, eluted towards the outlet channel of the packed bed column and was subsequently pumped back into the fermentor. After 48h of simultaneous fermentation and ex situ adsorption of rhamnolipids from the foam, 90% out of 5.5g of total rhamnolipids produced were found in ethanol eluate of the adsorbent material, indicating the suitability of this material for ex situ rhamnolipid capture from fermentation processes.

Flagellar Hooks and Hook Protein FlgE Participate in Host Microbe Interactions at Immunological Level

Host-microbe interactions determine the outcome of host responses to commensal and pathogenic microbes. Previously, two epithelial cell-binding peptides were found to be homologues of two sites (B, aa168–174; F, aa303–309) in the flagellar hook protein FlgE of Pseudomonas aeruginosa. Tertiary modeling predicted these sites at the interface of neighboring FlgE monomers in the fully formed hook. Recombinant FlgE protein stimulated proinflammatory cytokine production in a human cell line and in murine lung organoid culture as detected with real-time RT-PCR and ELISA assays. When administered to mice, FlgE induced lung inflammation and enhanced the Th2-biased humoral response to ovalbumin. A pull-down assay performed with FlgE-saturated resin identified caveolin-1 as an FlgE-binding protein, and caveolin-1 deficiency impaired FlgE-induced inflammation and downstream Erk1/2 pathway activation in lung organoids. Intact flagellar hooks from bacteria were also proinflammatory. Mutations to sites B and F impaired bacteria motility and proinflammatory potency of FlgE without altering adjuvanticity of FlgE. These findings suggest that the flagellar hook and FlgE are novel players in host-bacterial interactions at immunological level. Further studies along this direction would provide new opportunities for understanding and management of diseases related with bacterial infection.

The design of superhydrophobic stainless steel surfaces by controlling nanostructures: A key parameter to reduce the implantation of pathogenic bacteria

Reducing bacterial adhesion on substrates is fundamental for various industries. In this work, new superhydrophobic surfaces are created by electrodeposition of hydrophobic polymers (PEDOT-F₄ or PEDOT-H₈) on stainless steel with controlled topographical features, especially at a nano-scale. Results show that anti-bioadhesive and anti-biofilm properties require the control of the surface topographical features, and should be associated with a low adhesion of water onto the surface (Cassie-Baxter state) with limited crevice features at the scale of bacterial cells (nano-scale structures).

Pattern Formation by Staphylococcus epidermidis via Droplet Evaporation on Micropillars Arrays at a Surface

We evaluate the effect of epoxy surface structuring on the evaporation of water droplets containing Staphylococcus epidermidis (S. epidermidis). During evaporation, droplets with S. epidermidis cells yield to complex wetting patterns such as the zipping-wetting1-3 and the coffee-stain effects. Depending on the height of the microstructure, the wetting fronts propagate circularly or in a stepwise manner, leading to the formation of octagonal or square-shaped deposition patterns.4,5 We observed that the shape of the dried droplets has considerable influence on the local spatial distribution of S. epidermidis deposited between micropillars. These changes are attributed to an unexplored interplay between the zipping-wetting1 and the coffee-stain6 effects in polygonally shaped droplets containing S. epidermidis. Induced capillary flows during evaporation of S. epidermidis are modeled with polystyrene particles. Bacterial viability measurements for S. epidermidis show high viability of planktonic cells, but low biomass deposition on the microstructured surfaces. Our findings provide insights into design criteria for the development of microstructured surfaces on which bacterial propagation could be controlled, limiting the use of biocides.

Cross-regulation of Pseudomonas motility systems: the intimate relationship between flagella, pili and virulence

Pseudomonas aeruginosa navigates using two distinct forms of motility, swimming and twitching. A polar flagellum and Type 4 pili power these movements, respectively, allowing P. aeruginosa to attach to and colonize surfaces. Single cell imaging and particle tracking algorithms have revealed a wide range of bacterial surface behaviors which are regulated by second messengers cyclic-di-GMP and cAMP; the production of these signals is, in turn, responsive to the engagement of motility organelles with a surface. Innate immune defense systems, long known to recognize structural components of flagella, appear to respond to motility itself. The association of motility with both upregulation of virulence and induction of host defense mechanisms underlies the complex contributions of flagella and pili to P. aeruginosa pathogenesis.