Principles of c-di-GMP signalling in bacteria

Impaired denitrification of aerobic granules in response to micro/nanoplastic stress: Insights from interspecies interactions and electron transfer processes

The accumulation of micro/nanoplastics in wastewater significantly hinders denitrification in biological wastewater treatment systems, yet the intrinsic mechanisms are not fully understood. Herein, we combined signal molecule monitoring, electrochemical characterization and multi-omics analysis to investigate how quorum sensing (QS)-mediated microbial interactions influence denitrification in aerobic granular sludge systems. Results showed that after 90-day exposure to micro/nanoplastics, cross-talk between multiple signal molecules significantly declined, thereby disrupting the QS system to opportunely sense changes in the external environment. As a consequence of impaired QS, only 5 species exhibited up-regulation of the genes encoding amino acids and cofactors to sustain cross-feeding, while others, acting as "cheaters", relied on metabolites offered by cross-feeding but without reciprocating. This imbalance resulted in insufficient availability of metabolites, including redox-active metabolites such as riboflavin, and subsequently deteriorated the denitrification electron transfer process. Compared to the control group, the extracellular electron transfer capacity and denitrification electron transfer chain activity decreased to, respectively, 88.08 % and 63.33 % (microplastics-exposure) and 79.64 % and 63.75 % (nanoplastics-exposure), which directly contributed to the decline in nitrogen removal. Overall, this study provided deeper insights into the denitrification in complex microbial communities under the stress of micro/nanoplastics from the perspective of QS-mediated microbial social behavior.

Investigations on genomic, topological and structural properties of diguanylate cyclases involved in Vibrio cholerae biofilm signalling using in silico techniques: Promising drug targets in combating cholera

During various stages of its life cycle, Vibrio cholerae initiate biofilm signalling cascade. Intercellular high level of the signalling nucleotide 3'-5' cyclic dimeric guanosine monophosphate (c-di-GMP), synthesized by diguanylate cyclases (DGCs) from its precursor molecule GTP, is crucial for biofilm formation. Present study endeavours to in silico approaches in evaluating genomic, physicochemical, topological and functional properties of six c-di-GMP regulatory DGCs (CdgA, CdgH, CdgK, CdgL, CdgM, VpvC) of V. cholerae. Genomic investigations unveiled that codon preferences were inclined towards AU ending over GC ending codons and overall GC content ranged from 44.6 to 49.5 with codon adaptation index ranging from 0.707 to 0.783. Topological analyses deciphered the presence of transmembrane domains in all proteins. All the DGCs were acidic, hydrophilic and thermostable. Only CdgA, CdgH and VpvC were predicted to be stable during in vitro conditions. Non-polar amino acids with leucine being the most abundant amino acid among these DGCs with α-helix as the predominant secondary structure, responsible for forming the transmembrane regions by secondary structure analysis. Tertiary structures of the proteins were obtained by computation using AlphaFold and trRosetta. Predicted structures by both the servers were compared in various aspects using PROCHECK, ERRAT and Modfold8 servers. Selected 3D structures were refined using GalaxyRefine. InterPro Scan revealed presence of a conserved GGDEF domain in all DGCs and predicted the active site residues in the GGDEF domain. Molecular docking studies using CB-DOCK 2 tool revealed that among the DGCs, VpvC exhibited highest affinity for GTP (-5.6 kcal/mol), which was closely followed by CdgL (-5.5 kcal/mol). MD simulations depicted all DGC-GTP complexes to be stable due to its considerably low eigenvalues. Such studies are considered to provide maiden insights into the genomic and structural properties of V. cholerae DGCs, actively involved in biofilm signalling systems, and it is projected to be beneficial in the discovery of novel DGC inhibitors that can target and downregulate the c-di-GMP regulatory system to develop anti-biofilm strategies against the cholera pathogen.

Sensitive and Broadly Compatible Transcription Factor-Based Biosensor for Monitoring c-di-GMP Dynamics in Biofilms

Biofilms are ubiquitous and have many negative effects, for example, in infections or biocorrosion. Given the critical role of the second messenger cyclic di-GMP (c-di-GMP) in biofilm formation, targeting a reduction in intracellular concentrations of c-di-GMP is believed to be a key aspect in the development of biofilm mitigation strategies. To facilitate this effort, here, we developed a transcription factor (TF)-based biosensor that integrates the TF FleQ from Pseudomonas aeruginosa with a PR-Ppel tandem promoter. The dynamic range of the biosensor was optimized by fine-tuning the TF expression. The biosensor exhibited broad compatibility and effectiveness in detecting decreases in c-di-GMP levels across various biofilm model organisms, including strains lacking FleQ or its homologues, such as Escherichia coli, Shewanella oneidensis, Comamonas testosteroni, and Acinetobacter baumannii, as well as P. aeruginosa containing FleQ. Additionally, we monitored c-di-GMP levels in biofilms formed by P. aeruginosa and S. oneidensis through a ratiometric, image-based quantification method. The methodology used the green fluorescence protein (GFP) as a reporter for c-di-GMP levels and 4',6-diamidino-2-phenylindole (DAPI) or the monomeric red fluorescence protein (mRFP) as the indicator for biofilm biomass. The GFP/DAPI or GFP/mRFP ratio gives effective c-di-GMP per unit of biomass. This TF-based biosensor provides an important tool to study c-di-GMP dynamics, which facilitates efforts in developing biofilm control strategies and understanding regulatory networks for biofilm development.

c-di-GMP is required for swarming in E. coli, producing colanic acid that acts as surfactant

Many bacteria use flagella to swim individually through bulk liquid or swarm collectively over a semi-solid surface. In Escherichia coli, c-di-GMP inhibits swimming via the effector protein YcgR. We show in this study that, contrary to its effect on swimming, a certain threshold level of c-di-GMP is required for swarming. Gene expression profiles first indicated that several c-di-GMP synthases-dgcJ, dgcM, and dgcO-were upregulated during swarming. Of these, we found DgcO to play a critical role in promoting the production of colanic acid-one of the three major exopolysaccharides in E. coli. DgcO has been reported to increase poly-β-1,6-N-acetylglucosamine (PGA) synthesis in E. coli as well. We show that colanic acid has hitherto-unknown surfactant properties that are expected to aid swarming.IMPORTANCEIt is well established that, in bacteria, c-di-GMP inhibits flagella-driven motility at various points in the pathway. Concomitantly, elevated c-di-GMP levels induce the expression and synthesis of a variety of exopolysaccharides that enmesh the bacteria in a biofilm, thereby also interfering with the flagella function. This study reports the surprising finding that, in Escherichia coli, the exopolysaccharide colanic acid is required to enable surface navigation and that the diguanylate cyclase DgcO is employed for this purpose. For surface navigation, there appears to be a sweet spot where c-di-GMP levels are just right to produce polysaccharides that can serve as surfactants and wetting agents rather than promote the formation of biofilms.

Proteo-Transcriptomic Analysis Reveals the Mechanisms Underlying Escherichia coli Phenotypic Shifts Under Blue Light

Bacteria can adapt their lifestyles, including microbial growth, metabolism, and biofilm formation, in response to light signaling. However, the molecular pathways through which blue light affects the lifestyle of Escherichia coli (E. coli) remain incomplete and poorly understood. To address this gap, transcriptomic and proteomic approaches were employed to analyze the physiological differences of E. coli under dark and blue light conditions. Our results indicate that, compared to dark conditions, blue light attenuates flagellar assembly, reduces cell motility and communication, and decreases biofilm formation in E. coli. In addition, this study elucidates the signaling pathways involved in the blue light-mediated regulation of E. coli behavior, providing a theoretical framework for understanding how E. coli responds to blue light signaling to modulate biofilm formation for the production of food chemicals.

Carbon-Based Nanomaterials Alter the Behavior and Gene Expression Patterns of Bacteria

One of the most dangerous characteristics of bacteria is their propensity to form biofilms and their resistance to the drugs used in clinical practice today. The total number of genes that can be categorized as virulence genes ranges from a few hundred to more than a thousand. The bacteria employ a variety of mechanisms to regulate the expression of these genes in a coordinated manner during infection. The search for new agents with anti-virulence capacity is therefore crucial. Nanotechnology provides safe platforms for targeted therapies to combat a broad spectrum of microbial infections. As a new class of innovative materials, carbon-based nanomaterials (CBNs), which include carbon dots, carbon nanotubes, graphene, and fullerenes can have strong antibacterial activity. Exposure to CBNs has been shown to affect bacterial gene expression patterns. This study investigated the effect of CBNs on the repression of specific genes related to bacterial virulence/pathogenicity.

Inhibitors of Cyclic Dinucleotide Phosphodiesterases and Cyclic Oligonucleotide Ring Nucleases as Potential Drugs for Various Diseases

The phosphodiester linkage is found in DNA, RNA and many signaling molecules, such as cyclic mononucleotide, cyclic dinucleotides (CDNs) and cyclic oligonucleotides (cONs). Enzymes that cleave the phosphodiester linkage (nucleases and phosphodiesterases) play important roles in cell persistence and fitness and have therefore become targets for various diseased states. While various inhibitors have been developed for nucleases and cyclic mononucleotide phosphodiesterases, and some have become clinical successes, there is a paucity of inhibitors of the recently discovered phosphodiesterases or ring nucleases that cleave CDNs and cONs. Inhibitors of bacterial c-di-GMP or c-di-AMP phosphodiesterases have the potential to be used as anti-virulence compounds, while compounds that inhibit the degradation of 3',3'-cGAMP, cA3, cA4, cA6 could serve as antibiotic adjuvants as the accumulation of these second messengers leads to bacterial abortive infection. In humans, 2'3'-cGAMP plays critical roles in antiviral and antitumor responses. ENPP1 (the 2'3'-cGAMP phosphodiesterase) or virally encoded cyclic dinucleotide phosphodiesterases, such as poxin, however, blunt this response. Inhibitors of ENPP1 or poxin-like enzymes have the potential to be used as anticancer and antiviral agents, respectively. This review summarizes efforts made towards the discovery and development of compounds that inhibit CDN phosphodiesterases and cON ring nucleases.

Biofilm Dispersal in Bacillus velezensis FZB42 Is Regulated by the Second Messenger c-di-GMP

Cyclic diguanosine monophosphate (c-di-GMP) is a second messenger that plays a crucial role in regulating biofilm development, yet the role in Gram-positive bacteria remains elusive. Here, we demonstrated that dispersed cells from biofilms of Bacillus velezensis FZB42 exhibit a unique phenotype and gene expression compared to planktonic cells. Transcriptomic analysis revealed 1327 downregulated and 1298 upregulated genes, among which the c-di-GMP phosphodiesterase coding yuxH gene was remarkably upregulated. Deletion of the yuxH gene led to elevated c-di-GMP levels accompanied by reduced amounts of "actively dispersed cells" from the pellicle and the capacity of motility. Deletion of spoIIIJ, spo0J, and kinA resulted in increased c-di-GMP levels and reduced biofilm dispersal ability. Also, the level of c-di-GMP was increased when adding the cues of inhibition biofilm dispersal such as glucose and calcium ions. Collectively, these present findings suggest the c-di-GMP level is negatively correlated with biofilm dispersal in Bacillus velezensis FZB42, which sheds new light on biofilm regulation in Bacillus velezensis FZB42.

A novel transcriptional regulator, CdeR, modulates the type III secretion system via c-di-GMP signaling in Dickeya dadantii

Dickeya dadantii is a bacterial pathogen that causes soft rot disease in many plant species worldwide, including temperate, subtropical, and tropical regions. This bacterium employs the type III secretion system (T3SS) to manipulate host immune responses. Although cyclic-di-GMP (c-di-GMP), a ubiquitous bacterial second messenger, negatively regulates the expression of T3SS genes in D. dadantii, the underlying mechanism remains unclear. In this study, we identified a potential transcriptional regulator, CdeR, which regulates the T3SS involving c-di-GMP. Through transposon mutagenesis, we discovered that deletion of cdeR in a gcpD mutant background restored T3SS gene expression. GcpD is a diguanylate cyclase responsible for c-di-GMP synthesis, and its deletion led to high T3SS gene expression due to low c-di-GMP. Further analysis revealed that, in the gcpD mutant background, CdeR regulates T3SS by manipulating intracellular c-di-GMP levels, involving another diguanylate cyclase, GcpL, whose expression is upregulated by CdeR. Additionally, we found that removing helical regions within the Helix-Turn-Helix DNA-binding domain of CdeR completely disrupted its regulation of the T3SS, underscoring the essential role of this domain in CdeR's functional activity. This study is the first to identify CdeR as a potential transcriptional regulator involved in T3SS regulation. Our findings provide significant insights into the regulatory mechanisms of T3SS and highlight the complex interactions between bacterial second messengers and transcriptional regulators in pathogenic bacteria.IMPORTANCEBacterial pathogens, such as Dickeya dadantii, must adapt to diverse environmental and host conditions by utilizing intricate regulatory networks to control virulence. This study identifies CdeR, a novel transcriptional regulator, as a crucial factor in modulating the expression of the type III secretion system (T3SS), a key virulence mechanism. Importantly, we show that CdeR operates in a cyclic-di-GMP (c-di-GMP)-dependent manner, linking this second messenger to T3SS regulation in D. dadantii for the first time. Our findings reveal a sophisticated interaction between c-di-GMP signaling and transcriptional regulation, highlighting how these systems collectively drive bacterial virulence. This work advances our understanding of bacterial pathogenesis and opens new avenues for developing targeted strategies to mitigate soft rot disease in crops, potentially improving agricultural productivity and plant health.

NMR Solution Structure of the N-Terminal GSPII Domain from the Thermus Thermophilus Traffic ATPase PilF and Reconstruction of its c-di-GMP Binding Capability

The cyclic dinucleotide c-di-GMP is an important second messenger molecule in bacteria and interacts with a variety of receptor molecules including RNA and protein domains. An important class of c-di-GMP-binding protein domains are the general secretory pathway type II (GSPII) domains as exemplified by the N-terminal domain of the ATPase MshE from Vibrio cholerae (MshEN). MshEN binds monomeric c-di-GMP via two consecutive copies of a 24-residue sequence motif, which form a compact 4-α-helical bundle. The ATPase PilF from Thermus thermophilus regulates pilus formation, motility and DNA-uptake. Its N-terminal section contains three consecutive GSPII domains (GSPII-A-GSPII-C) all with considerable sequence homology to MshEN. While the GSPII-B and the GSPII-C domains bind c-di-GMP, the GSPII-A domain does not. To determine why it is incapable of c-di-GMP-binding we determined the NMR-solution structure of this domain. Our structure shows how small deviations in the consensus motif sequence, a stabilizing N-terminal helical capping motif and intersubdomain interactions absent in MshEN cooperate to prevent c-di-GMP-binding. By combining point mutations and truncations, we re-established the c-di-GMP binding capability. Our findings shed new light on the evolution and functional diversification of GSPII domains and the importance of sequence variations for protein activity in this domain family.

Staphylococcus aureus uses a GGDEF protein to recruit diacylglycerol kinase to the membrane for lipid recycling

Staphylococcus aureus is a Gram-positive pathogen responsible for numerous antibiotic-resistant infections. Identifying vulnerabilities in S. aureus is crucial for developing new antibiotics to treat these infections. With this in mind, we probed the function of GdpS, a conserved Staphylococcal membrane protein containing a cytoplasmic GGDEF domain. These domains are canonically involved in cyclic-di-GMP signaling processes, but S. aureus is not known to make cyclic-di-GMP. Using a transposon screen, we found that loss of GdpS is lethal when combined with disruption in synthesis of the glycolipid anchor of a cell surface polymer called lipoteichoic acid (LTA) or with deletion of genes important in cell division. Taking advantage of a small molecule that inhibits LTA glycolipid anchor synthesis, we selected for suppressors of ΔgdpS lethality. The most prevalent suppressors were hypermorphic alleles of dgkB, which encodes a soluble diacylglycerol (DAG) kinase required to recycle DAG to phosphatidylglycerol. By following up on these suppressors, we found that the GGDEF domain of GdpS interacts directly with DgkB, orienting its active site at the membrane to promote DAG recycling. DAG kinase hypermorphs also suppressed the lethality caused by combined loss of gdpS and cell division factors, highlighting the importance of lipid homeostasis for cell division. GdpS' positive regulation of DAG kinase function was dependent on the GGDEF domain but not its catalytic residues. As the sole conserved GGDEF-domain protein in Staphylococci, GdpS promotes an enzymatic process independent of cyclic-di-GMP signaling, revealing a new function for the ubiquitously conserved GGDEF domain.

Functional diversity of AI-2/LuxS system in lactic acid bacteria: Impacts on biofilm formation and environmental resilience

A key component of microbial communication, autoinducer-2 (AI-2) signaling, affects several physiological processes, including environmental adaptation and biofilm formation in lactic acid bacteria (LAB). The multifarious contribution of AI-2, synthesized by LuxS, in improving biofilms and tolerance to hostile conditions in LAB has been investigated in this review. The evolutionary conservation and diversity of AI-2 are shown by a phylogenetic analysis of luxS gene among several LAB species. Furthermore, AI-2 signaling in LAB improves resistance to unfavorable environmental factors, including pH fluctuations, temperature extremes, and antimicrobial agents. Lactic acid bacteria could set off defenses against harmful impacts from environmental stresses.

Biofilm detection in the food industry: Challenges in identifying biofilm eps markers and analytical techniques with insights for Listeria monocytogenes

Extracellular polymeric substances (EPS) in biofilms are promising targets for eradicating biofilms and monitoring their presence, especially in the food industry. For this understanding, the composition of the EPS matrix is crucial. Ideally, a biofilm marker is found serving both purposes, but such a compound has not yet been discovered. This review aims to identify general biofilm EPS markers distinct from planktonic cells, focusing on macromolecules in the EPS matrix. It also evaluates the feasibility of this goal across different bacterial groups and environmental conditions and discusses EPS analysis methods. This review digs deeper into the EPS matrix starting with an introduction to the EPS matrix itself and describing some of its influencing factors. Next, a brief description of cell-to-cell communication within biofilms is provided, as these interactions significantly influence the EPS matrix. The main part of this review describes the macromolecules inside the EPS matrix and attempts to find biofilm EPS markers applied to bacteria in general and specifically to Listeria monocytogenes as biofilms are a major contributor to its persistence. The last part of the review focuses on the analytical techniques available to characterize the EPS matrix. The review revealed that although multiple candidates showed great potential as biofilm markers, none were unique but ubiquitous in all bacteria tested. To achieve easy biofilm detection with current techniques, it's necessary to identify markers specific to the environmental conditions and common bacterial groups within each food category, sector, or facility, due to the lack of standardization in these techniques. This tailored approach ensures more accurate and effective biofilm monitoring. Moreover, the lack of standardized analytical techniques, including quantification techniques, complexifies studying the EPS matrix and developing monitoring and intervention strategies. Optimizing analytical techniques is crucial for this tailored approach, as it requires refined methods for detection, characterization, and quantification. This ensures the accurate identification of biofilm markers specific to environmental conditions and bacterial groups within each food sector.

A WYL domain transcription factor regulates Lactiplantibacillus plantarum intestinal colonization via perceiving c-di-GMP

Cyclic diguanosine monophosphate (c-di-GMP) functions as a crucial bacterial second messenger to control diverse biological functions. Although numerous studies have reported the health effects of Lactiplantibacillus plantarum, the regulatory role of c-di-GMP in L. plantarum remains elusive. Here we show that c-di-GMP functions as an important signal molecule for intestinal colonization of L. plantarum. The intracellular c-di-GMP pool in this probiotic is governed principally by the diguanylate cyclases DgcB, DgcC, and DgcD and the phosphodiesterases PdeA and PdeD. Moreover, we reveal that the WYL domain transcription factor MbpR is a c-di-GMP effector in L. plantarum WCFS1. MbpR reduces the transcription level of mucin-binding proteins (MucBPs) via binding to a special motif within the coding sequences. Perception of c-di-GMP by the WYL domain reversed the inhibitory effect of MbpR on the expression of MucBPs, resulting in increased adherence to intestinal epithelial cells by L. plantarum. Overall, our study provides evidence that a WYL domain transcription factor participates in probiotic colonization by sensing c-di-GMP.

c-di-GMP inhibits rRNA methylation and impairs ribosome assembly in the presence of kanamycin

Cyclic diguanosine monophosphate (c-di-GMP) is a ubiquitous bacterial secondary messenger with diverse functions. A previous Escherichia coli proteome microarray identified that c-di-GMP binds to the 23S rRNA methyltransferases RlmI and RlmE. Here we show that c-di-GMP inhibits RlmI activity in rRNA methylation assays, and that it modulates ribosome assembly in the presence of kanamycin. Molecular dynamics simulation and mutagenesis studies reveal that c-di-GMP binds to RlmI at residues R64, R103, G114, and K201. Structural simulations indicate that c-di-GMP quenches RlmI activity by inducing the closure of the catalytic pocket. We also show that c-di-GMP promotes antibiotic tolerance through RlmI. Binding and methylation assays indicate that the inhibitory effect of c-di-GMP on RlmI is conserved across various pathogenic bacteria. Our data suggest an unexpected role for c-di-GMP in regulating ribosome assembly under stress through the inhibition of rRNA methyltransferases.

100+ years of phase variation: the premier bacterial bet-hedging phenomenon

Stochastic, reversible switches in the expression of Salmonella flagella variants were first described by Andrewes in 1922. Termed phase variation (PV), subsequent research found that this phenomenon was widespread among bacterial species and controlled expression of major determinants of bacterial-host interactions. Underlying mechanisms were not discovered until the 1970s/1980s but were found to encompass intrinsic aspects of DNA processes (i.e. DNA slippage and recombination) and DNA modifications (i.e. DNA methylation). Despite this long history, discoveries are ongoing with expansions of the phase-variable repertoire into new organisms and novel insights into the functions of known loci and switching mechanisms. Some of these discoveries are somewhat controversial as the term 'PV' is being applied without addressing key aspects of the phenomenon such as whether mutations or epigenetic changes are reversible and generated prior to selection. Another 'missing' aspect of PV research is the impact of these adaptive switches in real-world situations. This review provides a perspective on the historical timeline of the discovery of PV, the current state-of-the-art, controversial aspects of classifying phase-variable loci and possible 'missing' real-world effects of this phenomenon.

Multifaceted Antipathogenic Activity of Two Novel Natural Products, Chermesiterpenoid B and Chermesiterpenoid B Seco Acid Methyl Ester, Against Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic human pathogen that causes both acute and chronic infections due to its virulence factors, biofilm formation and the ability to suppress the host immune system. Quorum sensing (QS) plays a key role in regulating these pathogenic traits and also downregulates the expression of peroxisome proliferator-activated receptor-γ (PPAR-γ) in host cells. In this study, we isolated two novel natural products from the jellyfish-derived fungus Penicillium chermesinum, chermesiterpenoid B (Che B) seco acid methyl ester (Che B ester) and Che B. Both compounds act as partial agonists of PPAR-γ and exhibit anti-QS activity. Che B ester and Che B were found to inhibit biofilm formation, reduce the production of proteases and decrease the infectivity of P. aeruginosa, all without affecting bacterial growth. In host cells, Che B ester and Che B reduced P. aeruginosa-induced inflammation by activating PPAR-γ. This multifaceted function makes these compounds promising candidates for developing new antipathogenic agents against bacterial infections with few side effects.

Time-resolved compositional and dynamics analysis of biofilm maturation and dispersal via solid-state NMR spectroscopy

Dispersal plays a crucial role in the development and ecology of biofilms. While extensive studies focused on elucidating the molecular mechanisms governing this process, few have characterized the associated temporal changes in composition and structure. Here, we employed solid-state nuclear magnetic resonance (NMR) techniques to achieve time-resolved characterization of Bacillus subtilis biofilms over a 5-day period. The mature biofilm, established within 48 h, undergoes significant degradation in following 72 h. The steepest decline of proteins precedes that of exopolysaccharides, likely reflecting their distinct spatial distribution. Exopolysaccharide sugar units display clustered temporal patterns, suggesting the presence of distinct polysaccharide types. A sharp rise in aliphatic carbon signals on day 4 probably corresponds to a surge in biosurfactant production. Different dynamic regimes respond differently to dispersal: the mobile domain exhibits increased rigidity, while the rigid domain remains stable. These findings provide novel insights and perspectives on the complex process of biofilm dispersal.

Revealing systematic changes in the transcriptome during the transition from exponential growth to stationary phase

The composition of bacterial transcriptomes is determined by the transcriptional regulatory network (TRN). The TRN regulates the transition from one physiological state to another. Here, we use independent component analysis to monitor the composition of the transcriptome during the transition from the exponential growth phase to the stationary phase. With Escherichia coli K-12 MG1655 as a model strain, we trigger the transition using carbon, nitrogen, and sulfur starvation. We find that (i) the transition to the stationary phase accompanies common transcriptome changes, including increased stringent responses and reduced production of cellular building blocks and energy regardless of the limiting element; (ii) condition-specific changes are strongly associated with transcriptional regulators (e.g., Crp, NtrC, CysB, Cbl) responsible for metabolizing the limiting element; and (iii) the shortage of each limiting element differentially affects the production of amino acids and extracellular polymers. This study demonstrates how the combination of genome-scale datasets and new data analytics reveals the fundamental characteristics of a key transition in the life cycle of bacteria.

IMPORTANCE

Nutrient limitations are critical environmental perturbations in bacterial physiology. Despite its importance, a detailed understanding of how bacterial transcriptomes are adjusted has been limited. By utilizing independent component analysis (ICA) to decompose transcriptome data, this study reveals key regulatory events that enable bacteria to adapt to nutrient limitations. The findings not only highlight common responses, such as the stringent response, but also condition-specific regulatory shifts associated with carbon, nitrogen, and sulfur starvation. The insights gained from this work advance our knowledge of bacterial physiology, gene regulation, and metabolic adaptation.

Harnessing hypoxia: bacterial adaptation and chronic infection in cystic fibrosis

The exquisite ability of bacteria to adapt to their environment is essential for their capacity to colonize hostile niches. In the cystic fibrosis (CF) lung, hypoxia is among several environmental stresses that opportunistic pathogens must overcome to persist and chronically colonize. Although the role of hypoxia in the host has been widely reviewed, the impact of hypoxia on bacterial pathogens has not yet been studied extensively. This review considers the bacterial oxygen-sensing mechanisms in three species that effectively colonize the lungs of people with CF, namely Pseudomonas aeruginosa, Burkholderia cepacia complex, and Mycobacterium abscessus and draws parallels between their three proposed oxygen-sensing two-component systems: BfiSR, FixLJ, and DosRS, respectively. Moreover, each species expresses regulons that respond to hypoxia: Anr, Lxa, and DosR, and encode multiple proteins that share similar homologies and function. Many adaptations that these pathogens undergo during chronic infection, including antibiotic resistance, protease expression, or changes in motility, have parallels in the responses of the respective species to hypoxia. It is likely that exposure to hypoxia in their environmental habitats predispose these pathogens to colonization of hypoxic niches, arming them with mechanisms than enable their evasion of the immune system and establish chronic infections. Overcoming hypoxia presents a new target for therapeutic options against chronic lung infections.

Surface remodeling and inversion of cell-matrix interactions underlie community recognition and dispersal in Vibrio cholerae biofilms

Biofilms are ubiquitous surface-associated bacterial communities embedded in an extracellular matrix. It is commonly assumed that biofilm cells are glued together by the matrix; however, how the specific biochemistry of matrix components affects the cell-matrix interactions and how these interactions vary during biofilm growth remain unclear. Here, we investigate cell-matrix interactions in Vibrio cholerae, the causative agent of cholera. We combine genetics, microscopy, simulations, and biochemical analyses to show that V. cholerae cells are not attracted to the main matrix component (Vibrio polysaccharide, VPS), but can be attached to each other and to the VPS network through surface-associated VPS and crosslinks formed by the protein Bap1. Downregulation of VPS production and surface trimming by the polysaccharide lyase RbmB cause surface remodeling as biofilms age, shifting the nature of cell-matrix interactions from attractive to repulsive and facilitating cell dispersal as aggregated groups. Our results shed light on the dynamics of diverse cell-matrix interactions as drivers of biofilm development.

The structural basis for high-affinity c-di-GMP binding to the GSPII-B domain of the traffic ATPase PilF from Thermus thermophilus

c-di-GMP is an important second messenger in bacteria regulating, for example motility, biofilm formation, cell wall biosynthesis, infectivity, and natural transformability. It binds to a multitude of intracellular receptors. This includes proteins containing general secretory pathway II (GSPII) domains such as the N-terminal domain of the Vibrio cholerae ATPase MshE (MshEN) which binds c-di-GMP with two copies of a 24-amino acids sequence motif. The traffic ATPase PilF from Thermus thermophilus is important for type IV pilus biogenesis, twitching motility, surface attachment, and natural DNA-uptake and contains three consecutive homologous GPSII domains. We show that only two of these domains bind c-di-GMP and define the structural basis for the exceptional high affinity of the GSPII-B domain for c-di-GMP, which is 83-fold higher than that of the prototypical MshEN domain. Our work establishes an extended consensus sequence for the c-di-GMP-binding motif and highlights the role of hydrophobic residues for high-affinity recognition of c-di-GMP. Our structure is the first example for a c-di-GMP-binding domain not relying on arginine residues for ligand recognition. We also show that c-di-GMP-binding induces local unwinding of an α-helical turn as well as subdomain reorientation to reinforce intermolecular contacts between c-di-GMP and the C-terminal subdomain. Abolishing c-di-GMP binding to GSPII-B reduces twitching motility and surface attachment but not natural DNA-uptake. Overall, our work contributes to a better characterization of c-di-GMP binding in this class of effector domains, allows the prediction of high-affinity c-di-GMP-binding family members, and advances our understanding of the importance of c-di-GMP binding for T4P-related functions.

Comparation on the responses and resilience of single-Anammox system and synergistic partial-denitrification/anammox system to long-term nutrient starvation: Performance and metagenomic insights

Starvation disturbance was a common problem in biological sewage treatment processes. However, understanding about the responses and resilience of different active anammox biomass in autotrophic and heterotrophic systems to long-term nutrient starvation remains limited. This study compared responses and potential recovery mechanisms of autotrophic single-Anammox and heterotrophic synergistic partial-denitrification/anammox (PD/anammox) systems to prolonged starvation (31-40 days). After starvation, total inorganic nitrogen (TIN) removal efficiency of single-Anammox and synergistic PD/anammox systems decreased to 62.16 % and 78.52 %, respectively, of their original level. After the nutrient resupply, the performance of both systems gradually recovered to a similar-to-pre-starvation level at the rate of 1.26 %/day and 1.89 %/day, respectively. Compared with single-Anammox system, complex synergistic relationship of microorganisms and effective quorum sensing (QS) regulation strategies might mitigate the negative influences were caused by starvation and ensure the performance quickly return of synergistic PD/anammox system. This study would contribute to promote the application of Anammox technology.

A widespread family of viral sponge proteins reveals specific inhibition of nucleotide signals in anti-phage defense

Cyclic oligonucleotide-based antiviral signaling systems (CBASS) are bacterial anti-phage defense operons that use nucleotide signals to control immune activation. Here we biochemically screen 57 diverse E. coli and Bacillus phages for the ability to disrupt CBASS immunity and discover anti-CBASS 4 (Acb4) from the Bacillus phage SPO1 as the founding member of a large family of >1,300 immune evasion proteins. A 2.1 Å crystal structure of Acb4 in complex with 3'3'-cGAMP reveals a tetrameric assembly that functions as a sponge to sequester CBASS signals and inhibit immune activation. We demonstrate Acb4 alone is sufficient to disrupt CBASS activation in vitro and enable immune evasion in vivo. Analyzing phages that infect diverse bacteria, we explain how Acb4 selectively targets nucleotide signals in host defense and avoids disruption of cellular homeostasis. Together, our results reveal principles of immune evasion protein evolution and explain a major mechanism phages use to inhibit host immunity.

ArgR regulates motility and virulence through positive control of flagellar genes and inhibition of diguanylate cyclase expression in Aeromonas veronii

Flagella are essential for biofilm formation, adhesion, virulence, and motility. In this study, the deletion of argR resulted in defects in flagellar synthesis and reduced motility, nevertheless, the underlying mechanism by which ArgR regulated bacterial motility remained unclear. ChIP-Seq and RNA-Seq analysis revealed that ArgR regulated the expression of flagellar genes, concluding two-component system flrBC and multitudinous flagellar structure genes. Specifically, ArgR bound to the ARG box in the flrBC promoter, positively regulating flrBC expression, which in turn promoted flagellar synthesis and enhanced motility. Additionally, in the absence of arginine, ArgR inhibited the expression of diguanylate cyclase, leading to reduced c-di-GMP levels, thereby alleviating its inhibitory effect on motility. Thus, ArgR coordinated two distinct pathways to regulate flagellar assembly and motility, ultimately affecting adhesion, virulence, and biofilm formation. In summary, this study elucidates the molecular mechanism by which ArgR regulates motility, highlighting its crucial role in bacterial virulence and offering new insights for the prevention and control of pathogenic bacteria.

Choline metabolism modulates cyclic-di-GMP signaling and virulence of Pseudomonas aeruginosa in a macrophage infection model

BACKGROUND

Bacterial pathogens frequently encounter host-derived metabolites during their colonization and invasion processes, which can serve as nutrients, antimicrobial agents, or signaling molecules for the pathogens. The essential nutrient choline (Cho) is widely known to be utilized by a diverse range of bacteria and may undergo conversion into the disease-associated metabolite trimethylamine (TMA). However, the impact of choline metabolism on bacterial physiology and virulence remains largely unexplored.

METHODS

Here, we employed an in vitro infection model to investigate the role of Cho in intracellular survival and virulence of Pseudomonas aeruginosa (P. aeruginosa). Additionally, a comprehensive RNA-seq based transcriptomic analysis and various phenotypic assays were performed to elucidate the impacts of Cho on P. aeruginosa.

RESULTS

We observed that the Cho metabolite glycine betaine (GB) effectively reduced intracellular levels of cyclic-di-GMP (c-di-GMP). Supplementation of Cho or GB in P. aeruginosa had thus affected c-di-GMP regulated phenotypes, such as pyoverdine production, biofilm formation, and mobility. Depletion of Cho metabolism through knockout of the betAB operon resulted in compromised intracellular survival of P. aeruginosa. Notably, the P. aeruginosa betAB mutant elicited a more robust protective inflammatory response compared to the wild-type strain.

CONCLUSION

Our study showed that P. aeruginosa Cho metabolism not only interferes host nutritional immunity, but also directly affect multiple virulence phenotypes through modulation of c-di-GMP signaling.

Capturing the micro-communities: Insights into biogenesis and architecture of bacterial biofilms

Biofilm is an assemblage of microorganisms embedded within the extracellular matrix that provides mechanical stability, nutrient absorption, antimicrobial resistance, cell-cell interactions, and defence against host immune system. Various biomolecules such as lipids, carbohydrates, protein polymers (amyloid), and eDNA are present in the matrix playing significant role in determining the distinctive properties of biofilm. The formation of biofilms contributes to resistance against antimicrobial therapy in most of the human infections and exacerbates existing diseases. Therefore, this field requires several state-of-the-art techniques to fully understand the 3-D organization of biofilms, their cell behaviour and responses to pharmaceutical treatments. Here, we explore the assembly and regulation of biofilm biogenesis in the context of matrix components and highlight the significance of high-resolution imaging and analysing techniques for monitoring complex biofilm architecture. Our review also emphasizes the novelty and advancements in techniques to visualise biofilm structure and composition, providing valuable insights to understand biofilm-related infections.

Distinct transcriptome and traits of freshly dispersed Pseudomonas aeruginosa cells

Bacteria assume two distinct lifestyles: the planktonic and biofilm modes of growth. Additionally, dispersion has emerged as a third phenotype, accompanied by the distinct phenotypes and the unique expression of >600 genes. Here, we asked whether the distinct phenotype of dispersed cells is already apparent within minutes of egressing from the biofilm. We used RNA-seq to show that the physiology of freshly dispersed cells from Pseudomonas aeruginosa biofilms is highly different from those of planktonic and biofilm cells, apparent by dispersed cells uniquely expressing 194 genes. Unique and differentially expressed genes relative to planktonic or biofilm cells include genes associated with type IV pili, pyoverdine, type III and type VI secretion systems, and antibiotic resistance that are downregulated in dispersed cells, whereas the transcript abundance of genes involved in swimming motility, Hxc type II secretion system and various other virulence factors, and metabolic and energy-generating pathways are increased, indicative of dispersion coinciding with an awakening and re-energizing of dispersed cells, and a switch in virulence, further apparent by freshly dispersed cells significantly subverting engulfment by macrophages. The findings suggest that dispersed cells display a distinct phenotype within minutes of egressing from the biofilm, with freshly dispersed cells already capable of efficiently evading phagocytosis.

IMPORTANCE

Dispersion is considered a transitionary phenotype, enabling bacteria to switch between the communal, biofilm lifestyle, where cells share resources and are protected from harmful conditions to the planktonic state. Here, we demonstrate that within minutes of leaving the biofilm, dispersed cells express genes and display phenotypic traits that are distinct from biofilms and planktonic cells. Our findings suggest that dispersed cells quickly adapt to a less structured and protected but more nutrient-rich environment, with this trade-off in environment coinciding with an awakening and a switch in virulence, specifically a switch from directly intoxicating host cells and potential competitors toward more broadly active virulence factors and strategies of evasion. To our knowledge, this is the first report of dispersed cells' distinct (trade-off) phenotype and their enhanced resilience so soon after egressing from the biofilm.

Methanogenic partner influences cell aggregation and signalling of Syntrophobacterium fumaroxidans

For several decades, the formation of microbial self-aggregates, known as granules, has been extensively documented in the context of anaerobic digestion. However, current understanding of the underlying microbial-associated mechanisms responsible for this phenomenon remains limited. This study examined morphological and biochemical changes associated with cell aggregation in model co-cultures of the syntrophic propionate oxidizing bacterium Syntrophobacterium fumaroxidans and hydrogenotrophic methanogens, Methanospirillum hungatei or Methanobacterium formicicum. Formerly, we observed that when syntrophs grow for long periods with methanogens, cultures tend to form aggregates visible to the eye. In this study, we maintained syntrophic co-cultures of S. fumaroxidans with either M. hungatei or M. formicicum for a year in a fed-batch growth mode to stimulate aggregation. Millimeter-scale aggregates were observed in both co-cultures within the first 5 months of cultivation. In addition, we detected quorum sensing molecules, specifically N-acyl homoserine lactones, in co-culture supernatants preceding the formation of macro-aggregates (with diameter of more than 20 μm). Comparative transcriptomics revealed higher expression of genes related to signal transduction, polysaccharide secretion and metal transporters in the late-aggregation state co-cultures, compared to the initial ones. This is the first study to report in detail both biochemical and physiological changes associated with the aggregate formation in syntrophic methanogenic co-cultures. KEYPOINTS: • Syntrophic co-cultures formed mm-scale aggregates within 5 months of fed-batch cultivation. • N-acyl homoserine lactones were detected during the formation of aggregates. • Aggregated co-cultures exhibited upregulated expression of adhesins- and polysaccharide-associated genes.

Bacterial communication intelligently regulates their interactions in anammox consortia under decreasing temperatures

Bacterial communication could affect their interactions, but whether this regulation has "intelligence" is still unknown. Here, we operated an anammox reactor under temperature gradient from 35 °C to 15 °C. As results, expression abundance of bacterial communication genes increased by 12 % significantly after temperature declined. Division of labor among distinct signal molecules was evidenced by complementary roles of acyl-homoserine lactones (AHLs) and diffusible signal factor (DSF) in affecting bacterial interactions and niche differentiation respectively. DSF based inter-and intra-communication helped bacteria match their investments and rewards during cross-feedings. When temperature was below 25 °C, transcription regulator Clp governed by DSF inclined to promote folate and molybdenum cofactor biosynthesis, which coincidentally benefited one anammox species more than another. Meanwhile, for the anammox species with lower benefits, Clp also inclined to decrease biosynthesis of costly tryptophan and vitamin B1 rewarding others. Interestingly, bacterial communication inclined to influence the bacteria with many cooperators in the community or with high capacity to export cofactors for cross-feedings when temperature decreased. As results, these bacteria were enriched which could lead to closer interactions in whole community to adapt to low temperatures. The discovered intelligence of bacterial communication opened another window for understanding bacterial sociobiology.

Antibiotic resistance in Pseudomonas aeruginosa: mechanisms and emerging treatment

Pseudomonas aeruginosa, able to survive on the surfaces of medical devices, is a life-threatening pathogen that mainly leads to nosocomial infection especially in immunodeficient and cystic fibrosis (CF) patients. The antibiotic resistance in P. aeruginosa has become a world-concerning problem, which results in reduced and ineffective therapy efficacy. Besides intrinsic properties to decrease the intracellular content and activity of antibiotics, P. aeruginosa develops acquired resistance by gene mutation and acquisition, as well as adaptive resistance under specific situations. With in-depth research on drug resistance mechanisms and the development of biotechnology, innovative strategies have emerged and yielded benefits such as screening for new antibiotics based on artificial intelligence technology, utilizing drugs synergistically, optimizing administration, and developing biological therapy. This review summarizes the recent advances in the mechanisms of antibiotic resistance and emerging treatments for combating resistance, aiming to provide a reference for the development of therapy against drug-resistant P. aeruginosa.

Phylogenetic distribution and characterization of conserved C-di-GMP metabolizing proteins in filamentous cyanobacterium Arthrospira

Cyclic diguanosine monophosphate (c-di-GMP) is a second messenger in bacteria that regulates multiple biological functions, including biofilm formation, virulence, and intercellular communication. However, c-di-GMP signaling is virtually unknown in economically important filamentous cyanobacteria, Arthrospira. In this study, we predicted 31 genes encoding GGDEF-domain proteins from A. platensis NIES39 as potential diguanylate cyclases (DGCs). Phylogenetic distribution analysis showed five genes (RS09460, RS04865, RS26155, M01840, and E02220) with highly conserved distribution across 25 Arthrospira strains. Adc1 encoded by RS09460 was further characterized as a typical DGC. By establishing the genetic transformation system of Arthrospira, we demonstrated that the overexpression of Adc1 promoted the production of extracellular polymeric substances (EPS), which in turn caused the aggregation of filaments. We also confirmed that RS04865 and RS26155 may encode active DGCs, while enzymatic activity assays showed that proteins encoded by M01840 and E02220 have phosphodiesterase (PDE) activity. Meta-analysis revealed that the expression profiles of RS09460 and RS04865 were unaffected under 31 conditions, suggesting that they may function as conserved genes in maintaining the basal level of c-di-GMP in Arthrospira. In summary, this report will provide the basis for further studies of c-di-GMP signal in Arthrospira.

Cyclic Diguanylate G-Quadruplex Inducer-Quorum Sensing Inhibitor Hybrids as Bifunctional Anti-biofilm and Anti-virulence Agents Against Pseudomonas aeruginosa

The release of virulence factors and biofilm formation by Pseudomonas aeruginosa are pivotal drivers of its severe pathogenicity and antibiotic resistance. Based on our prior findings, cyclic di-GMP (c-di-GMP) G-quadruplex inducers are promising biofilm inhibitors and that quorum sensing systems are central regulators of virulence, we aimed to design and synthesize c-di-GMP G-quadruplex inducer-quorum sensing inhibitor hybrids. These hybrids were envisioned as bifunctional agents with both antibiofilm and antivirulence capabilities. Hybrids A7 and A11, characterized by their quinoline and 3-indole rings, emerged as potent inhibitors. They achieve this dual action by inducing c-di-GMP G-quadruplex formation and disrupting the las and pqs signaling system. Additionally, hybrids A7 and A11 attenuated virulence factors and inhibited the motility phenotypes of P. aeruginosa. Furthermore, when tested in in vivo Caenorhabditis elegans infection models, these hybrids, in combination with antibiotics such as tetracycline, improved survival rates, all while maintaining a favorable biosafety profile.

Surface remodeling and inversion of cell-matrix interactions underlies community recognition and dispersal in Vibrio cholerae biofilms

Biofilms are ubiquitous surface-associated bacterial communities embedded in an extracellular matrix. While it is commonly assumed that biofilm-dwelling cells are glued together by the matrix, how the cell-matrix interaction depends on the specific biochemistry of the matrix components and how this interaction varies during biofilm growth remains unclear. Here, we investigated cell-matrix interactions in Vibrio cholerae ( Vc ), the causative agent of cholera. We combine genetics, microscopy, simulation, and biochemical tools to show that Vc cells are not attractive to V ibrio p oly s accharide (VPS), the main matrix component, but they can be bridged with each other and to the VPS network through crosslinking by Bap1. Downregulation of VPS and surface trimming by the polysaccharide lyase RbmB cause surface remodeling as biofilms age, shifting the nature of cell-matrix interactions from attractive to repulsive and facilitating cell dispersal as aggregated groups. Our results suggest a new conceptual model in understanding the intricate cell-matrix interaction as the major driver for biofilm development, which is potentially generalizable to certain other biofilm-forming species and exopolysaccharides.

BvgR is important for virulence-related phenotypes in Bordetella bronchiseptica

Bordetella bronchiseptica is a pathogenic bacterium that causes respiratory infections in mammals. Adhesins, toxins, and secretion systems necessary for infection are regulated by the two-component system BvgAS. When the BvgAS system is inactive, there is no transcription of virulence-activated genes, and virulence-repressed genes (vrg) are expressed. The regulation of some vrgs in B. bronchiseptica is dependent upon the virulence-activated gene bvgR. Although having a regulatory role, no DNA-binding domain is described for BvgR. Instead, it contains an EAL domain, usually found in cyclic-di-GMP (c-di-GMP)-specific phosphodiesterases. c-di-GMP is a bacterial second messenger that regulates multiple phenotypes in bacteria, including B. bronchiseptica. The current study aimed to deepen our knowledge about BvgR. We employed RNA-seq analysis to define the BvgR regulon, and then we investigated the phenotypes in which BvgR regulation might be involved such as biofilm formation, cytotoxicity, and virulence. Our result revealed that BvgR inhibits biofilm formation and flagellin expression in virulent phase. Although BvgR has long been considered a repressor protein, our results show that it also upregulates almost 100 genes. This regulation is likely indirect, as BvgR lacks a DNA-binding domain. Notably, among the upregulated genes, we identified 15 associated with the type three secretion system. Consistent with these findings, a B. bronchiseptica strain deficient in bvgR was less cytotoxic than the wild-type strain, elicited a milder immune response, and was less able to persist in the lower respiratory tract of mice.IMPORTANCEBordetella bronchiseptica is a harmful bacterium responsible for respiratory infections in mammals. Its ability to cause disease is tightly regulated by a system called BvgAS. In this study, we focused on understanding the role of a specific gene called bvgR in regulating B. bronchiseptica's virulence factors. Our findings revealed that BvgR, previously thought to primarily repress gene expression, actually plays a complex role in both activating and inhibiting various genes involved in bacterial virulence. This newfound understanding sheds light on the intricate mechanisms underlying B. bronchiseptica's ability to cause infections, providing valuable insights for developing strategies to combat these infections in humans and animals.

Cyclic Diguanylate in the Wild: Roles During Plant and Animal Colonization

Cyclic diguanylate (c-di-GMP) is a near-ubiquitous signaling molecule that regulates the motility-to-sessility transition in many bacterial species. Among the phenotypes influenced by c-di-GMP are biofilm formation, motility, cell cycle, and virulence. The hallmark phenotypes regulated by c-di-GMP-biofilm formation and motility-are key determinants of host-bacterial interactions. A large body of research has identified the roles of c-di-GMP in regulating phenotypes in culture. While numerous studies have investigated roles for c-di-GMP during the establishment and maintenance of pathogenic host-bacterial associations, considerably less attention has been devoted to defining the roles of c-di-GMP during beneficial and commensal associations. This review describes the known roles of c-di-GMP in regulating phenotypes that contribute to host colonization, with a focus on knowledge gaps and future prospects for examining c-di-GMP during beneficial colonization.

Gastrointestinal Biofilms: Endoscopic Detection, Disease Relevance, and Therapeutic Strategies

Gastrointestinal biofilms are matrix-enclosed, highly heterogenic and spatially organized polymicrobial communities that can cover large areas in the gastrointestinal tract. Gut microbiota dysbiosis, mucus disruption, and epithelial invasion are associated with pathogenic biofilms that have been linked to gastrointestinal disorders such as irritable bowel syndrome, inflammatory bowel diseases, gastric cancer, and colorectal cancer. Intestinal biofilms are highly prevalent in ulcerative colitis and irritable bowel syndrome patients, and most endoscopists will have observed such biofilms during colonoscopy, maybe without appreciating their biological and clinical importance. Gut biofilms have a protective extracellular matrix that renders them challenging to treat, and effective therapies are yet to be developed. This review covers gastrointestinal biofilm formation, growth, appearance and detection, biofilm architecture and signalling, human host defence mechanisms, disease and clinical relevance of biofilms, therapeutic approaches, and future perspectives. Critical knowledge gaps and open research questions regarding the biofilm's exact pathophysiological relevance and key hurdles in translating therapeutic advances into the clinic are discussed. Taken together, this review summarizes the status quo in gut biofilm research and provides perspectives and guidance for future research and therapeutic strategies.

Heme pocket hydrogen bonding residue interactions within the Pectobacterium Diguanylate cyclase-containing globin coupled sensor: A resonance Raman study

Heme-based sensor proteins are used by organisms to control signaling and physiological effects in response to their gaseous environment. Globin-coupled sensors (GCS) are oxygen-sensing proteins that are widely distributed in bacteria. These proteins consist of a heme globin domain linked by a middle domain to various output domains, including diguanylate cyclase domains, which are responsible for synthesizing c-di-GMP, a bacterial second messenger crucial for regulating biofilm formation. To understand the roles of heme pocket residues in controlling activity of the diguanylate cyclase domain, variants of the Pectobacterium carotovorum GCS (PccGCS) were characterized by enzyme kinetics and resonance Raman (rR) spectroscopy. Results of these studies have identified roles for hydrogen bonding and heme edge residues in modulating heme pocket conformation and flexibility. Better understanding of the ligand-dependent GCS signaling mechanism and the residues involved may allow for future development of methods to control O2-dependent c-di-GMP production.

Proteomics and EPS Compositional Analysis Reveals Desulfovibrio bisertensis SY-1 Induced Corrosion on Q235 Steel by Biofilm Formation

Microorganisms that exist in the seawater form microbial biofilms on materials used in marine construction, especially on metal surfaces submerged in seawater, where they form biofilms and cause severe corrosion. Biofilms are mainly composed of bacteria and their secreted polymeric substances. In order to understand how biofilms promote metal corrosion, planktonic and biofilm cells of Desulfovibrio bizertensis SY-1 (D. bizertensis) from Q235 steel were collected and analyzed as to their intracellular proteome and extracellular polymeric substances (EPS). The intracellular proteome analysis showed that the cellular proteins were strongly regulated in biofilm cells compared to planktonic cells, e.g., along with flagellar proteins, signaling-related proteins were significantly increased, whereas energy production and conversion proteins and DNA replication proteins were significantly regulated. The up-and-down regulation of proteins revealed that biofilm formation by bacteria on metal surfaces is affected by flagellar and signaling proteins. A significant decrease in DNA replication proteins indicated that DNA is no longer replicated and transcribed in mature biofilms, thus reducing energy consumption. Quantitative analysis and lectin staining of the biofilm on the metal's surface revealed that the bacteria secreted a substantial amount of EPS when they began to attach to the surface, and proteins dominated the main components of EPS. Further, the infrared analysis showed that the secondary structure of the proteins in the EPS of the biofilm was mainly dominated by β-sheet and 3-turn helix, which may help to enhance the adhesion of EPS. The functional groups of EPS analyzed using XPS showed that the C element of EPS in the biofilm mainly existed in the form of combinations with N. Furthermore, the hydroxyl structure in the EPS extracted from the biofilm had a stronger hydrogen bonding effect, which could maintain the stability of the EPS structure and biofilm. The study results revealed that D. bizertensis regulates the metabolic pathways and their secreted EPS structure to affect biofilm formation and cause metal corrosion, which has a certain reference significance for the study of the microbially influenced corrosion (MIC) mechanism.

New insights into the signal transduction mechanism of O2-sensing FixL and other biological heme-based sensor proteins

Recent structural and biophysical studies of O2-sensing FixL, NO-sensing soluble guanylate cyclase, and other biological heme-based sensing proteins have begun to reveal the details of their molecular mechanisms and shed light on how nature regulates important biological processes such as nitrogen fixation, blood pressure, neurotransmission, photosynthesis and circadian rhythm. The O2-sensing FixL protein from S. meliloti, the eukaryotic NO-sensing protein sGC, and the CO-sensing CooA protein from R. rubrum transmit their biological signals through gas-binding to the heme domain of these proteins, which inhibits or activates the regulatory, enzymatic domain. These proteins appear to propagate their signal by specific structural changes in the heme sensor domain initiated by the appropriate gas binding to the heme, which is then propagated through a coiled-coil linker or other domain to the regulatory, enzymatic domain that sends out the biological signal. The current understanding of the signal transduction mechanisms of O2-sensing FixL, NO-sensing sGC, CO-sensing CooA and other biological heme-based gas sensing proteins and their mechanistic themes are discussed, with recommendations for future work to further understand this rapidly growing area of biological heme-based gas sensors.

Small molecule communication of Legionella: the ins and outs of autoinducer and nitric oxide signaling

SUMMARYLegionella pneumophila is a Gram-negative environmental bacterium, which survives in planktonic form, colonizes biofilms, and infects protozoa. Upon inhalation of Legionella-contaminated aerosols, the opportunistic pathogen replicates within and destroys alveolar macrophages, thereby causing a severe pneumonia termed Legionnaires' disease. Gram-negative bacteria employ low molecular weight organic compounds as well as the inorganic gas nitric oxide (NO) for cell-cell communication. L. pneumophila produces, secretes, and detects the α-hydroxyketone compound Legionella autoinducer-1 (LAI-1, 3-hydroxypentadecane-4-one). LAI-1 is secreted by L. pneumophila in outer membrane vesicles and not only promotes communication among bacteria but also triggers responses from eukaryotic cells. L. pneumophila detects NO through three different receptors, and signaling through the volatile molecule translates into fluctuations of the intracellular second messenger cyclic-di-guanylate monophosphate. The LAI-1 and NO signaling pathways are linked via the pleiotropic transcription factor LvbR. In this review, we summarize current knowledge about inter-bacterial and inter-kingdom signaling through LAI-1 and NO by Legionella species.

Unravelling the Roles of Bacterial Nanomachines Bistability in Pathogens' Life Cycle

Bacterial nanomachines represent remarkable feats of evolutionary engineering, showcasing intricate molecular mechanisms that enable bacteria to perform a diverse array of functions essential to persist, thrive, and evolve within ecological and pathological niches. Injectosomes and bacterial flagella represent two categories of bacterial nanomachines that have been particularly well studied both at the molecular and functional levels. Among the diverse functionalities of these nanomachines, bistability emerges as a fascinating phenomenon, underscoring their dynamic and complex regulation as well as their contribution to shaping the bacterial community behavior during the infection process. In this review, we examine two closely related bacterial nanomachines, the type 3 secretion system, and the flagellum, to explore how the bistability of molecular-scale devices shapes the bacterial eco-pathological life cycle.

Intestinal biofilms: pathophysiological relevance, host defense, and therapeutic opportunities

SUMMARYThe human intestinal tract harbors a profound variety of microorganisms that live in symbiosis with the host and each other. It is a complex and highly dynamic environment whose homeostasis directly relates to human health. Dysbiosis of the gut microbiota and polymicrobial biofilms have been associated with gastrointestinal diseases, including irritable bowel syndrome, inflammatory bowel diseases, and colorectal cancers. This review covers the molecular composition and organization of intestinal biofilms, mechanistic aspects of biofilm signaling networks for bacterial communication and behavior, and synergistic effects in polymicrobial biofilms. It further describes the clinical relevance and diseases associated with gut biofilms, the role of biofilms in antimicrobial resistance, and the intestinal host defense system and therapeutic strategies counteracting biofilms. Taken together, this review summarizes the latest knowledge and research on intestinal biofilms and their role in gut disorders and provides directions toward the development of biofilm-specific treatments.

Heme pocket modulates protein conformation and diguanylate cyclase activity of a tetrameric globin coupled sensor

Bacteria use the second messenger cyclic dimeric guanosine monophosphate (c-di-GMP) to control biofilm formation and other key phenotypes in response to environmental signals. Changes in oxygen levels can alter c-di-GMP signaling through a family of proteins termed globin coupled sensors (GCS) that contain diguanylate cyclase domains. Previous studies have found that GCS diguanylate cyclase activity is controlled by ligand binding to the heme within the globin domain, with oxygen binding resulting in the greatest increase in catalytic activity. Herein, we present evidence that heme-edge residues control O2-dependent signaling in PccGCS, a GCS protein from Pectobacterium carotovorum, by modulating heme distortion. Using enzyme kinetics, resonance Raman spectroscopy, small angle X-ray scattering, and multi-wavelength analytical ultracentrifugation, we have developed an integrated model of the full-length PccGCS tetramer and have identified conformational changes associated with ligand binding, heme conformation, and cyclase activity. Taken together, these studies provide new insights into the mechanism by which O2 binding modulates activity of diguanylate cyclase-containing GCS proteins.

Stenotrophomonas maltophilia uses a c-di-GMP module to sense the mammalian body temperature during infection

The body temperature of Warm-blooded hosts impedes and informs responses of bacteria accustomed to cooler environments. The second messenger c-di-GMP modulates bacterial behavior in response to diverse, yet largely undiscovered, stimuli. A long-standing debate persists regarding whether a local or a global c-di-GMP pool plays a critical role. Our research on a Stenotrophomonas maltophilia strain thriving at around 28°C, showcases BtsD as a thermosensor, diguanylate cyclase, and effector. It detects 37°C and diminishes c-di-GMP synthesis, resulting in a responsive sequence: the periplasmic c-di-GMP level is decreased, the N-terminal region of BtsD disengages from c-di-GMP, activates the two-component signal transduction system BtsKR, and amplifies sod1-3 transcription, thereby strengthening the bacterium's pathogenicity and adaptation during infections in 37°C warm Galleria mellonella larvae. This revelation of a single-protein c-di-GMP module introduces unrecognized dimensions to the functional and structural paradigms of c-di-GMP modules and reshapes our understanding of bacterial adaptation and pathogenicity in hosts with a body temperature around 37°C. Furthermore, the discovery of a periplasmic c-di-GMP pool governing BtsD-BtsK interactions supports the critical role of a local c-di-GMP pool.

A comparative genomic and phenotypic study of Vibrio cholerae model strains using hybrid sequencing

Next-generation sequencing methods have become essential for studying bacterial biology and pathogenesis, often depending on high-quality, closed genomes. In this study, we utilized a hybrid sequencing approach to assemble the genome of C6706, a widely used Vibrio cholerae model strain. We present a manually curated annotation of the genome, enhancing user accessibility by linking each coding sequence to its counterpart in N16961, the first sequenced V. cholerae isolate and a commonly used reference genome. Comparative genomic analysis between V. cholerae C6706 and N16961 uncovered multiple genetic differences in genes associated with key biological functions. To determine whether these genetic variations result in phenotypic differences, we compared several phenotypes relevant to V. cholerae pathogenicity like genetic stability, acid sensitivity, biofilm formation and motility. Notably, V. cholerae N16961 exhibited greater motility and reduced biofilm formation compared to V. cholerae C6706. These phenotypic differences appear to be mediated by variations in quorum sensing and cyclic di-GMP signalling pathways between the strains. This study provides valuable insights into the regulation of biofilm formation and motility in V. cholerae.

The promotion of biofilm dispersion: a new strategy for eliminating foodborne pathogens in the food industry

Food safety is a critical global concern due to its direct impact on human health and overall well-being. In the food processing environment, biofilm formation by foodborne pathogens poses a significant problem as it leads to persistent and high levels of food contamination, thereby compromising the quality and safety of food. Therefore, it is imperative to effectively remove biofilms from the food processing environment to ensure food safety. Unfortunately, conventional cleaning methods fall short of adequately removing biofilms, and they may even contribute to further contamination of both equipment and food. It is necessary to develop alternative approaches that can address this challenge in food industry. One promising strategy in tackling biofilm-related issues is biofilm dispersion, which represents the final step in biofilm development. Here, we discuss the biofilm dispersion mechanism of foodborne pathogens and elucidate how biofilm dispersion can be employed to control and mitigate biofilm-related problems. By shedding light on these aspects, we aim to provide valuable insights and solutions for effectively addressing biofilm contamination issues in food industry, thus enhancing food safety and ensuring the well-being of consumers.

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.

Nitrifying niche in estuaries is expanded by the plastisphere

The estuarine plastisphere, a novel ecological habitat in the Anthropocene, has garnered global concerns. Recent geochemical evidence has pointed out its potential role in influencing nitrogen biogeochemistry. However, the biogeochemical significance of the plastisphere and its mechanisms regulating nitrogen cycling remain elusive. Using 15N- and 13C-labelling coupled with metagenomics and metatranscriptomics, here we unveil that the plastisphere likely acts as an underappreciated nitrifying niche in estuarine ecosystems, exhibiting a 0.9 ~ 12-fold higher activity of bacteria-mediated nitrification compared to surrounding seawater and other biofilms (stone, wood and glass biofilms). The shift of active nitrifiers from O2-sensitive nitrifiers in the seawater to nitrifiers with versatile metabolisms in the plastisphere, combined with the potential interspecific cooperation of nitrifying substrate exchange observed among the plastisphere nitrifiers, collectively results in the unique nitrifying niche. Our findings highlight the plastisphere as an emerging nitrifying niche in estuarine environment, and deepen the mechanistic understanding of its contribution to marine biogeochemistry.

Tyrosol blocks E. coli anaerobic biofilm formation via YbfA and FNR to increase antibiotic susceptibility

Bacteria within mature biofilms are highly resistant to antibiotics than planktonic cells. Oxygen limitation contributes to antibiotic resistance in mature biofilms. Nitric oxide (NO) induces biofilm dispersal; however, low NO levels stimulate biofilm formation, an underexplored process. Here, we introduce a mechanism of anaerobic biofilm formation by investigating the antibiofilm activity of tyrosol, a component in wine. Tyrosol inhibits E. coli and Pseudomonas aeruginosa biofilm formation by enhancing NO production. YbfA is identified as a target of tyrosol and its downstream targets are sequentially determined. YbfA activates YfeR, which then suppresses the anaerobic regulator FNR. This suppression leads to decreased NO production, elevated bis-(3'-5')-cyclic dimeric GMP levels, and finally stimulates anaerobic biofilm formation in the mature stage. Blocking YbfA with tyrosol treatment renders biofilm cells as susceptible to antibiotics as planktonic cells. Thus, this study presents YbfA as a promising antibiofilm target to address antibiotic resistance posed by biofilm-forming bacteria, with tyrosol acting as an inhibitor.