Effect of Shear Stress on Pseudomonas aeruginosa Isolated from the Cystic Fibrosis Lung

Structure and mechanism of the osmoregulated choline transporter BetT

The choline-glycine betaine pathway plays an important role in bacterial survival in hyperosmotic environments. Osmotic activation of the choline transporter BetT promotes the uptake of external choline for synthesizing the osmoprotective glycine betaine. Here, we report the cryo-electron microscopy structures of Pseudomonas syringae BetT in the apo and choline-bound states. Our structure shows that BetT forms a domain-swapped trimer with the C-terminal domain (CTD) of one protomer interacting with the transmembrane domain (TMD) of a neighboring protomer. The substrate choline is bound within a tryptophan prism at the central part of TMD. Together with functional characterization, our results suggest that in Pseudomonas species, including the plant pathogen P. syringae and the human pathogen Pseudomonas aeruginosa, BetT is locked at a low-activity state through CTD-mediated autoinhibition in the absence of osmotic stress, and its hyperosmotic activation involves the release of this autoinhibition.

Pseudomonas aeruginosa Biofilm Lifecycle: Involvement of Mechanical Constraints and Timeline of Matrix Production

Pseudomonas aeruginosa is an opportunistic pathogen causing acute and chronic infections, especially in immunocompromised patients. Its remarkable adaptability and resistance to various antimicrobial treatments make it difficult to eradicate. Its persistence is enabled by its ability to form a biofilm. Biofilm is a community of sessile micro-organisms in a self-produced extracellular matrix, which forms a scaffold facilitating cohesion, cell attachment, and micro- and macro-colony formation. This lifestyle provides protection against environmental stresses, the immune system, and antimicrobial treatments, and confers the capacity for colonization and long-term persistence, often characterizing chronic infections. In this review, we retrace the events of the life cycle of P. aeruginosa biofilm, from surface perception/contact to cell spreading. We focus on the importance of extracellular appendages, mechanical constraints, and the kinetics of matrix component production in each step of the biofilm life cycle.

Membrane fluidity homeostasis is required for tobramycin-enhanced biofilm in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen, which causes chronic infections, especially in cystic fibrosis (CF) patients where it colonizes the lungs via the build-up of biofilms. Tobramycin, an aminoglycoside, is often used to treat P. aeruginosa infections in CF patients. Tobramycin at sub-minimal inhibitory concentrations enhances both biofilm biomass and thickness in vitro; however, the mechanism(s) involved are still unknown. Herein, we show that tobramycin increases the expression and activity of SigX, an extracytoplasmic sigma factor known to be involved in the biosynthesis of membrane lipids and membrane fluidity homeostasis. The biofilm enhancement by tobramycin is not observed in a sigX mutant, and the sigX mutant displays increased membrane stiffness. Remarkably, the addition of polysorbate 80 increases membrane fluidity of sigX-mutant cells in biofilm, restoring the tobramycin-enhanced biofilm formation. Our results suggest the involvement of membrane fluidity homeostasis in biofilm development upon tobramycin exposure.IMPORTANCEPrevious studies have shown that sub-lethal concentrations of tobramycin led to an increase biofilm formation in the case of infections with the opportunistic pathogen Pseudomonas aeruginosa. We show that the mechanism involved in this phenotype relies on the cell envelope stress response, triggered by the extracytoplasmic sigma factor SigX. This phenotype was abolished in a sigX-mutant strain. Remarkably, we show that increasing the membrane fluidity of the mutant strain is sufficient to restore the effect of tobramycin. Altogether, our data suggest the involvement of membrane fluidity homeostasis in biofilm development upon tobramycin exposure.

A gain-of-function mutation in zinc cluster transcription factor Rob1 drives Candida albicans adaptive growth in the cystic fibrosis lung environment

Candida albicans chronically colonizes the respiratory tract of patients with Cystic Fibrosis (CF). It competes with CF-associated pathogens (e.g. Pseudomonas aeruginosa) and contributes to disease severity. We hypothesize that C. albicans undergoes specific adaptation mechanisms that explain its persistence in the CF lung environment. To identify the underlying genetic and phenotypic determinants, we serially recovered 146 C. albicans clinical isolates over a period of 30 months from the sputum of 25 antifungal-naive CF patients. Multilocus sequence typing analyses revealed that most patients were individually colonized with genetically close strains, facilitating comparative analyses between serial isolates. We strikingly observed differential ability to filament and form monospecies and dual-species biofilms with P. aeruginosa among 18 serial isolates sharing the same diploid sequence type, recovered within one year from a pediatric patient. Whole genome sequencing revealed that their genomes were highly heterozygous and similar to each other, displaying a highly clonal subpopulation structure. Data mining identified 34 non-synonymous heterozygous SNPs in 19 open reading frames differentiating the hyperfilamentous and strong biofilm-former strains from the remaining isolates. Among these, we detected a glycine-to-glutamate substitution at position 299 (G299E) in the deduced amino acid sequence of the zinc cluster transcription factor ROB1 (ROB1G299E), encoding a major regulator of filamentous growth and biofilm formation. Introduction of the G299E heterozygous mutation in a co-isolated weak biofilm-former CF strain was sufficient to confer hyperfilamentous growth, increased expression of hyphal-specific genes, increased monospecies biofilm formation and increased survival in dual-species biofilms formed with P. aeruginosa, indicating that ROB1G299E is a gain-of-function mutation. Disruption of ROB1 in a hyperfilamentous isolate carrying the ROB1G299E allele abolished hyperfilamentation and biofilm formation. Our study links a single heterozygous mutation to the ability of C. albicans to better survive during the interaction with other CF-associated microbes and illuminates how adaptive traits emerge in microbial pathogens to persistently colonize and/or infect the CF-patient airways.

The natriuretic peptide receptor agonist osteocrin disperses Pseudomonas aeruginosa biofilm

Biofilms are highly tolerant to antimicrobials and host immune defense, enabling pathogens to thrive in hostile environments. The diversity of microbial biofilm infections requires alternative and complex treatment strategies. In a previous work we demonstrated that the human Atrial Natriuretic Peptide (hANP) displays a strong anti-biofilm activity toward Pseudomonas aeruginosa and that the binding of hANP by the AmiC protein supports this effect. This AmiC sensor has been identified as an analog of the human natriuretic peptide receptor subtype C (h-NPRC). In the present study, we evaluated the anti-biofilm activity of the h-NPRC agonist, osteocrin (OSTN), a hormone that displays a strong affinity for the AmiC sensor at least in vitro. Using molecular docking, we identified a pocket in the AmiC sensor that OSTN reproducibly docks into, suggesting that OSTN might possess an anti-biofilm activity as well as hANP. This hypothesis was validated since we observed that OSTN dispersed established biofilm of P. aeruginosa PA14 strain at the same concentrations as hANP. However, the OSTN dispersal effect is less marked than that observed for the hANP (-61% versus -73%). We demonstrated that the co-exposure of P. aeruginosa preformed biofilm to hANP and OSTN induced a biofilm dispersion with a similar effect to that observed with hANP alone suggesting a similar mechanism of action of these two peptides. This was confirmed by the observation that OSTN anti-biofilm activity requires the activation of the complex composed by the sensor AmiC and the regulator AmiR of the ami pathway. Using a panel of both P. aeruginosa laboratory reference strains and clinical isolates, we observed that the OSTN capacity to disperse established biofilms is highly variable from one strain to another. Taken together, these results show that similarly to the hANP hormone, OSTN has a strong potential to be used as a tool to disperse P. aeruginosa biofilms.

Both Pseudomonas aeruginosa and Candida albicans Accumulate Greater Biomass in Dual-Species Biofilms under Flow

Microbe-microbe interactions can strongly influence growth and biofilm formation kinetics. For Pseudomonas aeruginosa and Candida albicans, which are found together in diverse clinical sites, including urinary and intravenous catheters and the lungs of individuals with cystic fibrosis (CF), we compared the kinetics of biofilm formation by each species in dual-species and single-species biofilms. We engineered fluorescent protein constructs for P. aeruginosa (producing mKO-κ) and C. albicans (producing mKate2) that did not alter growth and enabled single-cell resolution imaging by live-sample microscopy. Using these strains in an optically clear derivative of synthetic CF sputum medium, we found that both P. aeruginosa and C. albicans displayed increased biovolume accumulation—by three- and sixfold, respectively—in dual-species biofilms relative to single-species biofilms. This result was specific to the biofilm environment, as enhanced growth was not observed in planktonic cocultures. Stimulation of C. albicans biofilm formation occurred regardless of whether P. aeruginosa was added at the time of fungal inoculation or 24 h after the initiation of biofilm development. P. aeruginosa biofilm increases in cocultures did not require the Pel extracellular polysaccharide, phenazines, and siderophores known to influence C. albicans. P. aeruginosa mutants lacking Anr, LasR, and BapA were not significantly stimulated by C. albicans, but they still promoted a significant enhancement of biofilm development of the fungus, suggesting a fungal response to the presence of bacteria. Last, we showed that a set of P. aeruginosa clinical isolates also prompted an increase of biovolume by C. albicans in coculture.

IMPORTANCE There is an abundance of work on both P. aeruginosa and C. albicans in isolation, and quite some work as well on the way these two microbes interact. These studies do not, however, consider biofilm environments under flow, and our results here show that the expected outcome of interaction between these two pathogens can actually be reversed under flow, from pure antagonism to an increase in biomass on the part of both. Our work also highlights the importance of cellular-scale spatial structure in biofilms for understanding multispecies population dynamics.

Biofloc-Based Enhanced Survival of Litopenaeus vannamei Upon AHPND-Causing Vibrio parahaemolyticus Challenge Is Partially Mediated by Reduced Expression of Its Virulence Genes

The biofloc system is a relatively new aquaculture technology that offers practical solution to maintain culture water quality by recycling nutrients and improves the health status and resistance of shrimps against microbial infection, yet the mode of action involved remains unclear. This study aimed to unravel the underlying mechanism behind the protective effect of a biofloc system using Litopenaeus vannamei and acute hepatopancreatic necrosis disease (AHPND)-causing Vibrio parahaemolyticus M0904 strain as a host-pathogen model. The results showed that a biofloc system maintained at a C/N ratio of 15, improves the water quality and contributes to the nutrition of cultured animals as bioflocs might serve as an additional protein source. Furthermore, the study demonstrated that the biofloc system enhances the survival of L. vannamei upon challenge with a V. parahaemolyticus AHPND strain. Remarkably, the results highlight that in the biofloc system, AHPND-causing V. parahaemolyticus possibly switch from free-living virulent planktonic phenotype to a non-virulent biofilm phenotype, as demonstrated by a decreased transcription of flagella-related motility genes (flaA, CheR, and fliS), Pir toxin (PirB^VP), and AHPND plasmid genes (ORF14) and increased expression of the phenotype switching marker AlkPhoX gene in both in vitro and in vivo conditions. Taken together, results suggest that biofloc steer phenotype switching, contributing to the decreased virulence of V. parahaemolyticus AHPND strain toward shrimp postlarvae. This information reinforces our understanding about AHPND in a biofloc setting and opens the possibility to combat AHPND not only by trying to eliminate the AHPND-causing V. parahaemolyticus from the system but rather to steer the phenotypic switch.

Environmental conditions steer phenotypic switching in acute hepatopancreatic necrosis disease-causing Vibrio parahaemolyticus, affecting PirAVP /PirBVP toxins production

Bacteria in nature are widely exposed to differential fluid shears which are often a trigger for phenotypic switches. The latter mediates transcriptional and translation remodelling of cellular metabolism impacting among others virulence, antimicrobial resistance and stress resistance. In this study, we evaluated the role of fluid shear on phenotypic switch in an acute hepatopancreatic necrosis disease (AHPND)-causing Vibrio parahaemolyticus M0904 strain under both in vitro and in vivo conditions. The results showed that V. parahaemolyticus M0904 grown at lower shaking speed (110 rpm constant agitation, M0904/110), causing low fluid shear, develop cellular aggregates or floccules. These cells increased levan production (as verified by concanavalin binding) and developed differentially stained colonies on Congo red agar plates and resistance to antibiotics. In addition, the phenotypic switch causes a major shift in the protein secretome. At 120 rpm (M0904/120), PirA^VP /PirB^VP toxins are mainly produced, while at 110 rpm PirA^VP /PirB^VP toxins production is stopped and an alkaline phosphatase (ALP) PhoX becomes the dominant protein in the protein secretome. These observations are matched with a very strong reduction in virulence of M0904/110 towards two crustacean larvae, namely, Artemia and Macrobrachium. Taken together, our study provides substantial evidence for the existence of two phenotypic forms in AHPND V. parahaemolyticus strain displaying differential phenotypes. Moreover, as aerators and pumping devices are frequently used in shrimp aquaculture facilities, they can inflict fluid shear to the standing microbial agents. Hence, our study could provide a basis to understand the behaviour of AHPND-causing V. parahaemolyticus in aquaculture settings and open the possibility to monitor and control AHPND by steering phenotypes.

Modulatory effect of low-shear modeled microgravity on stress resistance, membrane lipid composition, virulence, and relevant gene expression in the food-borne pathogen Listeria monocytogenes

The present study investigated the influence of low-shear modeled microgravity (LSMMG) conditions on Listeria monocytogenes stress response (heat, cold, and acid), membrane fatty acid composition, and virulence potential as well as stress-/virulence-associated gene expression. The results showed that LSMMG-cultivated cells had lower survival rate and lower D-values under heat and acid stress conditions compared to cells grown under normal gravity (NG). Interestingly, the cold resistance was elevated in cells cultivated under LSMMG conditions when compared to NG conditions. A higher amount of anteiso-branched chain fatty acids and lower ratio of iso/anteiso were observed in LSMMG cultured cells, which would contribute to increased membrane fluidity. Under LSMMG conditions, upregulated expression of cold stress-related genes (cspA, cspB, and cspD) was noticed but no change in expression was observed for heat (dnaK, groES, clpC, clpP, and clpE) and acid stress-related genes (sigB). The LSMMG-grown cells showed inferior virulence capacity in terms of infection, cell cycle arrest, and apoptosis induction in Caco-2 cells compared to those grown under NG conditions. Approximately 3.65, 2.13, 4.02, and 2.65-fold downregulation of prfA, hly, inlA, and bsh genes, respectively, in LSMMG-cultured cells might be the reason for reduced virulence. In conclusion, these findings suggest that growth under LSMMG conditions stimulates alterations in L. monocytogenes stress/virulence response, perhaps due to changes in lipid composition and related genes expression.

Controlling chronic Pseudomonas aeruginosa infections by strategically interfering with the sensory function of SagS

The hybrid sensor SagS plays a central role in the formation of Pseudomonas aeruginosa biofilms, by enabling the switch from the planktonic to the biofilm mode of growth and by facilitating the transition of biofilm cells to a highly tolerant state. In this study, we examined the importance of the SagS key amino acid residues associated with biofilm formation (L154) and antibiotic tolerance (D105) in P. aeruginosa virulence. Recombinant P. aeruginosa ΔsagS and ΔsagS chromosomally expressing wild-type sagS, or its two variants D105A and L154A, were tested for their potential to form biofilms and cause virulence in plants and mouse models of acute and chronic pneumonia. Although mutation of sagS did not alter P. aeruginosa virulence during acute infections, a significant difference in pathogenicity of sagS mutants was observed during chronic infections, with the L154A variant showing reduced bacterial loads in the chronic pneumonia model, while interference with the D105 residue enhanced the susceptibility of P. aeruginosa biofilms during tobramycin treatment. Our findings suggest that interference with the biofilm or tolerance regulatory circuits of SagS affects P. aeruginosa pathogenicity in chronic but not acute infections, and reveal SagS to be a promising new target to treat P. aeruginosa biofilm infections.

Motility, Biofilm Formation and Antimicrobial Efflux of Sessile and Planktonic Cells of Achromobacter xylosoxidans

Achromobacter xylosoxidans is an innately multidrug-resistant bacterium capable of forming biofilms in the respiratory tract of cystic fibrosis (CF) patients. During the transition from the planktonic stage to biofilm growth, bacteria undergo a transcriptionally regulated differentiation. An isolate of A. xylosoxidans cultured from the sputum of a CF patient was separated into sessile and planktonic stages in vitro, and the transcriptomes were compared. The selected genes of interest were subsequently inactivated, and flagellar motility was found to be decisive for biofilm formation in vitro. The spectrum of a new resistance-nodulation-cell division (RND)-type multidrug efflux pump (AxyEF-OprN) was characterized by inactivation of the membrane fusion protein. AxyEF-OprN is capable of extruding some fluoroquinolones (levofloxacin and ciprofloxacin), tetracyclines (doxycycline and tigecycline) and carpabenems (ertapenem and imipenem), which are classes of antimicrobials that are widely used for treatment of CF pulmonary infections.

Transcriptional profiling of the mutualistic bacterium Vibrio fischeri and an hfq mutant under modeled microgravity

For long-duration space missions, it is critical to maintain health-associated homeostasis between astronauts and their microbiome. To achieve this goal it is important to more fully understand the host–symbiont relationship under the physiological stress conditions of spaceflight. To address this issue we examined the impact of a spaceflight analog, low-shear-modeled microgravity (LSMMG), on the transcriptome of the mutualistic bacterium Vibrio fischeri. Cultures of V. fischeri and a mutant defective in the global regulator Hfq (∆hfq) were exposed to either LSMMG or gravity conditions for 12 h (exponential growth) and 24 h (stationary phase growth). Comparative transcriptomic analysis revealed few to no significant differentially expressed genes between gravity and the LSMMG conditions in the wild type or mutant V. fischeri at exponential or stationary phase. There was, however, a pronounced change in transcriptomic profiles during the transition between exponential and stationary phase growth in both V. fischeri cultures including an overall decrease in gene expression associated with translational activity and an increase in stress response. There were also several upregulated stress genes specific to the LSMMG condition during the transition to stationary phase growth. The ∆hfq mutants exhibited a distinctive transcriptome profile with a significant increase in transcripts associated with flagellar synthesis and transcriptional regulators under LSMMG conditions compared to gravity controls. These results indicate the loss of Hfq significantly influences gene expression under LSMMG conditions in a bacterial symbiont. Together, these results improve our understanding of the mechanisms by which microgravity alters the physiology of beneficial host-associated microbes.

Insight into Pseudomonas aeruginosa pyocyanin production under low-shear modeled microgravity

Long-term space flight impairs the immune system of astronauts, rendering them vulnerable to opportunistic infections. Pseudomonas aeruginosa causes opportunistic infections, particularly in individuals with a compromised immune system; it can be a major health hazard for astronauts during space flight missions. Hence, the production of the most abundant redox active virulence factor, pyocyanin by P. aeruginosa, was assessed under low-shear modeled microgravity (LSMMG) conditions, simulated using a high aspect ratio vessel. Moreover, we evaluated changes in the expression of genes involved in pyocyanin biosynthesis and genes involved in the MexGHI-OpmD operon quorum sensing. Extracellular DNA and H₂O₂ production were measured, and their correlation with pyocyanin production was examined. Interestingly, the pyocyanin quantity was 2.58-fold lower in the LSMMG conditions compared to the normal gravity. LSMMG caused downregulation of the genes associated with pyocyanin biosynthesis. Interestingly, extracellular DNA and H₂O₂ release were significantly high in the normal gravity environment. Scanning electron microscopy revealed aggregation and elongated cells under LSMMG. Taken together, these findings suggest that LSMMG did not induce pyocyanin secretion in P. aeruginosa.

Modeling Host-Pathogen Interactions in the Context of the Microenvironment: Three-Dimensional Cell Culture Comes of Age

Tissues and organs provide the structural and biochemical landscapes upon which microbial pathogens and commensals function to regulate health and disease. While flat two-dimensional (2-D) monolayers composed of a single cell type have provided important insight into understanding host-pathogen interactions and infectious disease mechanisms, these reductionist models lack many essential features present in the native host microenvironment that are known to regulate infection, including three-dimensional (3-D) architecture, multicellular complexity, commensal microbiota, gas exchange and nutrient gradients, and physiologically relevant biomechanical forces (e.g., fluid shear, stretch, compression). A major challenge in tissue engineering for infectious disease research is recreating this dynamic 3-D microenvironment (biological, chemical, and physical/mechanical) to more accurately model the initiation and progression of host-pathogen interactions in the laboratory. Here we review selected 3-D models of human intestinal mucosa, which represent a major portal of entry for infectious pathogens and an important niche for commensal microbiota. We highlight seminal studies that have used these models to interrogate host-pathogen interactions and infectious disease mechanisms, and we present this literature in the appropriate historical context. Models discussed include 3-D organotypic cultures engineered in the rotating wall vessel (RWV) bioreactor, extracellular matrix (ECM)-embedded/organoid models, and organ-on-a-chip (OAC) models. Collectively, these technologies provide a more physiologically relevant and predictive framework for investigating infectious disease mechanisms and antimicrobial therapies at the intersection of the host, microbe, and their local microenvironments.

ANNOgesic: a Swiss army knife for the RNA-seq based annotation of bacterial/archaeal genomes

To understand the gene regulation of an organism of interest, a comprehensive genome annotation is essential. While some features, such as coding sequences, can be computationally predicted with high accuracy based purely on the genomic sequence, others, such as promoter elements or noncoding RNAs, are harder to detect. RNA sequencing (RNA-seq) has proven to be an efficient method to identify these genomic features and to improve genome annotations. However, processing and integrating RNA-seq data in order to generate high-resolution annotations is challenging, time consuming, and requires numerous steps. We have constructed a powerful and modular tool called ANNOgesic that provides the required analyses and simplifies RNA-seq-based bacterial and archaeal genome annotation. It can integrate data from conventional RNA-seq and differential RNA-seq and predicts and annotates numerous features, including small noncoding RNAs, with high precision. The software is available under an open source license (ISCL) at https://pypi.org/project/ANNOgesic/.

Transcriptomic analysis of longitudinal Burkholderia pseudomallei infecting the cystic fibrosis lung

The melioidosis bacterium, Burkholderia pseudomallei, is increasingly being recognised as a pathogen in patients with cystic fibrosis (CF). We have recently catalogued genome-wide variation of paired, isogenic B. pseudomallei isolates from seven Australasian CF cases, which were collected between 4 and 55 months apart. Here, we extend this investigation by documenting the transcriptomic changes in B. pseudomallei in five cases. Following growth in an artificial CF sputum medium, four of the five paired isolates exhibited significant differential gene expression (DE) that affected between 32 and 792 genes. The greatest number of DE events was observed between the strains from patient CF9, consistent with the hypermutator status of the latter strain, which is deficient in the DNA mismatch repair protein MutS. Two patient isolates harboured duplications that concomitantly increased expression of the β-lactamase-encoding gene penA, and a 35 kb deletion in another abolished expression of 29 genes. Convergent expression profiles in the chronically-adapted isolates identified two significantly downregulated and 17 significantly upregulated loci, including the resistance-nodulation-division (RND) efflux pump BpeEF-OprC, the quorum-sensing hhqABCDE operon, and a cyanide- and pyocyanin-insensitive cytochrome bd quinol oxidase. These convergent pathoadaptations lead to increased expression of pathways that may suppress competing bacterial and fungal pathogens, and that enhance survival in oxygen-restricted environments, the latter of which may render conventional antibiotics less effective in vivo. Treating chronically adapted B. pseudomallei infections with antibiotics designed to target anaerobic infections, such as the nitroimidazole class of antibiotics, may significantly improve pathogen eradication attempts by exploiting this Achilles heel.

Intrapulmonary percussive ventilation improves lung function in cystic fibrosis patients chronically colonized with Pseudomonas aeruginosa: a pilot cross-over study

High levels of shear stress can prevent and disrupt Pseudomonas aeruginosa biofilm formation in vitro. Intrapulmonary percussive ventilation (IPV) could be used to introduce shear stress into the lungs of cystic fibrosis (CF) patients to disrupt biofilms in vivo. We performed a first-of-its-kind pilot clinical study to evaluate short-term IPV therapy at medium (200 bursts per minute, bpm) and high frequency (400 bpm) as compared to autogenic drainage (AD) on lung function and the behavior of P. aeruginosa in the CF lung in four patients who are chronically colonized by P. aeruginosa. A significant difference between the three treatment groups was observed for both the forced expiratory volume in 1 s (FEV1) and the forced vital capacity (FVC) (p < 0.05). More specifically, IPV at high frequency significantly increased FEV1 and FVC compared to AD (p < 0.05) and IPV at medium frequency (p < 0.001). IPV at high frequency enhanced the expression levels of P. aeruginosa planktonic marker genes, which was less pronounced with IPV at medium frequency or AD. In conclusion, IPV at high frequency could potentially alter the behavior of P. aeruginosa in the CF lung and improve lung function.

TRIAL REGISTRATION

The trail was retrospectively registered at the ISRCTN registry on 6 June 2013, under trial registration number ISRCTN75391385.

Swimming and rafting of E.coli microcolonies at air-liquid interfaces

The dynamics of swimming microorganisms is strongly affected by solid‐liquid and air‐liquid interfaces. In this paper, we characterize the motion of both single bacteria and microcolonies at an air‐liquid interface. Both of them follow circular trajectories. Single bacteria preferentially show a counter‐clockwise motion, in agreement with previous experimental and theoretical findings. Instead, no preferential rotation direction is observed for microcolonies suggesting that their motion is due to a different physical mechanism. We propose a simple mechanical model where the microcolonies move like rafts constrained to the air‐liquid interface. Finally, we observed that the microcolony growth is due to the aggregation of colliding single‐swimmers, suggesting that the microcolony formation resembles a condensation process where the first nucleus originates by the collision between two single‐swimmers. Implications of microcolony splitting and aggregation on biofilm growth and dispersion at air‐liquid interface are discussed.

An Update on the Sociomicrobiology of Quorum Sensing in Gram-Negative Biofilm Development

Bacteria are social creatures that are able to interact and coordinate behaviors with each other in a multitude of ways. The study of such group behaviors in microbes was coined “sociomicrobiology” in 2005. Two such group behaviors in bacteria are quorum sensing (QS) and biofilm formation. At a very basic level, QS is the ability to sense bacterial density via cell-to-cell signaling using self-produced signals called autoinducers, and biofilms are aggregates of cells that are attached to one another via a self-produced, extracellular matrix. Since cells in biofilm aggregates are in close proximity, biofilms represent an ecologically relevant environment for QS. While QS is known to affect biofilm formation in both Gram-negative and Gram-positive species, in this review, we will focus exclusively on Gram-negative bacteria, with an emphasis on Pseudomonas aeruginosa. We will begin by describing QS systems in P. aeruginosa and how they affect P. aeruginosa biofilm formation. We then expand our review to other Gram-negative bacteria and conclude with interesting questions with regard to the effect of biofilms on QS.

Spatial Organization Plasticity as an Adaptive Driver of Surface Microbial Communities

Biofilms are dynamic habitats which constantly evolve in response to environmental fluctuations and thereby constitute remarkable survival strategies for microorganisms. The modulation of biofilm functional properties is largely governed by the active remodeling of their three-dimensional structure and involves an arsenal of microbial self-produced components and interconnected mechanisms. The production of matrix components, the spatial reorganization of ecological interactions, the generation of physiological heterogeneity, the regulation of motility, the production of actives enzymes are for instance some of the processes enabling such spatial organization plasticity. In this contribution, we discussed the foundations of architectural plasticity as an adaptive driver of biofilms through the review of the different microbial strategies involved. Moreover, the possibility to harness such characteristics to sculpt biofilm structure as an attractive approach to control their functional properties, whether beneficial or deleterious, is also discussed.

Calling all hosts: Bacterial communication in situ

Bacteria are cosmopolitan organisms that in recent years have demonstrated many roles in maintaining host equilibrium. In this review, we discuss three roles bacteria can occupy in a host: pathogenic, symbiotic, and transient, with a specific focus on how bacterial small molecules contribute to homeostasis or dysbiosis. First, we will dissect how small molecules produced by pathogenic bacteria can be used as a source for communication during colonization and as protection against host immune responses. The ability to achieve a higher level of organization through small molecule communication gives pathogenic bacteria an opportunity for increased virulence and fitness. Conversely, in symbiotic relationships with hosts, small molecules are used in the initial acquisition, colonization, and maintenance of this beneficial population. Chemical signals can come from both the host and symbiont, and it is often observed that these interKingdom symbioses result in coevolution of both species involved. Furthermore, the transition from transient to commensal or opportunistic likely relies on molecular mechanisms. The small molecules utilized and produced by transient bacteria are desirable for both the immune and nutritional benefits they provide to the host. Finally, the advantages and disadvantages of modern analytical techniques that are available to researchers in order to study small molecules in situ is an important aspect of this review. It is our opinion that small molecules produced by bacteria are central to many biological processes and a larger focus on uncovering the function and identity of these small molecules is required to gain a deeper understanding of host-microbe associations.