Genome-wide transcriptional profile of Escherichia coli in response to high levels of the second messenger 3',5'-cyclic diguanylic acid

Structures of the multi-domain oxygen sensor DosP: remote control of a c-di-GMP phosphodiesterase by a regulatory PAS domain

The heme-based direct oxygen sensor DosP degrades c-di-GMP, a second messenger nearly unique to bacteria. In stationary phase Escherichia coli, DosP is the most abundant c-di-GMP phosphodiesterase. Ligation of O2 to a heme-binding PAS domain (hPAS) of the protein enhances the phosphodiesterase through an allosteric mechanism that has remained elusive. We determined six structures of full-length DosP in its aerobic or anaerobic conformations, with or without c-di-GMP. DosP is an elongated dimer with the regulatory heme and phosphodiesterase separated by nearly 180 Å. In the absence of substrate, regardless of the heme status, DosP presents an equilibrium of two distinct conformations. Binding of substrate induces DosP to adopt a single, ON-state or OFF-state conformation depending on its heme status. Structural and biochemical studies of this multi-domain sensor and its mutants provide insights into signal regulation of second-messenger levels.

Zinc effects on bacteria: insights from Escherichia coli by multi-omics approach

IMPORTANCE

A long-term exposure of bacteria to zinc oxide and zinc oxide nanoparticles leads to major alterations in bacterial morphology and physiology. These included biochemical and physiological processes promoting the emergence of strains with multi-drug resistance and virulence traits. After the removal of zinc pressure, bacterial phenotype reversed back to the original state; however, certain changes at the genomic, transcriptomic, and proteomic level remained. Why is this important? The extensive and intensive use of supplements in animal feed effects the intestinal microbiota of livestock and this may negatively impact the health of animals and people. Therefore, it is crucial to understand and monitor the impact of feed supplements on intestinal microorganisms in order to adequately assess and prevent potential health risks.

Functional expansion of the natural inorganic phosphorus starvation response system in Escherichia coli

Phosphorus, an indispensable nutrient, plays an essential role in cell composition, metabolism, and signal transduction. When inorganic phosphorus (Pi) is scarce, the Pi starvation response in E. coli is activated to increase phosphorus acquisition and drive the cells into a non-growing state to reduce phosphorus consumption. In the six decades of research history, the initiation, output, and shutdown processes of the Pi starvation response have been extensively studied. Simultaneously, Pi starvation has been used in biosensor development, recombinant protein production, and natural product biosynthesis. In this review, we focus on the output process and the applications of the Pi starvation response that have not been summarized before. Meanwhile, based on the current status of mechanistic studies and applications, we propose practical strategies to develop the natural Pi starvation response into a multifunctional and standardized regulatory system in four aspects, including response threshold, temporal expression, intensity range, and bifunctional regulation, which will contribute to its broader application in more fields such as industrial production, medical analysis, and environmental protection.

Structures of biological heme-based sensors of oxygen

Since their initial discovery some 30 years ago, heme-based O₂ sensors have been extensively studied. Among many other lessons, we have learned that they have adapted a wide variety of folds to bind heme for O₂ sensing, and they can couple those sensory domains to transducer domains with many different activities. There is no question that we have learned a great deal about those systems by solving X-ray structures of the truncated pieces of larger multi-domain proteins. All of the studies have, for example, hinted at the importance of protein residues, which were further investigated, usually by site-directed mutagenesis of the full-length proteins together with physico-chemical measurements and enzymatic studies. The biochemistry has suggested that the sensing functions of heme-based O₂ sensors involve not only the entire proteins but also, and quite often, their associated regulatory partners and targets. Here we critically examine the state of knowledge for some well-studied sensors and discuss outstanding questions regarding their structures. For the near future, we may foresee many large complexes with sensor proteins being solved by cryo-EM, to enhance our understanding of their mechanisms.

Phenotypic and transcriptional changes in Escherichia coli K12 in response to simulated microgravity on the EagleStat, a new 2D microgravity analog for bacterial studies

Understanding the impacts of microgravity on bacteria is vital for successful long duration space missions. In this environment, bacteria have been shown to become more virulent, more resistant to antibiotics and to regulate biofilm formation. Since the study of these phenomena under true microgravity is cost- and time challenging, the use of ground-based analogs might allow researchers to test hypotheses before planning and executing experiments in the space environment. We designed and developed a 2D clinostat with capabilities robust enough for bacterial studies to allow for multiple simultaneous replicates of treatment and control conditions, thus permitting the generation of growth curves, in a single run. We used computational fluid dynamics (CFD), biofilm growth measurement and differential gene expression analysis on Escherichia coli cultures grown to late exponential phase (24 h) to validate the system's ability to simulate microgravity conditions. The CFD model with a rotational speed of 8 rpm projected cells growing homogeneously distributed along the tube, while the static condition showed the accumulation of the cells at the bottom of the container. These results were empirically validated with cultures on nutrient broth. Additionally, crystal violet assays showed that higher biofilm biomass grew on the internal walls of the gravity control tubes, compared to the simulated microgravity treatment. In contrast, when cells from both treatments were grown under standard conditions, those exposed to simulated microgravity formed significantly more biofilms than their gravity counterparts. Consistent with this result, transcriptome analysis showed the upregulation of several gene families related to biofilm formation and development such as cells adhesion, aggregation and regulation of cell motility, which provides a potential transcriptional explanation for the differential phenotype observed. Our results show that when operated under parameters for simulated microgravity, our 2D clinostat creates conditions that maintain a proportion of the cells in a constant free-falling state, consistent with the effect of microgravity. Also, the high-throughput nature of our instrument facilitates, significantly, bacterial experiments that require multiple sampling timepoints and small working volumes, making this new instrument extremely efficient.

Zetaproteobacteria Pan-Genome Reveals Candidate Gene Cluster for Twisted Stalk Biosynthesis and Export

Twisted stalks are morphologically unique bacterial extracellular organo-metallic structures containing Fe(III) oxyhydroxides that are produced by microaerophilic Fe(II)-oxidizers belonging to the Betaproteobacteria and Zetaproteobacteria. Understanding the underlying genetic and physiological mechanisms of stalk formation is of great interest based on their potential as novel biogenic nanomaterials and their relevance as putative biomarkers for microbial Fe(II) oxidation on ancient Earth. Despite the recognition of these special biominerals for over 150 years, the genetic foundation for the stalk phenotype has remained unresolved. Here we present a candidate gene cluster for the biosynthesis and secretion of the stalk organic matrix that we identified with a trait-based analyses of a pan-genome comprising 16 Zetaproteobacteria isolate genomes. The “stalk formation in Zetaproteobacteria” (sfz) cluster comprises six genes (sfz1-sfz6), of which sfz1 and sfz2 were predicted with functions in exopolysaccharide synthesis, regulation, and export, sfz4 and sfz6 with functions in cell wall synthesis manipulation and carbohydrate hydrolysis, and sfz3 and sfz5 with unknown functions. The stalk-forming Betaproteobacteria Ferriphaselus R-1 and OYT-1, as well as dread-forming Zetaproteobacteria Mariprofundus aestuarium CP-5 and Mariprofundus ferrinatatus CP-8 contain distant sfz gene homologs, whereas stalk-less Zetaproteobacteria and Betaproteobacteria lack the entire gene cluster. Our pan-genome analysis further revealed a significant enrichment of clusters of orthologous groups (COGs) across all Zetaproteobacteria isolate genomes that are associated with the regulation of a switch between sessile and motile growth controlled by the intracellular signaling molecule c-di-GMP. Potential interactions between stalk-former unique transcription factor genes, sfz genes, and c-di-GMP point toward a c-di-GMP regulated surface attachment function of stalks during sessile growth.

Synthetic Biological Approaches for Optogenetics and Tools for Transcriptional Light-Control in Bacteria

Light has become established as a tool not only to visualize and investigate but also to steer biological systems. This review starts by discussing the unique features that make light such an effective control input in biology. It then gives an overview of how light-control came to progress, starting with photoactivatable compounds and leading up to current genetic implementations using optogenetic approaches. The review then zooms in on optogenetics, focusing on photosensitive proteins, which form the basis for optogenetic engineering using synthetic biological approaches. As the regulation of transcription provides a highly versatile means for steering diverse biological functions, the focus of this review then shifts to transcriptional light regulators, which are presented in the biotechnologically highly relevant model organism Escherichia coli.

Escherichia coli DosC and DosP: a role of c-di-GMP in compartmentalized sensing by degradosomes

The Escherichia coli operon dosCP, also called yddV-yddU, co-expresses two heme proteins, DosC and DosP, both of which are direct oxygen sensors but paradoxically have opposite effects on the levels of the second messenger c-di-GMP. DosC is a diguanylate cyclase that synthesizes c-di-GMP from GTP, whereas DosP is a phosphodiesterase that linearizes c-di-GMP to pGpG. Both proteins are associated with the large degradosome enzyme complex that regulates many bacterial genes post-transcriptionally by processing or degrading the corresponding RNAs. Moreover, the c-di-GMP directly binds to PNPase, a key degradosome enzyme, and enhances its activity. This review combines biochemical, biophysical, and genetic findings on DosC and DosP, a task that has not been undertaken until now, partly because of the varied nomenclature. The DosC and DosP system is examined in the context of the current knowledge of degradosomes and considered as a possible prototype for the compartmentalization of sensing by E. coli.

c-di-GMP Regulates Various Phenotypes and Insecticidal Activity of Gram-Positive Bacillus thuringiensis

C-di-GMP has been well investigated to play significant roles in the physiology of many Gram-negative bacteria. However, its effect on Gram-positive bacteria is less known. In order to more understand the c-di-GMP functions in Gram-positive bacteria, we have carried out a detailed study on the c-di-GMP-metabolizing enzymes and their physiological functions in Bacillus thuringiensis, a Gram-positive entomopathogenic bacterium that has been applied as an insecticide successfully. We performed a systematic study on the ten putative c-di-GMP-synthesizing enzyme diguanylate cyclases (DGCs) and c-di-GMP-degrading enzyme phosphodiesterases (PDEs) in B. thuringiensis BMB171, and artificially elevated the intracellular c-di-GMP level in BMB171 by deleting one or more pde genes. We found increasing level of intracellular c-di-GMP exhibits similar activities as those in Gram-negative bacteria, including altered activities in cell motility, biofilm formation, and cell-cell aggregation. Unexpectedly, we additionally found a novel function exhibited by the increasing level of c-di-GMP to promote the insecticidal activity of this bacterium against Helicoverpa armigera. Through whole-genome transcriptome profile analyses, we found that 4.3% of the B. thuringiensis genes were differentially transcribed when c-di-GMP level was increased, and 77.3% of such gene products are involved in some regulatory pathways not reported in other bacteria to date. In summary, our study represents the first comprehensive report on the c-di-GMP-metabolizing enzymes, their effects on phenotypes, and the transcriptome mediated by c-di-GMP in an important Gram-positive bacterium.

Engineering an E. coli Near-Infrared Light Sensor

Optogenetics is a technology wherein researchers combine light and genetically engineered photoreceptors to control biological processes with unrivaled precision. Near-infrared (NIR) wavelengths (>700 nm) are desirable optogenetic inputs due to their low phototoxicity and spectral isolation from most photoproteins. The bacteriophytochrome photoreceptor 1 (BphP1), found in several purple photosynthetic bacteria, senses NIR light and activates transcription of photosystem promoters by binding to and inhibiting the transcriptional repressor PpsR2. Here, we examine the response of a library of output promoters to increasing levels of Rhodopseudomonas palustris PpsR2 expression, and we identify that of Bradyrhizobium sp. BTAi1 crtE as the most strongly repressed in Escherichia coli. Next, we optimize Rps. palustris bphP1 and ppsR2 expression in a strain engineered to produce the required chromophore biliverdin IXα in order to demonstrate NIR-activated transcription. Unlike a previously engineered bacterial NIR photoreceptor, our system does not require production of a second messenger, and it exhibits rapid response dynamics. It is also the most red-shifted bacterial optogenetic tool yet reported by approximately 50 nm. Accordingly, our BphP1-PpsR2 system has numerous applications in bacterial optogenetics.

New Insights into the Non-orthodox Two Component Rcs Phosphorelay System

The Rcs phosphorelay system, a non-orthodox two-component regulatory system, integrates environmental signals, regulates gene expression, and alters the physiological behavior of members of the Enterobacteriaceae family of Gram-negative bacteria. Recent studies of Rcs system focused on protein interactions, functions, and the evolution of Rcs system components and its auxiliary regulatory proteins. Herein we review the latest advances on the Rcs system proteins, and discuss the roles that the Rcs system plays in the environmental adaptation of various Enterobacteriaceae species.

Oxygen-Dependent Globin Coupled Sensor Signaling Modulates Motility and Virulence of the Plant Pathogen Pectobacterium carotovorum

Bacterial pathogens utilize numerous signals to identify the presence of their host and coordinate changes in gene expression that allow for infection. Within plant pathogens, these signals typically include small molecules and/or proteins from their plant hosts and bacterial quorum sensing molecules to ensure sufficient bacterial cell density for successful infection. In addition, bacteria use environmental signals to identify conditions when the host defenses are weakened and potentially to signal entry into an appropriate host/niche for infection. A globin coupled sensor protein (GCS), termed PccGCS, within the soft rot bacterium Pectobacterium carotovorum ssp. carotovorum WPP14 has been identified as an O₂ sensor and demonstrated to alter virulence factor excretion and control motility, with deletion of PccGCS resulting in decreased rotting of a potato host. Using small molecules that modulate bacterial growth and quorum sensing, PccGCS signaling also has been shown to modulate quorum sensing pathways, resulting in the PccGCS deletion strain being more sensitive to plant-derived phenolic acids, which can function as quorum sensing inhibitors, and exhibiting increased N-acylhomoserine lactone (AHL) production. These findings highlight a role for GCS proteins in controlling key O₂-dependent phenotypes of pathogenic bacteria and suggest that modulating GCS signaling to limit P. carotovorum motility may provide a means to decrease rotting of plant hosts.

The importance of conserved amino acids in heme-based globin-coupled diguanylate cyclases

Globin-coupled diguanylate cyclases contain globin, middle, and diguanylate cyclase domains that sense O₂ to synthesize c-di-GMP and regulate bacterial motility, biofilm formation, and virulence. However, relatively few studies have extensively examined the roles of individual residues and domains of globin-coupled diguanylate cyclases, which can shed light on their signaling mechanisms and provide drug targets. Here, we report the critical residues of two globin-coupled diguanylate cyclases, EcGReg from Escherichia coli and BpeGReg from Bordetella pertussis, and show that their diguanylate cyclase activity requires an intact globin domain. In the distal heme pocket of the globin domain, residues Phe42, Tyr43, Ala68 (EcGReg)/Ser68 (BpeGReg), and Met69 are required to maintain full diguanylate cyclase activity. The highly conserved amino acids His223/His225 and Lys224/Lys226 in the middle domain of EcGReg/BpeGReg are essential to diguanylate cyclase activity. We also identified sixteen important residues (Leu300, Arg306, Asp333, Phe337, Lys338, Asn341, Asp342, Asp350, Leu353, Asp368, Arg372, Gly374, Gly375, Asp376, Glu377, and Phe378) in the active site and inhibitory site of the diguanylate cyclase domain of EcGReg. Moreover, BpeGReg₂₆₆ (residues 1–266) and BpeGReg₂₉₆ (residues 1–296), which only contain the globin and middle domains, can inhibit bacterial motility. Our findings suggest that the distal residues of the globin domain affect diguanylate cyclase activity and that BpeGReg may interact with other c-di-GMP-metabolizing proteins to form mixed signaling teams.

Oxygen and Bis(3',5')-cyclic Dimeric Guanosine Monophosphate Binding Control Oligomerization State Equilibria of Diguanylate Cyclase-Containing Globin Coupled Sensors

Bacteria sense their environment to alter phenotypes, including biofilm formation, to survive changing conditions. Heme proteins play important roles in sensing the bacterial gaseous environment and controlling the switch between motile and sessile (biofilm) states. Globin coupled sensors (GCS), a family of heme proteins consisting of a globin domain linked by a central domain to an output domain, are often found with diguanylate cyclase output domains that synthesize c-di-GMP, a major regulator of biofilm formation. Characterization of diguanylate cyclase-containing GCS proteins from Bordetella pertussis and Pectobacterium carotovorum demonstrated that cyclase activity is controlled by ligand binding to the heme within the globin domain. Both O₂ binding to the heme within the globin domain and c-di-GMP binding to a product-binding inhibitory site (I-site) within the cyclase domain control oligomerization states of the enzymes. Changes in oligomerization state caused by c-di-GMP binding to the I-site also affect O₂ kinetics within the globin domain, suggesting that shifting the oligomer equilibrium leads to broad rearrangements throughout the protein. In addition, mutations within the I-site that eliminate product inhibition result in changes to the accessible oligomerization states and decreased catalytic activity. These studies provide insight into the mechanism by which ligand binding to the heme and I-site controls activity of GCS proteins and suggests a role for oligomerization-dependent activity in vivo.

The role of the globin-coupled sensor YddV in a mature E. coli biofilm population

Biofilm-associated infections are hard to treat because of their high antibiotic resistance and the presence of a very persistent subpopulation of bacteria. The second messenger molecule cyclic di-guanosine monophosphate (c-di-GMP) plays a very important role in this biofilm physiology. Here, we evaluated the role of YddV, an enzyme with a c-di-GMP synthesis function, in the formation and maturation of Escherichia coli biofilms. Our results suggest that YddV stimulates biofilm growth via its role in the production of c-di-GMP and this likely by influencing the production of matrix (e.g. poly-N-acetylglucosamine (PGA)). However, lowering the YddV expression did not alter the biofilm formation since there was no significant difference between the biofilm phenotypes of WT E. coli and YddV-knockout bacteria. Additionally, YddV expression had no significant influence on the amount of persister cells within the biofilm population, questioning the use of YddV as therapeutic target.

Mini-review: Biofilm responses to oxidative stress

Biofilms constitute the predominant microbial style of life in natural and engineered ecosystems. Facing harsh environmental conditions, microorganisms accumulate reactive oxygen species (ROS), potentially encountering a dangerous condition called oxidative stress. While high levels of oxidative stress are toxic, low levels act as a cue, triggering bacteria to activate effective scavenging mechanisms or to shift metabolic pathways. Although a complex and fragmentary picture results from current knowledge of the pathways activated in response to oxidative stress, three main responses are shown to be central: the existence of common regulators, the production of extracellular polymeric substances, and biofilm heterogeneity. An investigation into the mechanisms activated by biofilms in response to different oxidative stress levels could have important consequences from ecological and economic points of view, and could be exploited to propose alternative strategies to control microbial virulence and deterioration.

Structural analysis of an oxygen-regulated diguanylate cyclase

Cyclic di-GMP is a bacterial second messenger that is involved in switching between motile and sessile lifestyles. Given the medical importance of biofilm formation, there has been increasing interest in understanding the synthesis and degradation of cyclic di-GMPs and their regulation in various bacterial pathogens. Environmental cues are detected by sensing domains coupled to GGDEF and EAL or HD-GYP domains that have diguanylate cyclase and phosphodiesterase activities, respectively, producing and degrading cyclic di-GMP. The Escherichia coli protein DosC (also known as YddV) consists of an oxygen-sensing domain belonging to the class of globin sensors that is coupled to a C-terminal GGDEF domain via a previously uncharacterized middle domain. DosC is one of the most strongly expressed GGDEF proteins in E. coli, but to date structural information on this and related proteins is scarce. Here, the high-resolution structural characterization of the oxygen-sensing globin domain, the middle domain and the catalytic GGDEF domain in apo and substrate-bound forms is described. The structural changes between the iron(III) and iron(II) forms of the sensor globin domain suggest a mechanism for oxygen-dependent regulation. The structural information on the individual domains is combined into a model of the dimeric DosC holoprotein. These findings have direct implications for the oxygen-dependent regulation of the activity of the cyclase domain.

The expanding roles of c-di-GMP in the biosynthesis of exopolysaccharides and secondary metabolites

The cyclic dinucleotide c-di-GMP has emerged in the last decade as a prevalent intracellular messenger that orchestrates the transition between the motile and sessile lifestyles of many bacterial species. The motile-to-sessile transition is often associated with the formation of extracellular matrix-encased biofilm, an organized community of bacterial cells that often contributes to antibiotic resistance and host-pathogen interaction. It is increasingly clear that c-di-GMP controls motility, biofilm formation and bacterial pathogenicity partially through regulating the production of exopolysaccharides (EPS) and small-molecule secondary metabolites. This review summarizes our current understanding of the regulation of EPS biosynthesis by c-di-GMP in a diversity of bacterial species and highlights the emerging role of c-di-GMP in the biosynthesis of small-molecule secondary metabolites.

Probing the ligand recognition and discrimination environment of the globin-coupled oxygen sensor protein YddV by FTIR and time-resolved step-scan FTIR spectroscopy

We present time-resolved step-scan FTIR evidence for the role of the distal Y43 and L65 residues in controlling the ligand dynamics in the signal transducer protein YddV.

Bacterial diguanylate cyclases: structure, function and mechanism in exopolysaccharide biofilm development

The ubiquitous bacterial cyclic di-guanosine monophosphate (c-di-GMP) emerges as an important messenger for the control of many bacterial cellular functions including virulence, motility, bioluminescence, cellulose biosynthesis, adhesion, secretion, community behaviour, biofilm formation and cell differentiation. The synthesis of this cyclic nucleotide arises from external stimuli on various signalling domains within the N-terminal region of a dimeric diguanylate cyclase. This initiates the condensation of two molecules of guanosine triphosphate juxtaposed to each other within the C-terminal region of the enzyme. The biofilm from pathogenic microbes is highly resistant to antimicrobial agents suggesting that diguanylate cyclase and its product - c-di-GMP - are key biomedical targets for the inhibition of biofilm development. Furthermore the formation and long-term stability of the aerobic granule, a superior biofilm for biological wastewater treatment, can be controlled by stimulation of c-di-GMP. Any modulation of the synthetic pathways for c-di-GMP is clearly advantageous in terms of medical, industrial and/or environmental bioremediation implications. This review discusses the structure and reaction of individual diguanylate cyclase enzymes with a focus on new directions in c-di-GMP research. Specific attention is made on the molecular mechanisms that control bacterial exopolysaccharide biofilm formation and aerobic granules.

Cooperative substrate binding by a diguanylate cyclase

XAC0610, from Xanthomonas citri subsp. citri, is a large multi-domain protein containing one GAF (cGMP-specific phosphodiesterases, adenylyl cyclases and FhlA) domain, four PAS (Per-Arnt-Sim) domains and one GGDEF domain. This protein has a demonstrable in vivo and in vitro diguanylate cyclase (DGC) activity that leads to the production of cyclic di-GMP (c-di-GMP), a ubiquitous bacterial signaling molecule. Analysis of a XacΔ0610 knockout strain revealed that XAC0610 plays a role in the regulation of Xac motility and resistance to H2O2. Site-directed mutagenesis of a conserved DGC lysine residue (Lys759 in XAC0610) resulted in a severe reduction in XAC0610 DGC activity. Furthermore, experimental and in silico analyses suggest that XAC0610 is not subject to allosteric product inhibition, a common regulatory mechanism for DGC activity control. Instead, steady-state kinetics of XAC0610 DGC activity revealed a positive cooperative effect of the GTP substrate with a dissociation constant for the binding of the first GTP molecule (K1) approximately 5× greater than the dissociation constant for the binding of the second GTP molecule (K2). We present a general kinetics scheme that should be used when analyzing DGC kinetics data and propose that cooperative GTP binding could be a common, though up to now overlooked, feature of these enzymes that may in some cases offer a physiologically relevant mechanism for regulation of DGC activity in vivo.

Phosphodiesterase DosP increases persistence by reducing cAMP which reduces the signal indole

Persisters are bacteria that are highly tolerant to antibiotics due to their dormant state and are of clinical significance owing to their role in infections. Given that the population of persisters increases in biofilms and that cyclic diguanylate (c-di-GMP) is an intracellular signal that increases biofilm formation, we sought to determine whether c-di-GMP has a role in bacterial persistence. By examining the effect of 30 genes from Escherichia coli, including diguanylate cyclases that synthesize c-di-GMP and phosphodiesterases that breakdown c-di-GMP, we determined that DosP (direct oxygen sensing phosphodiesterase) increases persistence by over a thousand fold. Using both transcriptomic and proteomic approaches, we determined that DosP increases persistence by decreasing tryptophanase activity and thus indole. Corroborating this effect, addition of indole reduced persistence. Despite the role of DosP as a c-di-GMP phosphodiesterase, the decrease in tryptophanase activity was found to be a result of cyclic adenosine monophosphate (cAMP) phosphodiesterase activity. Corroborating this result, the reduction of cAMP via CpdA, a cAMP-specific phosphodiesterase, increased persistence and reduced indole levels similarly to DosP. Therefore, phosphodiesterase DosP increases persistence by reducing the interkingdom signal indole via reduction of the global regulator cAMP.

BdcA, a protein important for Escherichia coli biofilm dispersal, is a short-chain dehydrogenase/reductase that binds specifically to NADPH

The Escherichia coli protein BdcA (previously referred to as YjgI) plays a key role in the dispersal of cells from bacterial biofilms, and its constitutive activation provides an attractive therapeutic target for dismantling these communities. In order to investigate the function of BdcA at a molecular level, we integrated structural and functional studies. Our 2.05 Å structure of BdcA shows that it is a member of the NAD(P)(H)-dependent short-chain dehydrogenase/reductase (SDR) superfamily. Structural comparisons with other members of the SDR family suggested that BdcA binds NADP(H). This was demonstrated experimentally using thermal denaturation studies, which showed that BcdA binds specifically to NADPH. Subsequent ITC experiments further confirmed this result and reported a Kd of 25.9 µM. Thus, BdcA represents the newest member of the limited number of oxidoreductases shown to be involved in quorum sensing and biofilm dispersal.

Importance of Biofilms in Urinary Tract Infections: New Therapeutic Approaches

Bacterial biofilms play an important role in urinary tract infections (UTIs), being responsible for persistence infections causing relapses and acute prostatitis. Bacterial forming biofilm are difficult to eradicate due to the antimicrobial resistant phenotype that this structure confers being combined therapy recommended for the treatment of biofilm-associated infections. However, the presence of persistent cells showing reduced metabolism that leads to higher levels of antimicrobial resistance makes the search for new therapeutic tools necessary. Here, a review of these new therapeutic approaches is provided including catheters coated with hydrogels or antibiotics, nanoparticles, iontophoresis, biofilm enzyme inhibitors, liposomes, bacterial interference, bacteriophages, quorum sensing inhibitors, low-energy surface acoustic waves, and antiadhesion agents. In conclusion, new antimicrobial drugs that inhibit bacterial virulence and biofilm formation are needed.

Roles of cyclic Di-GMP and the Gac system in transcriptional control of the genes coding for the Pseudomonas putida adhesins LapA and LapF

LapA and LapF are large extracellular proteins that play a relevant role in biofilm formation by Pseudomonas putida. Current evidence favors a sequential model in which LapA is first required for the initial adhesion of individual bacteria to a surface, while LapF participates in later stages of biofilm development. In agreement with this model, lapF transcription was previously shown to take place at late times of growth and to respond to the stationary-phase sigma factor RpoS. We have now analyzed the transcription pattern of lapA and other regulatory elements that influence expression of both genes. The lapA promoter shows a transient peak of activation early during growth, with a second increase in stationary phase that is independent of RpoS. The same pattern is observed in biofilms although expression is not uniform in the population. Both lapA and lapF are under the control of the two-component regulatory system GacS/GacA, and their transcription also responds to the intracellular levels of the second messenger cyclic diguanylate (c-di-GMP), although in surprisingly reverse ways. Whereas expression from the lapA promoter increases with high levels of c-di-GMP, the opposite is true for lapF. The transcriptional regulator FleQ is required for the modulation of lapA expression by c-di-GMP but has a minor influence on lapF. This work represents a further step in our understanding of the regulatory interactions controlling biofilm formation in P. putida.

Cyclic di-GMP: the first 25 years of a universal bacterial second messenger

Twenty-five years have passed since the discovery of cyclic dimeric (3'→5') GMP (cyclic di-GMP or c-di-GMP). From the relative obscurity of an allosteric activator of a bacterial cellulose synthase, c-di-GMP has emerged as one of the most common and important bacterial second messengers. Cyclic di-GMP has been shown to regulate biofilm formation, motility, virulence, the cell cycle, differentiation, and other processes. Most c-di-GMP-dependent signaling pathways control the ability of bacteria to interact with abiotic surfaces or with other bacterial and eukaryotic cells. Cyclic di-GMP plays key roles in lifestyle changes of many bacteria, including transition from the motile to the sessile state, which aids in the establishment of multicellular biofilm communities, and from the virulent state in acute infections to the less virulent but more resilient state characteristic of chronic infectious diseases. From a practical standpoint, modulating c-di-GMP signaling pathways in bacteria could represent a new way of controlling formation and dispersal of biofilms in medical and industrial settings. Cyclic di-GMP participates in interkingdom signaling. It is recognized by mammalian immune systems as a uniquely bacterial molecule and therefore is considered a promising vaccine adjuvant. The purpose of this review is not to overview the whole body of data in the burgeoning field of c-di-GMP-dependent signaling. Instead, we provide a historic perspective on the development of the field, emphasize common trends, and illustrate them with the best available examples. We also identify unresolved questions and highlight new directions in c-di-GMP research that will give us a deeper understanding of this truly universal bacterial second messenger.

Archaeal biofilms: the great unexplored

Biofilms are currently viewed as the most common form in which microorganisms exist in nature. Bacterial biofilms play important roles in disease and industrial applications, and they have been studied in great detail. Although it is well accepted that archaea are not only the extremists they were thought to be as they occupy nearly every habitat where also bacteria are found, it is surprising how little molecular details are known about archaeal biofilm formation. Therefore, we aim to highlight the available information and indicate open questions in this field.

Genetic analysis of the role of yfiR in the ability of Escherichia coli CFT073 to control cellular cyclic dimeric GMP levels and to persist in the urinary tract

During urinary tract infections (UTIs), uropathogenic Escherichia coli must maintain a delicate balance between sessility and motility to achieve successful infection of both the bladder and kidneys. Previous studies showed that cyclic dimeric GMP (c-di-GMP) levels aid in the control of the transition between motile and nonmotile states in E. coli. The yfiRNB locus in E. coli CFT073 contains genes for YfiN, a diguanylate cyclase, and its activity regulators, YfiR and YfiB. Deletion of yfiR yielded a mutant that was attenuated in both the bladder and the kidneys when tested in competition with the wild-type strain in the murine model of UTI. A double yfiRN mutant was not attenuated in the mouse model, suggesting that unregulated YfiN activity and likely increased cytoplasmic c-di-GMP levels cause a survival defect. Curli fimbriae and cellulose production were increased in the yfiR mutant. Expression of yhjH, a gene encoding a proven phosphodiesterase, in CFT073 ΔyfiR suppressed the overproduction of curli fimbriae and cellulose and further verified that deletion of yfiR results in c-di-GMP accumulation. Additional deletion of csgD and bcsA, genes necessary for curli fimbriae and cellulose production, respectively, returned colonization levels of the yfiR deletion mutant to wild-type levels. Peroxide sensitivity assays and iron acquisition assays displayed no significant differences between the yfiR mutant and the wild-type strain. These results indicate that dysregulation of c-di-GMP production results in pleiotropic effects that disable E. coli in the urinary tract and implicate the c-di-GMP regulatory system as an important factor in the persistence of uropathogenic E. coli in vivo.

Cyclic di-AMP is critical for Listeria monocytogenes growth, cell wall homeostasis, and establishment of infection

Listeria monocytogenes infection leads to robust induction of an innate immune signaling pathway referred to as the cytosolic surveillance pathway (CSP), characterized by expression of beta interferon (IFN-β) and coregulated genes. We previously identified the IFN-β stimulatory ligand as secreted cyclic di-AMP. Synthesis of c-di-AMP in L. monocytogenes is catalyzed by the diadenylate cyclase DacA, and multidrug resistance transporters are necessary for secretion. To identify additional bacterial factors involved in L. monocytogenes detection by the CSP, we performed a forward genetic screen for mutants that induced altered levels of IFN-β. One mutant that stimulated elevated levels of IFN-β harbored a transposon insertion in the gene lmo0052. Lmo0052, renamed here PdeA, has homology to a cyclic di-AMP phosphodiesterase, GdpP (formerly YybT), of Bacillus subtilis and is able to degrade c-di-AMP to the linear dinucleotide pApA. Reduction of c-di-AMP levels by conditional depletion of the di-adenylate cyclase DacA or overexpression of PdeA led to marked decreases in growth rates, both in vitro and in macrophages. Additionally, mutants with altered levels of c-di-AMP had different susceptibilities to peptidoglycan-targeting antibiotics, suggesting that the molecule may be involved in regulating cell wall homeostasis. During intracellular infection, increases in c-di-AMP production led to hyperactivation of the CSP. Conditional depletion of dacA also led to increased IFN-β expression and a concomitant increase in host cell pyroptosis, a result of increased bacteriolysis and subsequent bacterial DNA release. These data suggest that c-di-AMP coordinates bacterial growth, cell wall stability, and responses to stress and plays a crucial role in the establishment of bacterial infection.

Listeria monocytogenes is a Gram-positive intracellular pathogen and the causative agent of the food-borne illness listeriosis. Upon infection, L. monocytogenes stimulates expression of IFN-β and coregulated genes dependent upon host detection of a secreted bacterial signaling nucleotide, c-di-AMP. Using a forward genetic screen for mutants that induced high levels of host IFN-β expression, we identified a c-di-AMP phosphodiesterase, PdeA, that degrades c-di-AMP. Here we characterize L. monocytogenes mutants that express enhanced or diminished levels of c-di-AMP. Decreased c-di-AMP levels by conditional depletion of the diadenylate cyclase (DacA) or overexpression of PdeA attenuated bacterial growth and led to bacteriolysis, suggesting that its production is essential for viability and may regulate cell wall metabolism. Mutants lacking PdeA had a distinct transcriptional profile, which may provide insight into additional roles for the molecule. This work demonstrates that c-di-AMP is a critical signaling molecule required for bacterial replication, cell wall stability, and pathogenicity.

Study of the response regulator Rrp1 reveals its regulatory role in chitobiose utilization and virulence of Borrelia burgdorferi

Life cycle alternation between arthropod and mammals forces the Lyme disease spirochete, Borrelia burgdorferi, to adapt to different host milieus by utilizing diverse carbohydrates. Glycerol and chitobiose are abundantly present in the Ixodes tick. B. burgdorferi can utilize glycerol as a carbohydrate source for glycolysis and chitobiose to produce N-acetylglucosamine (GlcNAc), a key component of the bacterial cell wall. A recent study reported that Rrp1, a response regulator that synthesizes cyclic diguanylate (c-di-GMP), governs glycerol utilization in B. burgdorferi. In this report, we found that the rrp1 mutant had growth defects and formed membrane blebs that led to cell lysis when GlcNAc was replaced by chitobiose in the growth medium. The gene chbC encodes a key chitobiose transporter of B. burgdorferi. We found that the expression level of chbC was significantly repressed in the mutant and that constitutive expression of chbC in the mutant successfully rescued the growth defect, indicating a regulatory role of Rrp1 in chitobiose uptake. Immunoblotting and transcriptional studies revealed that Rrp1 is required for the activation of bosR and rpoS and that its impact on chbC is most likely mediated by the BosR-RpoS regulatory pathway. Tick-mouse infection studies showed that although the rrp1 mutant failed to establish infection in mice via tick bite, exogenous supplementation of GlcNAc into unfed ticks partially rescued the infection. The finding reported here provides us with new insight into the regulatory role of Rrp1 in carbohydrate utilization and virulence of B. burgdorferi.

The c-di-GMP phosphodiesterase VmpA absent in Escherichia coli K12 strains affects motility and biofilm formation in the enterohemorrhagic O157:H7 serotype

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a foodborne pathogen that resists the acidic gastric environment, colonizes the gut epithelium, and causes hemorrhagic colitis and hemolytic-uremic syndrome, especially in children. The genomic island OI-47 of E. coli O157:H7 contains a gene, z1528, encoding an EAL-domain protein potentially involved in c-di-GMP hydrolysis that is absent in non-pathogenic E. coli. This gene, designated vmpA, is co-transcribed with ycdT, which is present in non pathogenic E. coli and encodes a diguanylate cyclase involved in c-di-GMP synthesis. To test for vmpA function, we constructed a vmpA knockout mutant. We also overexpressed vmpA, purified the VmpA protein and assayed for its activity in vitro. We found that VmpA possesses c-di-GMP phosphodiesterase activity and that the vmpA mutation results in increased biofilm formation, and reduced swimming motility, which is consistent with the function determined in vitro. Unexpectedly, suppressor mutations arise frequently in the vmpA background suggesting that VmpA plays an important regulatory role in E. coli O157:H7. These findings represent an example of remarkable flexibility in the organization of c-di-GMP signaling pathways in closely related species.

Cyclic diguanylate inversely regulates motility and aggregation in Clostridium difficile

Clostridium difficile-associated disease is increasing in incidence and is costly to treat. Our understanding of how this organism senses its entry into the host and adapts for growth in the large bowel is limited. The small-molecule second messenger cyclic diguanylate (c-di-GMP) has been extensively studied in gram-negative bacteria and has been shown to modulate motility, biofilm formation, and other processes in response to environmental signals, yet little is known about the functions of this signaling molecule in gram-positive bacteria or in C. difficile specifically. In the current study, we investigated the function of the second messenger c-di-GMP in C. difficile. To determine the role of c-di-GMP in C. difficile, we ectopically expressed genes encoding a diguanylate cyclase enzyme, which synthesizes c-di-GMP, or a phosphodiesterase enzyme, which degrades c-di-GMP. This strategy allowed us to artificially elevate or deplete intracellular c-di-GMP, respectively, and determine that c-di-GMP represses motility in C. difficile, consistent with previous studies in gram-negative bacteria, in which c-di-GMP has a negative effect on myriad modes of bacterial motility. Elevated c-di-GMP levels also induced clumping of C. difficile cells, which may signify that C. difficile is capable of forming biofilms in the host. In addition, we directly quantified, for the first time, c-di-GMP production in a gram-positive bacterium. This work demonstrates the effect of c-di-GMP on the motility of a gram-positive bacterium and on aggregation of C. difficile, which may be relevant to the function of this signaling molecule during infection.

The pyrimidine nucleotide biosynthetic pathway modulates production of biofilm determinants in Escherichia coli

Bacteria are often found in multicellular communities known as biofilms, which constitute a resistance form against environmental stresses. Extracellular adhesion and cell aggregation factors, responsible for bacterial biofilm formation and maintenance, are tightly regulated in response to physiological and environmental cues. We show that, in Escherichia coli, inactivation of genes belonging to the de novo uridine monophosphate (UMP) biosynthetic pathway impairs production of curli fibers and cellulose, important components of the bacterial biofilm matrix, by inhibiting transcription of the csgDEFG operon, thus preventing production of the biofilm master regulator CsgD protein. Supplementing growth media with exogenous uracil, which can be converted to UMP through the pyrimidine nucleotide salvage pathway, restores csgDEFG transcription and curli production. In addition, however, exogenous uracil triggers cellulose production, particularly in strains defective in either carB or pyrB genes, which encode enzymes catalyzing the first steps of de novo UMP biosynthesis. Our results indicate the existence of tight and complex links between pyrimidine metabolism and curli/cellulose production: transcription of the csgDEFG operon responds to pyrimidine nucleotide availability, while cellulose production is triggered by exogenous uracil in the absence of active de novo UMP biosynthesis. We speculate that perturbations in the UMP biosynthetic pathways allow the bacterial cell to sense signals such as starvation, nucleic acids degradation, and availability of exogenous pyrimidines, and to adapt the production of the extracellular matrix to the changing environmental conditions.

Transcriptional analysis and functional characterization of XCC1294 gene encoding a GGDEF domain protein in Xanthomonas campestris pv. campestris

The nucleotide cyclic di-GMP is a second messenger in bacteria that regulates a range of cellular functions including the virulence of pathogens. GGDEF is a protein domain involved in the synthesis of cyclic di-GMP. The genome of the crucifer pathogen Xanthomonas campestris pv. campestris (Xcc) encodes 21 proteins with a GGDEF domain. Clp, a homolog of the model transcription factor Crp of Escherichia coli, is a global regulator in Xcc. The aim of this study is to identify genes encoding GGDEF domain proteins whose expression is regulated by Clp. Results of reporter assay and RT-PCR analysis suggested that Clp regulates the expression of a set of genes encoding proteins harboring GGDEF domain. The transcription initiation site of XCC1294, one of the Clp regulated gene encoding a GGDEF domain protein, was mapped. Promoter analysis and gel retardation assay indicated that the transcription of XCC1294 is positively and directly regulated by Clp. Furthermore, transcription of XCC1294 was subject to catabolite repression and affected by several stress conditions. We also showed that mutation of XCC1294 results in enhanced surface attachment. In addition, transcription of three putative adhesin genes (xadA, fhaC, and yapH) was increased in the XCC1294 mutant. Taken together, the data presented here indicate that Clp positively regulates expression of XCC1294, and that XCC1294 serves a regulator of bacterial attachment and regulates different adhesin genes expression.

Comparative analyses imply that the enigmatic Sigma factor 54 is a central controller of the bacterial exterior

Background

Sigma-54 is a central regulator in many pathogenic bacteria and has been linked to a multitude of cellular processes like nitrogen assimilation and important functional traits such as motility, virulence, and biofilm formation. Until now it has remained obscure whether these phenomena and the control by Sigma-54 share an underlying theme.

Results

We have uncovered the commonality by performing a range of comparative genome analyses. A) The presence of Sigma-54 and its associated activators was determined for all sequenced prokaryotes. We observed a phylum-dependent distribution that is suggestive of an evolutionary relationship between Sigma-54 and lipopolysaccharide and flagellar biosynthesis. B) All Sigma-54 activators were identified and annotated. The relation with phosphotransfer-mediated signaling (TCS and PTS) and the transport and assimilation of carboxylates and nitrogen containing metabolites was substantiated. C) The function annotations, that were represented within the genomic context of all genes encoding Sigma-54, its activators and its promoters, were analyzed for intra-phylum representation and inter-phylum conservation. Promoters were localized using a straightforward scoring strategy that was formulated to identify similar motifs. We found clear highly-represented and conserved genetic associations with genes that concern the transport and biosynthesis of the metabolic intermediates of exopolysaccharides, flagella, lipids, lipopolysaccharides, lipoproteins and peptidoglycan.

Conclusion

Our analyses directly implicate Sigma-54 as a central player in the control over the processes that involve the physical interaction of an organism with its environment like in the colonization of a host (virulence) or the formation of biofilm.

The bacterial second messenger c-di-GMP: mechanisms of signalling

Cyclic-di-GMP (c-di-GMP) regulates many important bacterial processes. Freely diffusible intracellular c-di-GMP is determined by the action of metabolizing enzymes that allow integration of numerous input signals. c-di-GMP specifically regulates multiple cellular processes by binding to diverse target molecules. This review highlights important questions in research into the mechanisms of c-di-GMP signalling and its role in bacterial physiology.

Cyclic diguanylate turnover mediated by the sole GGDEF/EAL response regulator in Pseudomonas putida: its role in the rhizosphere and an analysis of its target processes

GGDEF and EAL/HD-GYP protein domains are responsible for the synthesis and hydrolysis of the bacterial secondary messenger cyclic diguanylate (c-di-GMP) through their diguanylate cyclase and phosphodiesterase activities, respectively. Forty-three genes in Pseudomonas putida KT2440 are putatively involved in the turnover of c-di-GMP. Of them only rup4959 (locus PP4959) encodes a GGDEF/EAL response regulator, which was identified in a genome wide analysis as preferentially induced while this bacterium colonizes roots and adjacent soil areas (the rhizosphere). By using fusions to reporter genes it was confirmed that the rup4959 promoter is active in the rhizosphere and inducible by corn plant root exudates and microaerobiosis. Transcription of rup4959 was strictly dependent on the alternative transcriptional factor σ(S) . The inactivation of the rup4959-4957 operon altered the expression of 22 genes in the rhizosphere and had a negative effect upon oligopeptide utilization and biofilm formation. In multicopy or when overexpressed, rup4959 enhanced adhesin LapA-dependent biofilm formation, the development of wrinkly colony morphology, and increased Calcofluor stainable exopolysaccharides (EPS). Under these conditions the inhibition of swarming motility was total and plant root tip colonization considerably less efficient, whereas swimming was partially diminished. This pleiotropic phenotype, which correlated with an increase in the global level of c-di-GMP, was not acquired with increased levels of Rup4959 catalytic mutant at GGDEF as a proof of this response regulator exhibiting diguanylate cyclase activity. A screen for mutants in putative targets of c-di-GMP led to the identification of a surface polysaccharide specific to KT2440, which is encoded by the genes cluster PP3133-PP3141, as essential for phenotypes associated with increased c-di-GMP. Cellulose and alginate were discarded as the overproduced EPS, and lipopolysaccharide (LPS) core and O-antigen were found to be essential for the development of wrinkly colony morphology.

Engineering global regulator Hha of Escherichia coli to control biofilm dispersal

The global transcriptional regulator Hha of Escherichia coli controls biofilm formation and virulence. Previously, we showed that Hha decreases initial biofilm formation; here, we engineered Hha for two goals: to increase biofilm dispersal and to reduce biofilm formation. Using random mutagenesis, Hha variant Hha13D6 (D22V, L40R, V42I and D48A) was obtained that causes nearly complete biofilm dispersal (96%) by increasing apoptosis without affecting initial biofilm formation. Hha13D6 caused cell death probably by the activation of proteases since Hha‐mediated dispersal was dependent on protease HslV. Hha variant Hha24E9 (K62X) was also obtained that decreased biofilm formation by inducing gadW, glpT and phnF but that did not alter biofilm dispersal. Hence, Hha may be engineered to influence both biofilm dispersal and formation.

Engineering a novel c-di-GMP-binding protein for biofilm dispersal

Bacteria prefer to grow attached to themselves or an interface, and it is important for an array of applications to make biofilms disperse. Here we report simultaneously the discovery and protein engineering of BdcA (formerly YjgI) for biofilm dispersal using the universal signal 3,5-cyclic diguanylic acid (c-di-GMP). The bdcA deletion reduced biofilm dispersal, and production of BdcA increased biofilm dispersal to wild-type level. Since BdcA increases motility and extracellular DNA production while decreasing exopolysaccharide, cell length and aggregation, we reasoned that BdcA decreases the concentration of c-di-GMP, the intracellular messenger that controls cell motility through flagellar rotation and biofilm formation through synthesis of curli and cellulose. Consistently, c-di-GMP levels increase upon deleting bdcA, and purified BdcA binds c-di-GMP but does not act as a phosphodiesterase. Additionally, BdcR (formerly YjgJ) is a negative regulator of bdcA. To increase biofilm dispersal, we used protein engineering to evolve BdcA for greater c-di-GMP binding and found that the single amino acid change E50Q causes nearly complete removal of biofilms via dispersal without affecting initial biofilm formation.

GGDEF proteins YeaI, YedQ, and YfiN reduce early biofilm formation and swimming motility in Escherichia coli

The second messenger 3'-5'-cyclic diguanylic acid (c-di-GMP) promotes biofilm formation, and c-di-GMP is synthesized by diguanylate cyclases (characterized by a GGDEF domain) and degraded by phosphodiesterases. Here, we evaluated the effect of the 12 E. coli GGDEF-only proteins on biofilm formation and motility. Deletions of the genes encoding the GGDEF proteins YeaI, YedQ, YfiN, YeaJ, and YneF increased swimming motility as expected for strains with reduced c-di-GMP. Alanine substitution in the EGEVF motif of YeaI abolished its impact on swimming motility. In addition, extracellular DNA (eDNA) was increased as expected for the deletions of yeaI (tenfold), yedQ (1.8-fold), and yfiN (3.2-fold). As a result of the significantly enhanced motility, but contrary to current models of decreased biofilm formation with decreased diguanylate cyclase activity, early biofilm formation increased dramatically for the deletions of yeaI (30-fold), yedQ (12-fold), and yfiN (18-fold). Our results indicate that YeaI, YedQ, and YfiN are active diguanylate cyclases that reduce motility, eDNA, and early biofilm formation and contrary to the current paradigm, the results indicate that c-di-GMP levels should be reduced, not increased, for initial biofilm formation so c-di-GMP levels must be regulated in a temporal fashion in biofilms.

The Rcs phosphorelay: more than just a two-component pathway

The Rcs phosphorelay is a complex signaling pathway found in many, but not all, members of the Enterobacteriaceae. The complexity of this pathway is due to the direct involvement of three proteins (RcsC, RcsD and RcsB) in the phosphorelay and the presence of multiple accessory proteins with important roles in modulating the inputs and outputs associated with this signaling pathway. This article will discuss the various inputs and outputs associated with the Rcs phosphorelay and also present a model suggesting an important role for this signaling pathway in the temporal control of virulence in Salmonella enterica and biofilm formation in Escherichia coli.

A global metabolic shift is linked to Salmonella multicellular development

Bacteria can elaborate complex patterns of development that are dictated by temporally ordered patterns of gene expression, typically under the control of a master regulatory pathway. For some processes, such as biofilm development, regulators that initiate the process have been identified but subsequent phenotypic changes such as stress tolerance do not seem to be under the control of these same regulators. A hallmark feature of biofilms is growth within a self-produced extracellular matrix. In this study we used metabolomics to compare Salmonella cells in rdar colony biofilms to isogenic csgD deletion mutants that do not produce an extracellular matrix. The two populations show distinct metabolite profiles. Even though CsgD controls only extracellular matrix production, metabolite signatures associated with cellular adaptations associated with stress tolerances were present in the wild type but not the mutant cells. To further explore these differences we examine the temporal gene expression of genes implicated in biofilm development and stress adaptations. In wild type cells, genes involved in a metabolic shift to gluconeogenesis and various stress-resistance pathways exhibited an ordered expression profile timed with multicellular development even though they are not CsgD regulated. In csgD mutant cells, the ordered expression was lost. We conclude that the induction of these pathways results from production of, and growth within, a self produced matrix rather than elaboration of a defined genetic program. These results predict that common physiological properties of biofilms are induced independently of regulatory pathways that initiate biofilm formation.

3',5'-Cyclic diguanylic acid: a small nucleotide that makes big impacts

3',5'-Cyclic diguanylic acid (c-di-GMP) is a naturally occurring small cyclic dinucleotide found in bacteria. There has been a recent surge of interest in the two-component signalling networks involving this molecule. This tutorial review introduces the biosynthesis of c-di-GMP, particularly the conserved domain features involved in its enzymatic synthesis and degradation, cellular functions and phenotypes regulated by c-di-GMP through c-di-GMP-binding proteins. The chemical synthesis and structural studies of c-di-GMP are also summarized. Two potential applications of c-di-GMP, i.e. bacterial biofilm formation and immunostimulation, are surveyed. Recent observations on c-di-GMP-binding riboswitches are also introduced.

The diguanylate cyclase YddV controls production of the exopolysaccharide poly-N-acetylglucosamine (PNAG) through regulation of the PNAG biosynthetic pgaABCD operon

In Gram-negative bacteria, production of adhesion factors and extracellular polysaccharides (EPS) is promoted by the activity of diguanylate cyclases (DGCs), a class of enzymes able to catalyse the synthesis of the signal molecule bis-(3',5')-cyclic di-guanylic acid (c-di-GMP). In this report we show that in Escherichia coli, overexpression of the YddV protein, but not of other DGCs such as AdrA and YcdT, induces the production of the EPS poly-N-acetylglucosamine (PNAG) by stimulating expression of pgaABCD, the PNAG-biosynthetic operon. Stimulation of PNAG production and activation of pgaABCD expression by the YddV protein are abolished by inactivation of its GGDEF motif, responsible for DGC activity. Consistent with the effects of YddV overexpression, inactivation of the yddV gene negatively affects pgaABCD transcription and PNAG-mediated biofilm formation. pgaABCD regulation by the yddV gene also takes place in a mutant carrying a partial deletion of the csrA gene, which encodes the main regulator of pgaABCD expression, suggesting that YddV does not regulate pgaABCD through modulation of CsrA activity. Our results demonstrate that PNAG production does not simply respond to intracellular c-di-GMP concentration, but specifically requires the DGC activity of the YddV protein, thus supporting the notion that in E. coli, c-di-GMP biosynthesis by a given DGC protein triggers regulatory events that lead to activation of specific sets of EPS biosynthetic genes or proteins.