Surface expression of O-specific lipopolysaccharide in Escherichia coli requires the function of the TolA protein

A Dye-Assisted Paper-Based Assay to Rapidly Differentiate the Stress of Chlorophenols and Heavy Metals on Enterococcus faecalis and Escherichia coli

Biological toxicity testing plays an essential role in identifying the possible negative effects induced by substances such as organic pollutants or heavy metals. As an alternative to conventional methods of toxicity detection, paper-based analytical device (PAD) offers advantages in terms of convenience, quick results, environmental friendliness, and cost-effectiveness. However, detecting the toxicity of both organic pollutants and heavy metals is challenging for a PAD. Here, we show the evaluation of biotoxicity testing for chlorophenols (pentachlorophenol, 2,4-dichlorophenol, and 4-chlorophenol) and heavy metals (Cu²⁺, Zn²⁺, and Pb²⁺) by a resazurin-integrated PAD. The results were achieved by observing the colourimetric response of bacteria (Enterococcus faecalis and Escherichia coli) to resazurin reduction on the PAD. The toxicity responses of E. faecalis-PAD and E. coli-PAD to chlorophenols and heavy metals can be read within 10 min and 40 min, respectively. Compared to the traditional growth inhibition experiments for toxicity measuring which takes at least 3 h, the resazurin-integrated PAD can recognize toxicity differences between studied chlorophenols and between studied heavy metals within 40 min.

Lipopolysaccharide-Induced Immunological Tolerance in Monocyte-Derived Dendritic Cells

Bacterial lipopolysaccharides (LPS), also referred to as endotoxins, are major outer surface membrane components present on almost all Gram-negative bacteria and are major determinants of sepsis-related clinical complications including septic shock. LPS acts as a strong stimulator of innate or natural immunity in a wide variety of eukaryotic species ranging from insects to humans including specific effects on the adaptive immune system. However, following immune stimulation, lipopolysaccharide can induce tolerance which is an essential immune-homeostatic response that prevents overactivation of the inflammatory response. The tolerance induced by LPS is a state of reduced immune responsiveness due to persistent and repeated challenges, resulting in decreased expression of pro-inflammatory modulators and up-regulation of antimicrobials and other mediators that promote a reduction of inflammation. The presence of environmental-derived LPS may play a key role in decreasing autoimmune diseases and gut tolerance to the plethora of ingested antigens. The use of LPS may be an important immune adjuvant as demonstrated by the promotion of IDO1 increase when present in the fusion protein complex of CTB-INS (a chimera of the cholera toxin B subunit linked to proinsulin) that inhibits human monocyte-derived DC (moDC) activation, which may act through an IDO1-dependent pathway. The resultant state of DC tolerance can be further enhanced by the presence of residual E. coli lipopolysaccharide (LPS) which is almost always present in partially purified CTB-INS preparations. The approach to using an adjuvant with an autoantigen in immunotherapy promises effective treatment for devastating tissue-specific autoimmune diseases like multiple sclerosis (MS) and type 1 diabetes (T1D).

Functional Diversity of TonB-Like Proteins in the Heterocyst-Forming Cyanobacterium Anabaena sp. PCC 7120

The TonB-dependent transport of scarcely available substrates across the outer membrane is a conserved feature in Gram-negative bacteria. The plasma membrane-embedded TonB-ExbB-ExbD accomplishes complex functions as an energy transducer by physically interacting with TonB-dependent outer membrane transporters (TBDTs). TonB mediates structural rearrangements in the substrate-loaded TBDTs that are required for substrate translocation into the periplasm. In the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120, four TonB-like proteins have been identified. Out of these TonB3 accomplishes the transport of ferric schizokinen, the siderophore which is secreted by Anabaena to scavenge iron. In contrast, TonB1 (SjdR) is exceptionally short and not involved in schizokinen transport. The proposed function of SjdR in peptidoglycan structuring eliminates the protein from the list of TonB proteins in Anabaena. Compared with the well-characterized properties of SjdR and TonB3, the functions of TonB2 and TonB4 are yet unknown. Here, we examined tonB2 and tonB4 mutants for siderophore transport capacities and other specific phenotypic features. Both mutants were not or only slightly affected in schizokinen transport, whereas they showed decreased nitrogenase activity in apparently normal heterocysts. Moreover, the cellular metal concentrations and pigment contents were altered in the mutants, most pronouncedly in the tonB2 mutant. This strain showed an altered susceptibility toward antibiotics and SDS and formed cell aggregates when grown in liquid culture, a phenotype associated with an elevated lipopolysaccharide (LPS) production. Thus, the TonB-like proteins in Anabaena appear to take over distinct functions, and the mutation of TonB2 strongly influences outer membrane integrity.

IMPORTANCE The genomes of many organisms encode more than one TonB protein, and their number does not necessarily correlate with that of TonB-dependent outer membrane transporters. Consequently, specific as well as redundant functions of the different TonB proteins have been identified. In addition to a role in uptake of scarcely available nutrients, including iron complexes, TonB proteins are related to virulence, flagellum assembly, pilus localization, or envelope integrity, including antibiotic resistance. The knowledge about the function of TonB proteins in cyanobacteria is limited. Here, we compare the four TonB proteins of Anabaena sp. strain PCC 7120, providing evidence that their functions are in part distinct, since mutants of these proteins exhibit specific features but also show some common impairments.

The multifarious roles of Tol-Pal in Gram-negative bacteria

In the 1960s several groups reported the isolation and preliminary genetic mapping of Escherichia coli strains tolerant towards the action of colicins. These pioneering studies kick-started two new fields in bacteriology; one centred on how bacteriocins like colicins exploit the Tol (or more commonly Tol-Pal) system to kill bacteria, the other on the physiological role of this cell envelope-spanning assembly. The following half century has seen significant advances in the first of these fields whereas the second has remained elusive, until recently. Here, we review work that begins to shed light on Tol-Pal function in Gram-negative bacteria. What emerges from these studies is that Tol-Pal is an energised system with fundamental, interlinked roles in cell division – coordinating the re-structuring of peptidoglycan at division sites and stabilising the connection between the outer membrane and underlying cell wall. This latter role is achieved by Tol-Pal exploiting the proton motive force to catalyse the accumulation of the outer membrane peptidoglycan associated lipoprotein Pal at division sites while simultaneously mobilising Pal molecules from around the cell. These studies begin to explain the diverse phenotypic outcomes of tol-pal mutations, point to other cell envelope roles Tol-Pal may have and raise many new questions.

Molecular Dialogues between Early Divergent Fungi and Bacteria in an Antagonism versus a Mutualism

Animals and plants interact with microbes by engaging specific surveillance systems, regulatory networks, and response modules that allow for accommodation of mutualists and defense against antagonists. Antimicrobial defense responses are mediated in both animals and plants by innate immunity systems that owe their functional similarities to convergent evolution. Like animals and plants, fungi interact with bacteria. However, the principles governing these relations are only now being discovered. In a study system of host and nonhost fungi interacting with a bacterium isolated from the host, we found that bacteria used a common gene repertoire to engage both partners. In contrast, fungal responses to bacteria differed dramatically between the host and nonhost. These findings suggest that as in animals and plants, the genetic makeup of the fungus determines whether bacterial partners are perceived as mutualists or antagonists and what specific regulatory networks and response modules are initiated during each encounter.

Fungal-bacterial symbioses range from antagonisms to mutualisms and remain one of the least understood interdomain interactions despite their ubiquity as well as ecological and medical importance. To build a predictive conceptual framework for understanding interactions between fungi and bacteria in different types of symbioses, we surveyed fungal and bacterial transcriptional responses in the mutualism between Rhizopus microsporus (Rm) (ATCC 52813, host) and its Mycetohabitans (formerly Burkholderia) endobacteria versus the antagonism between a nonhost Rm (ATCC 11559) and Mycetohabitans isolated from the host, at two time points, before and after partner physical contact. We found that bacteria and fungi sensed each other before contact and altered gene expression patterns accordingly. Mycetohabitans did not discriminate between the host and nonhost and engaged a common set of genes encoding known as well as novel symbiosis factors. In contrast, responses of the host versus nonhost to endobacteria were dramatically different, converging on the altered expression of genes involved in cell wall biosynthesis and reactive oxygen species (ROS) metabolism. On the basis of the observed patterns, we formulated a set of hypotheses describing fungal-bacterial interactions and tested some of them. By conducting ROS measurements, we confirmed that nonhost fungi increased production of ROS in response to endobacteria, whereas host fungi quenched their ROS output, suggesting that ROS metabolism contributes to the nonhost resistance to bacterial infection and the host ability to form a mutualism. Overall, our study offers a testable framework of predictions describing interactions of early divergent Mucoromycotina fungi with bacteria.

The lipoprotein Pal stabilises the bacterial outer membrane during constriction by a mobilisation-and-capture mechanism

Coordination of outer membrane constriction with septation is critical to faithful division in Gram-negative bacteria and vital to the barrier function of the membrane. This coordination requires the recruitment of the peptidoglycan-binding outer-membrane lipoprotein Pal at division sites by the Tol system. Here, we show that Pal accumulation at Escherichia coli division sites is a consequence of three key functions of the Tol system. First, Tol mobilises Pal molecules in dividing cells, which otherwise diffuse very slowly due to their binding of the cell wall. Second, Tol actively captures mobilised Pal molecules and deposits them at the division septum. Third, the active capture mechanism is analogous to that used by the inner membrane protein TonB to dislodge the plug domains of outer membrane TonB-dependent nutrient transporters. We conclude that outer membrane constriction is coordinated with cell division by active mobilisation-and-capture of Pal at division septa by the Tol system.

Structural changes of bacterial nanocellulose pellicles induced by genetic modification of Komagataeibacter hansenii ATCC 23769

Bacterial nanocellulose (BNC) synthesized by Komagataeibacter hansenii is a polymer that recently gained an attention of tissue engineers, since its features make it a suitable material for scaffolds production. Nevertheless, it is still necessary to modify BNC to improve its properties in order to make it more suitable for biomedical use. One approach to address this issue is to genetically engineer K. hansenii cells towards synthesis of BNC with modified features. One of possible ways to achieve that is to influence the bacterial movement or cell morphology. In this paper, we described for the first time, K. hansenii ATCC 23769 motA+ and motB+ overexpression mutants, which displayed elongated cell phenotype, increased motility, and productivity. Moreover, the mutant cells produced thicker ribbons of cellulose arranged in looser network when compared to the wild-type strain. In this paper, we present a novel development in obtaining BNC membranes with improved properties using genetic engineering tools.

Proteomic Analysis of Resistance of Gram-Negative Bacteria to Chlorhexidine and Impacts on Susceptibility to Colistin, Antimicrobial Peptides, and Ceragenins

Use of chlorhexidine in clinical settings has led to concerns that repeated exposure of bacteria to sub-lethal doses of chlorhexidine might result in chlorhexidine resistance and cross resistance with other cationic antimicrobials including colistin, endogenous antimicrobial peptides (AMPs) and their mimics, ceragenins. We have previously shown that colistin-resistant Gram-negative bacteria remain susceptible to AMPs and ceragenins. Here, we investigated the potential for cross resistance between chlorhexidine, colistin, AMPs and ceragenins by serial exposure of standard strains of Gram-negative bacteria to chlorhexidine to generate resistant populations of organisms. Furthermore, we performed a proteomics study on the chlorhexidine-resistant strains and compared them to the wild-type strains to find the pathways by which bacteria develop resistance to chlorhexidine. Serial exposure of Gram-negative bacteria to chlorhexidine resulted in four- to eight-fold increases in minimum inhibitory concentrations (MICs). Chlorhexidine-resistant organisms showed decreased susceptibility to colistin (8- to 32-fold increases in MICs) despite not being exposed to colistin. In contrast, chlorhexidine-resistant organisms had the same MICs as the original strains when tested with representative AMPs (LL-37 and magainin I) and ceragenins (CSA-44 and CSA-131). These results imply that there may be a connection between the emergence of highly colistin-resistant Gram-negative pathogens and the prevalence of chlorhexidine usage. Yet, use of chlorhexidine may not impact innate immune defenses (e.g., AMPs) and their mimics (e.g., ceragenins). Here, we also show that chlorhexidine resistance is associated with upregulation of proteins involved in the assembly of LPS for outer membrane biogenesis and virulence factors in Pseudomonas aeruginosa. Additionally, resistance to chlorhexidine resulted in elevated expression levels of proteins associated with chaperones, efflux pumps, flagella and cell metabolism. This study provides a comprehensive overview of the evolutionary proteomic changes in P. aeruginosa following exposure to chlorhexidine and colistin. These results have important clinical implications considering the continuous application of chlorhexidine in hospitals that could influence the emergence of colistin-resistant strains.

Similarities and Differences between Colicin and Filamentous Phage Uptake by Bacterial Cells

Gram-negative bacteria have evolved a complex envelope to adapt and survive in a broad range of ecological niches. This physical barrier is the first line of defense against noxious compounds and viral particles called bacteriophages. Colicins are a family of bactericidal proteins produced by and toxic to Escherichia coli and closely related bacteria. Filamentous phages have a complex structure, composed of at least five capsid proteins assembled in a long thread-shaped particle, that protects the viral DNA. Despite their difference in size and complexity, group A colicins and filamentous phages both parasitize multiprotein complexes of their sensitive host for entry. They first bind to a receptor located at the surface of the target bacteria before specifically recruiting components of the Tol system to cross the outer membrane and find their way through the periplasm. The Tol system is thought to use the proton motive force of the inner membrane to maintain outer membrane integrity during the life cycle of the cell. This review describes the sequential docking mechanisms of group A colicins and filamentous phages during their uptake by their bacterial host, with a specific focus on the translocation step, promoted by interactions with the Tol system.

Gram-negative bacterial membrane vesicle release in response to the host-environment: different threats, same trick?

Bacteria are confronted with a multitude of stressors when occupying niches within the host. These stressors originate from host defense mechanisms, other bacteria during niche competition or result from physiological challenges such as nutrient limitation. To counteract these stressors, bacteria have developed a stress-induced network to mount the adaptations required for survival. These stress-induced adaptations include the release of membrane vesicles from the bacterial envelope. Membrane vesicles can provide bacteria with a plethora of immediate and ultimate benefits for coping with environmental stressors. This review addresses how membrane vesicles aid Gram-negative bacteria to cope with host-associated stress factors, focusing on vesicle biogenesis and the physiological functions. As many of the pathways, that drive vesicle biogenesis, confer we propose that shedding of membrane vesicles by Gram-negative bacteria entails an integrated part of general stress responses.

Electrostatic interactions between the CTX phage minor coat protein and the bacterial host receptor TolA drive the pathogenic conversion of Vibrio cholerae

Vibrio cholerae is a natural inhabitant of aquatic environments and converts to a pathogen upon infection by a filamentous phage, CTXΦ, that transmits the cholera toxin-encoding genes. This toxigenic conversion of V. cholerae has evident implication in both genome plasticity and epidemic risk, but the early stages of the infection have not been thoroughly studied. CTXΦ transit across the bacterial periplasm requires binding between the minor coat protein named pIII and a bacterial inner-membrane receptor, TolA, which is part of the conserved Tol-Pal molecular motor. To gain insight into the TolA-pIII complex, we developed a bacterial two-hybrid approach, named Oxi-BTH, suited for studying the interactions between disulfide bond-folded proteins in the bacterial cytoplasm of an Escherichia coli reporter strain. We found that two of the four disulfide bonds of pIII are required for its interaction with TolA. By combining Oxi-BTH assays, NMR, and genetic studies, we also demonstrate that two intermolecular salt bridges between TolA and pIII provide the driving forces of the complex interaction. Moreover, we show that TolA residue Arg-325 involved in one of the two salt bridges is critical for proper functioning of the Tol-Pal system. Our results imply that to prevent host evasion, CTXΦ uses an infection strategy that targets a highly conserved protein of Gram-negative bacteria essential for the fitness of V. cholerae in its natural environment.

The Burkholderia cenocepacia peptidoglycan-associated lipoprotein is involved in epithelial cell attachment and elicitation of inflammation

The Burkholderia cepacia complex (Bcc) is a group of Gram-negative opportunistic pathogens causing infections in people with cystic fibrosis (CF). Bcc is highly antibiotic resistant, making conventional antibiotic treatment problematic. The identification of novel targets for anti-virulence therapies should improve therapeutic options for infected CF patients. We previously identified that the peptidoglycan-associated lipoprotein (Pal) was immunogenic in Bcc infected CF patients; however, its role in Bcc pathogenesis is unknown. The virulence of a pal deletion mutant (Δpal) in Galleria mellonella was 88-fold reduced (p < .001) compared to wild type. The lipopolysaccharide profiles of wild type and Δpal were identical, indicating no involvement of Pal in O-antigen transport. However, Δpal was more susceptible to polymyxin B. Structural elucidation by X-ray crystallography and calorimetry demonstrated that Pal binds peptidoglycan fragments. Δpal showed a 1.5-fold reduced stimulation of IL-8 in CF epithelial cells relative to wild type (p < .001), demonstrating that Pal is a significant driver of inflammation. The Δpal mutant had reduced binding to CFBE41o^- cells, but adhesion of Pal-expressing recombinant E. coli to CFBE41o^- cells was enhanced compared to wild-type E. coli (p < .0001), confirming that Pal plays a direct role in host cell attachment. Overall, Bcc Pal mediates host cell attachment and stimulation of cytokine secretion, contributing to Bcc pathogenesis.

Exposure of Salmonella enterica Serovar Typhimurium to Three Humectants Used in the Food Industry Induces Different Osmoadaptation Systems

Common salt (NaCl) is frequently used by the food industry to add flavor and to act as a humectant in order to reduce the water content of a food product. The improved health awareness of consumers is leading to a demand for food products with reduced salt content; thus, manufacturers require alternative water activity-reducing agents which elicit the same general effects as NaCl. Two examples include KCl and glycerol. These agents lower the water activity of a food matrix and also contribute to limit the growth of the microbiota, including foodborne pathogens. Little is currently known about how foodborne pathogens respond to these water activity-lowering agents. Here we examined the response of Salmonella enterica serovar Typhimurium 4/74 to NaCl, KCl, and glycerol at three time points, using a constant water activity level, compared with the response of a control inoculum. All conditions induced the upregulation of gluconate metabolic genes after 6 h of exposure. Bacteria exposed to NaCl and KCl demonstrated the upregulation of the osmoprotective transporter mechanisms encoded by the proP, proU, and osmU (STM1491 to STM1494) genes. Glycerol exposure elicited the downregulation of these osmoadaptive mechanisms but stimulated an increase in lipopolysaccharide and membrane protein-associated genes after 1 h. The most extensive changes in gene expression occurred following exposure to KCl. Because many of these genes were of unknown function, further characterization may identify KCl-specific adaptive processes that are not stimulated by NaCl. This study shows that the response of S. Typhimurium to different humectants does not simply reflect reduced water activity and likely involves systems that are linked to specific humectants.

Structure and function of the Escherichia coli Tol-Pal stator protein TolR

TolR is a 15-kDa inner membrane protein subunit of the Tol-Pal complex in Gram-negative bacteria, and its function is poorly understood. Tol-Pal is recruited to cell division sites where it is involved in maintaining the integrity of the outer membrane. TolR is related to MotB, the peptidoglycan (PG)-binding stator protein from the flagellum, suggesting it might serve a similar role in Tol-Pal. The only structure thus far reported for TolR is of the periplasmic domain from Haemophilus influenzae in which N- and C-terminal residues had been deleted (TolR(62–133), Escherichia coli numbering). H. influenzae TolR(62–133) is a symmetrical dimer with a large deep cleft at the dimer interface. Here, we present the 1.7-Å crystal structure of the intact periplasmic domain of E. coli TolR (TolR(36–142)). E. coli TolR(36–142) is also dimeric, but the architecture of the dimer is radically different from that of TolR(62–133) due to the intertwining of its N and C termini. TolR monomers are rotated ∼180° relative to each other as a result of this strand swapping, obliterating the putative PG-binding groove seen in TolR(62–133). We found that removal of the strand-swapped regions (TolR(60–133)) exposes cryptic PG binding activity that is absent in the full-length domain. We conclude that to function as a stator in the Tol-Pal complex dimeric TolR must undergo large scale structural remodeling reminiscent of that proposed for MotB, where the N- and C-terminal sequences unfold in order for the protein to both reach and bind the PG layer ∼90 Å away from the inner membrane.

Targeting glioblastoma via intranasal administration of Ff bacteriophages

Bacteriophages (phages) are ubiquitous viruses that control the growth and diversity of bacteria. Although they have no tropism to mammalian cells, accumulated evidence suggests that phages are not neutral to the mammalian macro-host and can promote immunomodulatory and anti-tumorigenic activities. Here we demonstrate that Ff phages that do not display any proteins or peptides could inhibit the growth of subcutaneous glioblastoma tumors in mice and that this activity is mediated in part by lipopolysaccharide molecules attached to their virion. Using the intranasal route, a non-invasive approach to deliver therapeutics directly to the CNS, we further show that phages rapidly accumulate in the brains of mice and could attenuate progression of orthotopic glioblastoma. Taken together, this study provides new insight into phages non-bacterial activities and demonstrates the feasibility of delivering Ff phages intranasally to treat brain malignancies.

Comparative gene expression of Wigglesworthia inhabiting non-infected and Trypanosoma brucei gambiense-infected Glossina palpalis gambiensis flies

Tsetse flies (Glossina sp.) that transmit trypanosomes causing human (and animal) African trypanosomiasis (HAT and AAT, respectively) harbor symbiotic microorganisms, including the obligate primary symbiont Wigglesworthia glossinidia. A relationship between Wigglesworthia and tsetse fly infection by trypanosomes has been suggested, as removal of the symbiont results in a higher susceptibility to midgut infection in adult flies. To investigate this relationship and to decipher the role of W. glossinidia in the fly's susceptibility to trypanosome infection, we challenged flies with trypanosomes and subsequently analyzed and compared the transcriptomes of W. glossinidia from susceptible and refractory tsetse flies at three time points (3, 10, and 20 days). More than 200 W. glossinidia genes were found to be differentially expressed between susceptible and refractory flies. The high specificity of these differentially expressed genes makes it possible to distinguish Wigglesworthia inhabiting these two distinct groups of flies. Furthermore, gene expression patterns were observed to evolve during the infection time course, such that very few differentially expressed genes were found in common in Wigglesworthia from the 3-, 10- and 20-day post-feeding fly samples. The overall results clearly demonstrate that the taking up of trypanosomes by flies, regardless of whether flies proceed with the developmental program of Trypanosoma brucei gambiense, strongly alters gene expression in Wigglesworthia. These results therefore provide a novel framework for studies that aim to decrease or even abolish tsetse fly vector competence.

Polar localization of Escherichia coli chemoreceptors requires an intact Tol-Pal complex

Subcellular biomolecular localization is critical for the metabolic and structural properties of the cell. The functional implications of the spatiotemporal distribution of protein complexes during the bacterial cell cycle have long been acknowledged; however, the molecular mechanisms for generating and maintaining their dynamic localization in bacteria are not completely understood. Here we demonstrate that the trans-envelope Tol-Pal complex, a widely conserved component of the cell envelope of Gram-negative bacteria, is required to maintain the polar positioning of chemoreceptor clusters in Escherichia coli. Localization of the chemoreceptors was independent of phospholipid composition of the membrane and the curvature of the cell wall. Instead, our data indicate that chemoreceptors interact with components of the Tol-Pal complex and that this interaction is required to polarly localize chemoreceptor clusters. We found that disruption of the Tol-Pal complex perturbs the polar localization of chemoreceptors, alters cell motility, and affects chemotaxis. We propose that the E. coli Tol-Pal complex restricts mobility of the chemoreceptor clusters at the cell poles and may be involved in regulatory mechanisms that co-ordinate cell division and segregation of the chemosensory machinery.

Bacterial self-defence: how Escherichia coli evades serum killing

The ability to survive the bactericidal action of serum is advantageous to extraintestinal pathogenic Escherichia coli that gain access to the bloodstream. Evasion of the innate defences present in serum, including complement and antimicrobial peptides, involves multiple factors. Serum resistance mechanisms utilized by E. coli include the production of protective extracellular polysaccharide capsules and expression of factors that inhibit or interfere with the complement cascade. Recent studies have also highlighted the importance of structural integrity of the cell envelope in serum survival. These survival strategies are outlined in this review with particular attention to novel findings and recent insights into well-established resistance mechanisms.

Microcompartments and protein machines in prokaryotes

The prokaryotic cell was once thought of as a 'bag of enzymes' with little or no intracellular compartmentalization. In this view, most reactions essential for life occurred as a consequence of random molecular collisions involving substrates, cofactors and cytoplasmic enzymes. Our current conception of a prokaryote is far from this view. We now consider a bacterium or an archaeon as a highly structured, nonrandom collection of functional membrane-embedded and proteinaceous molecular machines, each of which serves a specialized function. In this article we shall present an overview of such microcompartments including (1) the bacterial cytoskeleton and the apparati allowing DNA segregation during cell division; (2) energy transduction apparati involving light-driven proton pumping and ion gradient-driven ATP synthesis; (3) prokaryotic motility and taxis machines that mediate cell movements in response to gradients of chemicals and physical forces; (4) machines of protein folding, secretion and degradation; (5) metabolosomes carrying out specific chemical reactions; (6) 24-hour clocks allowing bacteria to coordinate their metabolic activities with the daily solar cycle, and (7) proteinaceous membrane compartmentalized structures such as sulfur granules and gas vacuoles. Membrane-bound prokaryotic organelles were considered in a recent Journal of Molecular Microbiology and Biotechnology written symposium concerned with membranous compartmentalization in bacteria [J Mol Microbiol Biotechnol 2013;23:1-192]. By contrast, in this symposium, we focus on proteinaceous microcompartments. These two symposia, taken together, provide the interested reader with an objective view of the remarkable complexity of what was once thought of as a simple noncompartmentalized cell.

Overexpression of the Escherichia coli TolQ protein leads to a null-FtsN-like division phenotype

Mutations involving the Tol-Pal complex of Escherichia coli result in a subtle phenotype in which cells chain when grown under low-salt conditions. Here, the nonpolar deletion of individual genes encoding the cytoplasmic membrane-associated components of the complex (TolQ, TolR, TolA) produced a similar phenotype. Surprisingly, the overexpression of one of these proteins, TolQ, resulted in a much more overt phenotype in which cells occurred as elongated rods coupled in long chains when grown under normal salt conditions. Neither TolR nor TolA overexpression produced a phenotype, nor was the presence of either protein required for the TolQ-dependent phenotype. Consistent with their native membrane topology, the amino-terminal domain of TolQ specifically associated in vivo with the periplasmic domain of FtsN in a cytoplasm-based two-hybrid analysis. Further, the concomitant overexpression of FtsN rescued the TolQ-dependent phenotype, suggesting a model wherein the overexpression of TolQ sequesters FtsN, depleting this essential protein from the divisome during Gram-negative cell division. The role of the Tol-Pal system in division is discussed.

Over-expression of the cytoplasmic membrane protein TolQ resulted in a division phenotype similar to that seen in cells depleted for FtsN. Two hybrid analysis suggested that TolQ and FtsN physically interact through domains that localize in the periplasmic space; while the concurrent over-expression of FtsN alleviated the TolQ over-expression phenotype. Together these results suggest a model wherein over-expressed TolQ sequesters FtsN, disrupting normal cell division.

Prc contributes to Escherichia coli evasion of classical complement-mediated serum killing

Escherichia coli is a common Gram-negative organism that causes bacteremia. Prc, a bacterial periplasmic protease, and its homologues are known to be involved in the pathogenesis of Gram-negative bacterial infections. The present study examined the role of Prc in E. coli bacteremia and characterized the ability of the prc mutant of the pathogenic E. coli strain RS218 to cause bacteremia and survive in human serum. The prc mutant of RS218 exhibited a decreased ability to cause a high level of bacteremia and was more sensitive to serum killing than strain RS218. This sensitivity was due to the mutant's decreased ability to avoid the activation of the antibody-dependent and -independent classical complement cascades as well as its decreased resistance to killing mediated by the membrane attack complex, the end product of complement system activation. The demonstration of Prc in the evasion of classical complement-mediated serum killing of pathogenic E. coli makes this factor a potential target for the development of therapeutic and preventive measures against E. coli bacteremia.

Pseudomonas biofilm matrix composition and niche biology

Biofilms are a predominant form of growth for bacteria in the environment and in the clinic. Critical for biofilm development are adherence, proliferation, and dispersion phases. Each of these stages includes reinforcement by, or modulation of, the extracellular matrix. Pseudomonas aeruginosa has been a model organism for the study of biofilm formation. Additionally, other Pseudomonas species utilize biofilm formation during plant colonization and environmental persistence. Pseudomonads produce several biofilm matrix molecules, including polysaccharides, nucleic acids, and proteins. Accessory matrix components shown to aid biofilm formation and adaptability under varying conditions are also produced by pseudomonads. Adaptation facilitated by biofilm formation allows for selection of genetic variants with unique and distinguishable colony morphology. Examples include rugose small-colony variants and wrinkly spreaders (WS), which over produce Psl/Pel or cellulose, respectively, and mucoid bacteria that over produce alginate. The well-documented emergence of these variants suggests that pseudomonads take advantage of matrix-building subpopulations conferring specific benefits for the entire population. This review will focus on various polysaccharides as well as additional Pseudomonas biofilm matrix components. Discussions will center on structure-function relationships, regulation, and the role of individual matrix molecules in niche biology.

The caulobacter Tol-Pal complex is essential for outer membrane integrity and the positioning of a polar localization factor

Cell division in Caulobacter crescentus involves constriction and fission of the inner membrane (IM) followed about 20 min later by fission of the outer membrane (OM) and daughter cell separation. In contrast to Escherichia coli, the Caulobacter Tol-Pal complex is essential. Cryo-electron microscopy images of the Caulobacter cell envelope exhibited outer membrane disruption, and cells failed to complete cell division in TolA, TolB, or Pal mutant strains. In wild-type cells, components of the Tol-Pal complex localize to the division plane in early predivisional cells and remain predominantly at the new pole of swarmer and stalked progeny upon completion of division. The Tol-Pal complex is required to maintain the position of the transmembrane TipN polar marker, and indirectly the PleC histidine kinase, at the cell pole, but it is not required for the polar maintenance of other transmembrane and membrane-associated polar proteins tested. Coimmunoprecipitation experiments show that both TolA and Pal interact directly or indirectly with TipN. We propose that disruption of the trans-envelope Tol-Pal complex releases TipN from its subcellular position. The Caulobacter Tol-Pal complex is thus a key component of cell envelope structure and function, mediating OM constriction at the final step of cell division as well as the positioning of a protein localization factor.

Characterization of the orf1-tolQRA operon in Pseudomonas aeruginosa

The tol-pal genes play important roles in maintaining outer membrane integrity, transmembrane transportation, and cell division in Gram-negative bacteria. In Pseudomonas aeruginosa, an important human opportunistic pathogen, the tol-oprL genes are organized uniquely in three operons, orf1-tolQRA, tolB and oprL-orf2, and are regulated by iron availability. Similarity between TolQRA and the iron transport system ExbBD-TonB also exists in P. aeruginosa and they can replace each other imperfectly. It is of importance to investigate the regulation and functions of this membrane complex. In the present study, we characterized the promoters and expression profiles of the orf1-tolQRA operon and investigated the function of Orf1. Primer extension was carried out by using both isotope-labeled and florescence labeled primers and the expression profiles were determined using both lacZ and luxCDABE-based transcriptional fusions. Our results revealed two distinct promoters at the upstream region of tolQRA; the one located in front of orf1 was constitutive whereas the other within the orf1 coding region was iron regulated. Expression profiles indicate the tol genes were also downregulated by the quorum-sensing systems during the late stage of growth. Unlike tolQ and tolA, we were able to construct a viable orf1 knockout strain, and the mutant exhibited altered cell and colony morphology, providing first evidence that Orf1 plays a non-essential role in the Tol-OprL complex in P. aeruginosa.

Peptidoglycan-associated lipoprotein (Pal) of Gram-negative bacteria: function, structure, role in pathogenesis and potential application in immunoprophylaxis

The protein Pal (peptidoglycan-associated lipoprotein) is anchored in the outer membrane (OM) of Gram-negative bacteria and interacts with Tol proteins. Tol-Pal proteins form two complexes: the first is composed of three inner membrane Tol proteins (TolA, TolQ and TolR); the second consists of the TolB and Pal proteins linked to the cell's OM. These complexes interact with one another forming a multiprotein membrane-spanning system. It has recently been demonstrated that Pal is essential for bacterial survival and pathogenesis, although its role in virulence has not been clearly defined. This review summarizes the available data concerning the structure and function of Pal and its role in pathogenesis.

Energy-dependent immunity protein release during tol-dependent nuclease colicin translocation

Nuclease colicins bind their target receptor in the outer membrane of sensitive cells in the form of a high affinity complex with their cognate immunity proteins. Upon cell entry the immunity protein is lost from the complex by means that are poorly understood. We have developed a sensitive fluorescence assay that has enabled us to study the molecular requirements for immunity protein release. Nuclease colicins use members of the tol operon for their translocation across the outer membrane. We have demonstrated that the amino-terminal 80 residues of the colicin E9 molecule, which is the region that interacts with TolB, are essential for immunity protein release. Using tol deletion strains we analyzed the cellular components necessary for immunity protein release and found that in addition to a requirement for tolB, the tolA deletion strain was most affected. Complementation studies showed that the mutation H22A, within the transmembrane segment of TolA, abolishes immunity protein release. Investigation of the energy requirements demonstrated that the proton motive force of the cytoplasmic membrane is critical. Taken together these results demonstrate for the first time a clear energy requirement for the uptake of a nuclease colicin complex and suggest that energy transduced from the cytoplasmic membrane to the outer membrane by TolA could be the driving force for immunity protein release and concomitant translocation of the nuclease domain.

Deletion of tolA in Salmonella Typhimurium generates an attenuated strain with vaccine potential

The Gram-negative Tol-Pal system of envelope proteins plays a key role in maintaining outer membrane integrity and contributes to the virulence of several pathogens. We have investigated the role of one of these proteins, TolA, in the biology of Salmonella enterica serovar Typhimurium. Deletion of tolA rendered strain SL1344 more susceptible to killing by bile and human serum. In addition the mutant had impaired membrane integrity and displayed alterations in LPS production. The tolA mutant was highly attenuated in mouse infections via the oral and intravenous routes. Importantly, each phenotype displayed by the mutant was complemented by provision of tolA in trans. The tolA gene therefore contributes to virulence, membrane integrity, LPS production and bile and serum resistance in S. enterica serovar Typhimurium SL1344. Finally, immunization with the tolA mutant provided significant protection against subsequent challenge with wild-type SL1344. The Tol-Pal system is therefore a potential target in the development of novel attenuated live vaccines against Salmonella and other Gram-negative pathogens.

Regulation of lipopolysaccharide O antigen expression in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative bacterium that is ubiquitously found in the environment. It is an important opportunistic pathogen in immunocompromised patients and causes life-threatening lung infections in individuals with cystic fibrosis. A prominent virulence factor for many Gram-negative bacteria, including P. aeruginosa, is lipopolysaccharide (LPS), which is an immunodominant antigen located in the outer portion of the outer membrane. P. aeruginosa produces two O antigens that are attached to lipid A + core: a B-band O antigen and an A-band O polysaccharide. The B-band O antigen-repeating unit of LPS is responsible for serotype specificity; strains lacking O antigen have been shown to be less virulent in animal models of infection. What is less well understood is how the O antigen chain length is regulated and why P. aeruginosa and some other bacteria show two preferred O antigen lengths. P. aeruginosa encodes two genes encoding O antigen chain length regulators. These genes, wzz1 and wzz2, influence the expression of the long and very long chain lengths, respectively. The long chain length appears more important for resistance to the action of sera and virulence in a mouse model of infection, while the very long chain length appears to be more sensitive to environmental stress conditions. Studies in other bacteria point to regulation at the level of transcription and complex formation as being involved in determining the O antigen chain length and may provide clues to the regulation in P. aeruginosa.

HorB (HP0127) is a gastric epithelial cell adhesin

BACKGROUND

The Helicobacter pylori protein HorB (encoded by HP0127) is a member of a paralogous family that includes the adhesins BabA, AlpA, AlpB, and HopZ, which contribute to adhesion to gastric epithelial cells. Of the verified H. pylori porins, the HorB sequence is most similar to that of HopE, but the function of HorB is unknown. The aim of our study was to investigate the role of HorB in H. pylori gastric epithelial cell adhesion.

MATERIALS AND METHODS

We disrupted the horB gene in H. pylori and measured the adhesion to gastric epithelial cells (AGS cells). We then assessed the effect that HorB disruption had on lipopolysaccharide (LPS) O-chain production and Lewis x and Lewis y antigen expression. A HorB mutant in the mouse-adapted strain H. pylori SS1 was created by marker exchange and mouse stomach colonization was quantified. Using reverse transcription polymerase chain reaction, human gastric biopsy material from H. pylori-infected patients was then examined for expression of the horB gene.

RESULTS

Disruption of the horB gene reduced H. pylori adhesion by more than twofold. Adhesion in the horB knockout strain was restored to wild-type levels by re-introduction of HorB into the chromosome. Disruption of HorB reduced production of LPS O-chains and lowered the level of expression of Lewis x and Lewis y antigens. Insertional mutagenesis of the horB gene in H. pylori SS1 reduced mouse stomach colonization threefold. Finally, expression of the horB gene was detected in human gastric biopsy material from H. pylori-infected patients.

CONCLUSIONS

From these data we conclude that HorB has a role in H. pylori adhesion during infection.

Colicin biology

Colicins are proteins produced by and toxic for some strains of Escherichia coli. They are produced by strains of E. coli carrying a colicinogenic plasmid that bears the genetic determinants for colicin synthesis, immunity, and release. Insights gained into each fundamental aspect of their biology are presented: their synthesis, which is under SOS regulation; their release into the extracellular medium, which involves the colicin lysis protein; and their uptake mechanisms and modes of action. Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The structures of some colicins are known at the atomic level and are discussed. Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or the TonB system. The components of each system are known, and their implication in the functioning of the system is described. Colicins then reach their lethal target and act either by forming a voltage-dependent channel into the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA. The mechanisms of inhibition by specific and cognate immunity proteins are presented. Finally, the use of colicins as laboratory or biotechnological tools and their mode of evolution are discussed.

Mutational Analyses Define Helix Organization and Key Residues of a Bacterial Membrane Energy-transducing Complex

In Gram-negative bacteria, many biological processes are coupled to inner membrane ion gradients. Ions transit at the interface of helices of integral membrane proteins, generating mechanical energy to drive energetic processes. To better understand how ions transit through these channels, we used a model system involved in two different processes, one of which depends on inner membrane energy. The Tol machinery of the Escherichia coli cell envelope is dedicated to maintaining outer membrane stability, a process driven by the proton-motive force. The Tol system is parasitized by bacterial toxins called colicins, which are imported through the outer membrane using an energy-independent process. Herein, we mutated TolQ and TolR transmembrane residues, and we analyzed the mutants for outer membrane stability, colicin import and protein complex formation. We identified residues involved in the assembly of the complex, and a new class of discriminative mutations that conferred outer membrane destabilization identical to a tol deletion mutant, but which remained fully sensitive to colicins. Further genetic approaches revealed transmembrane helix interactions and organization in the bilayer, and suggested that most of the discriminative residues are located in a putative aqueous ion channel. We discuss a model for the function of related bacterial molecular motors.

Interplay of the Wzx translocase and the corresponding polymerase and chain length regulator proteins in the translocation and periplasmic assembly of lipopolysaccharide o antigen

Genetic evidence suggests that a family of bacterial and eukaryotic integral membrane proteins (referred to as Wzx and Rft1, respectively) mediates the transbilayer movement of isoprenoid lipid-linked glycans. Recent work in our laboratory has shown that Wzx proteins involved in O-antigen lipopolysaccharide (LPS) assembly have relaxed specificity for the carbohydrate structure of the O-antigen subunit. Furthermore, the proximal sugar bound to the isoprenoid lipid carrier, undecaprenyl-phosphate (Und-P), is the minimal structure required for translocation. In Escherichia coli K-12, N-acetylglucosamine (GlcNAc) is the proximal sugar of the O16 and enterobacterial common antigen (ECA) subunits. Both O16 and ECA systems have their respective translocases, WzxO16 and WzxE, and also corresponding polymerases (WzyO16 and WzyE) and O-antigen chain-length regulators (WzzO16 and WzzE), respectively. In this study, we show that the E. coli wzxE gene can fully complement a wzxO16 translocase deletion mutant only if the majority of the ECA gene cluster is deleted. In addition, we demonstrate that introduction of plasmids expressing either the WzyE polymerase or the WzzE chain-length regulator proteins drastically reduces the O16 LPS-complementing activity of WzxE. We also show that this property is not unique to WzxE, since WzxO16 and WzxO7 can cross-complement translocase defects in the O16 and O7 antigen clusters only in the absence of their corresponding Wzz and Wzy proteins. These genetic data are consistent with the notion that the translocation of O-antigen and ECA subunits across the plasma membrane and the subsequent assembly of periplasmic O-antigen and ECA Und-PP-linked polymers depend on interactions among Wzx, Wzz, and Wzy, which presumably form a multiprotein complex.

Escherichia coliTolA tolerates multiple amino-acid substitutions as revealed by screening randomized variants for membrane integrity and phage receptor function

Escherichia coli TolA is a cytoplasmic membrane protein required for outer membrane integrity and the translocation of F-specific filamentous (Ff) bacteriophage DNA. Both phage infection and membrane integrity depend on several TolA interactions, e.g. those of the TolA C-terminal domain (TolAIII). Membrane integrity involves interaction with two host proteins and phage translocation requires direct interaction with the N-terminal domain (N1) of Ff phage protein g3p. Although cocrystallization of TolAIII and N1g3p has identified several contact points, it is still uncertain which residues are selectively involved in the different TolA functions. Thus, four different limited substitution libraries of TolA were created, targeting contacts at positions 415-420. These libraries were introduced into the tolA strain K17DE3tolA/F(+) and several variants, containing complementing, multiple amino-acid substitutions, were identified. However, most randomized variants did not complement the tolA strain K17DE3tolA/F(+). The TolA variants that restored sensitivity to phage infection displayed a considerable sequence variation, while the few variants that restored tolerance to detergent were from the same library. A comparison of the generated residue variation and natural variation, suggests that structural dependence overrides contact residue dependence. Thus, library screening can be efficient in identifying TolA variants with different functionally associated characteristics.

A protein network for phospholipid synthesis uncovered by a variant of the tandem affinity purification method in Escherichia coli

In prokaryotes, acyl carrier protein (ACP) is a cofactor central to a myriad of syntheses, including fatty acid and phospholipid synthesis. To fulfill its function, ACP must therefore interact with a multitude of different enzymes, which includes the thioesterase YbgC. We found a specific interaction between ACP and YbgC whose thioesterase activity has been demonstrated in vitro on acyl-CoA derivatives, but whose physiological function in bacteria remains unknown. Therefore, YbgC could be a thioesterase active on some specific acyl-ACPs. We then assigned a function to the ACP/YbgC pair by employing a proteomic approach derived from tandem affinity purification, the split tag method. This technique allowed us to purify proteins interacting with ACP and YbgC proteins at the same time. Interactions with PlsB, a sn-glycerol-3-phosphate acyltransferase and PssA, a phosphatidylserine synthase, were identified and validated, showing that YbgC is involved in phospholipid metabolism. Furthermore, using an in vivo bacterial two-hybrid interaction analysis, we showed for the first time that enzymes of the phospholipid synthesis pathway form a complex in the inner membrane. Taken together, these results describe an integrated protein network that could be involved in the coordination of phospholipid metabolism.

The Pseudomonas fluorescens SBW25 wrinkly spreader biofilm requires attachment factor, cellulose fibre and LPS interactions to maintain strength and integrity

The wrinkly spreader (WS) isolate of Pseudomonas fluorescens SBW25 forms a substantial biofilm at the air-liquid interface. The biofilm is composed of an extracellular partially acetylated cellulose-fibre matrix, and previous mutagenesis of WS with mini-Tn5 had identified both the regulatory and cellulose-biosynthetic operons. One uncharacterized WS mutant, WS-5, still expressed cellulose but produced very weak biofilms. In this work, the mini-Tn5 insertion site in WS-5 has been identified as being immediately upstream of the tol-pal operon. Like Tol-Pal mutants of other Gram-negative bacteria, WS-5 showed a "leaky-membrane" phenotype, including the serendipitous ability to utilize sucrose, increased uptake of the hydrophilic dye propidium iodide, and the loss of lipopolysaccharide (LPS) expression. WS-5 cells were altered in relative hydrophobicity, and showed poorer recruitment and maintenance in the biofilm than WS. The WS-5 biofilm was also less sensitive to chemical interference during development. However, growth rate, cellulose expression and attachment were not significantly different between WS and WS-5. Finally, WS-5 biofilms could be partially complemented with WS-4, a biofilm- and attachment-deficient mutant that expressed LPS, resulting in a mixed biofilm with significantly increased strength. These findings show that a major component of the WS air-liquid biofilm strength results from the interactions between LPS and the cellulose matrix of the biofilm--and that in the WS biofilm, cellulose fibres, attachment factor and LPS are required for biofilm development, strength and integrity.

Functional genome annotation through phylogenomic mapping

Accurate determination of functional interactions among proteins at the genome level remains a challenge for genomic research. Here we introduce a genome-scale approach to functional protein annotation--phylogenomic mapping--that requires only sequence data, can be applied equally well to both finished and unfinished genomes, and can be extended beyond single genomes to annotate multiple genomes simultaneously. We have developed and applied it to more than 200 sequenced bacterial genomes. Proteins with similar evolutionary histories were grouped together, placed on a three dimensional map and visualized as a topographical landscape. The resulting phylogenomic maps display thousands of proteins clustered in mountains on the basis of coinheritance, a strong indicator of shared function. In addition to systematic computational validation, we have experimentally confirmed the ability of phylogenomic maps to predict both mutant phenotype and gene function in the delta proteobacterium Myxococcus xanthus.

Defective O-antigen polymerization in tolA and pal mutants of Escherichia coli in response to extracytoplasmic stress

We have previously shown that the TolA protein is required for the correct surface expression of the Escherichia coli O7 antigen lipopolysaccharide (LPS). In this work, delta tolA and delta pal mutants of E. coli K-12 W3110 were transformed with pMF19 (encoding a rhamnosyltransferase that reconstitutes the expression of O16-specific LPS), pWQ5 (encoding the Klebsiella pneumoniae O1 LPS gene cluster), or pWQ802 (encoding the genes necessary for the synthesis of Salmonella enterica O:54). Both DeltatolA and delta pal mutants exhibited reduced surface expression of O16 LPS as compared to parental W3110, but no significant differences were observed in the expression of K. pneumoniae O1 LPS and S. enterica O:54 LPS. Therefore, TolA and Pal are required for the correct surface expression of O antigens that are assembled in a wzy (polymerase)-dependent manner (like those of E. coli O7 and O16) but not for O antigens assembled by wzy-independent pathways (like K. pneumoniae O1 and S. enterica O:54). Furthermore, we show that the reduced surface expression of O16 LPS in delta tolA and delta pal mutants was associated with a partial defect in O-antigen polymerization and it was corrected by complementation with intact tolA and pal genes, respectively. Using derivatives of W3110 delta tolA and W3110 delta pal containing lacZ reporter fusions to fkpA and degP, we also demonstrate that the RpoE-mediated extracytoplasmic stress response is upregulated in these mutants. Moreover, an altered O16 polymerization was also detected under conditions that stimulate RpoE-mediated extracytoplasmic stress responses in tol+ and pal+ genetic backgrounds. A Wzy derivative with an epitope tag at the C-terminal end of the protein was stable in all the mutants, ruling out stress-mediated proteolysis of Wzy. We conclude that the absence of TolA and Pal elicits a sustained extracytoplasmic stress response that in turn reduces O-antigen polymerization but does not affect the stability of the Wzy O-antigen polymerase.

Solution Structure of the E.coli TolA C-terminal Domain Reveals Conformational Changes upon Binding to the Phage g3p N-terminal Domain

The Tol-Pal system of Escherichia coli is a macromolecular complex located in the cell envelope. It is involved in maintaining the integrity of the outer membrane and is required for the uptake of two different types of macromolecules, which are bacteriotoxins (colicins) and DNA of filamentous bacteriophages. The TolA protein plays a central role in these import mechanisms. Its C-terminal domain (TolAIII) is involved in the translocation step via direct interaction with the N-terminal domain of colicins and the N-terminal domain of the phage minor coat gene 3 protein (g3pN1). Extreme behaviours of TolAIII have been previously observed, since the structure of TolAIII either remained unaffected or adopted disordered conformation upon binding to different pore-forming colicins. Here, we have solved the 3D structure of free TolAIII by heteronuclear NMR spectroscopy and compared it to the crystal structure of TolAIII bound to g3pN1 in order to study the effect of g3pN1 on the tertiary structure of TolAIII. Backbone 1H, 15N and 13C resonances of the g3pN1-bound TolAIII were also assigned and used to superimpose the solution structure of free TolAIII on the crystal structure of the g3pN1-TolAIII fusion protein. This allowed us to track conformational changes of TolAIII upon binding. While the global fold of free TolAIII is mainly identical to that of g3pN1-bound TolAIII, shift of secondary structures does occur. Thus, TolAIII, which interacts also in vivo with Pal and TolB, is able to adapt its conformation upon binding to various partners. Possible models for protein binding mechanisms are discussed to explain this so-far unobserved behaviour of TolAIII.

Improved methods for producing outer membrane vesicles in Gram-negative bacteria

Outer membrane vesicle formation occurs during Gram-negative bacterial growth. However, natural production of large amounts of outer membrane vesicles has only been described in a few bacterial genera. The purified vesicles of some bacterial pathogens have shown potential applications in vaccinology and in antibiotic therapy. This study focused on the development of a gene expression system able to induce production of large amounts of outer membrane vesicles. The Tol-Pal system of Escherichia coli, required to maintain outer membrane integrity, is composed of five cell envelope proteins, TolA, TolB, TolQ, TolR and Pal. Tol proteins are parasitized by filamentous bacteriophages and by colicins. The phage infection process and colicin import require, respectively, the N-terminal domain of the minor coat g3p protein and the translocation domain of colicins, with both domains interacting with Tol proteins. In this study, we show that the periplasmic production of either Tol, g3p or colicin domains was able to specifically destabilize the E. coli or Shigella flexneri cell envelope and to induce production of high amounts of vesicles. This technique was further found to work efficiently in Salmonella enterica serovar Typhimurium.

Deletion analyses of the peptidoglycan-associated lipoprotein Pal reveals three independent binding sequences including a TolA box

The Tol-Pal system of the Escherichia coli cell envelope is composed of five proteins. TolQ, TolR and TolA form a complex in the inner membrane, whereas TolB is a periplasmic protein interacting with Pal, the peptidoglycan-associated lipoprotein anchored to the outer membrane. This system is required for outer membrane integrity and has been shown to form a trans-envelope bridge linking inner and outer membranes. The TolA-Pal interaction plays an important role in the function of this system and has been found to depend on the proton motive force and the TolQ and TolR proteins. The Pal lipoprotein interacts with many components, such as TolA, TolB, OmpA, the major lipoprotein and the murein layer. In this study, six pal deletions were constructed. The analyses of the resulting Pal protein functions and interactions defined an N-terminal region of 40 residues, which can be deleted without any cell-damaging effect, and three independent regions required for its interaction with TolA, OmpA and TolB or the peptidoglycan. The analyses of the integrity of the cells producing the various Pal lipoproteins revealed strong outer membrane destabilization only when binding regions were deleted. Furthermore, a conserved polypeptide sequence located downstream of the peptidoglycan binding motif of Pal was required for the TolA-Pal interaction and for the maintenance of outer membrane stability.

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Role of Pseudomonas putida tol-oprL gene products in uptake of solutes through the cytoplasmic membrane

Proteins of the Tol-Pal (Tol-OprL) system play a key role in the maintenance of outer membrane integrity and cell morphology in gram-negative bacteria. Here we describe an additional role for this system in the transport of various carbon sources across the cytoplasmic membrane. Growth of Pseudomonas putida tol-oprL mutant strains in minimal medium with glycerol, fructose, or arginine was impaired, and the growth rate with succinate, proline, or sucrose as the carbon source was lower than the growth rate of the parental strain. Assays with radiolabeled substrates revealed that the rates of uptake of these compounds by mutant cells were lower than the rates of uptake by the wild-type strain. The pattern and amount of outer membrane protein in the P. putida tol-oprL mutants were not changed, suggesting that the transport defect was not in the outer membrane. Consistently, the uptake of radiolabeled glucose and glycerol in spheroplasts was defective in the P. putida tol-oprL mutant strains, suggesting that there was a defect at the cytoplasmic membrane level. Generation of a proton motive force appeared to be unaffected in these mutants. To rule out the possibility that the uptake defect was due to a lack of specific transporter proteins, the PutP symporter was overproduced, but this overproduction did not enhance proline uptake in the tol-oprL mutants. These results suggest that the Tol-OprL system is necessary for appropriate functioning of certain uptake systems at the level of the cytoplasmic membrane.

The mechanism of bacterial infection by filamentous phages involves molecular interactions between TolA and phage protein 3 domains

The early events in filamentous bacteriophage infection of gram-negative bacteria are mediated by the gene 3 protein (g3p) of the virus. This protein has a sophisticated domain organization consisting of two N-terminal domains and one C-terminal domain, separated by flexible linkers. The molecular interactions between these domains and the known bacterial coreceptor protein (TolA) were studied using a biosensor technique, and we report here on interactions of the viral coat protein with TolA, as well as on interactions between the TolA molecules. We detected an interaction between the pilus binding second domain (N2) of protein 3 and the bacterial TolA. This novel interaction was found to depend on the periplasmatic domain of TolA (TolAII). Furthermore, extensive interaction was detected between TolA molecules, demonstrating that bacterial TolA has the ability to interact functionally with itself during phage infection. The kinetics of g3p binding to TolA is also different from that of bacteriocins, since both N-terminal domains of g3p were found to interact with TolA. The multiple roles for each of the separate g3p and TolA domains imply a delicate interaction network during the phage infection process and a model for the infection mechanism is hypothesized.

Transcriptional organization of the Pseudomonas putida tol-oprL genes

Proteins of the Tol system play a key role in the maintenance of outer membrane integrity and cell morphology in gram-negative bacteria. In Pseudomonas putida, the seven genes, orf1, tolQ, tolR, tolA, tolB, oprL, and orf2, which encode the proteins of this complex, are clustered in a 5.8-kb region of chromosomal DNA. Analysis of polar mutations, reverse transcriptase PCR assays, and transcriptional fusion constructs with a promoterless lacZ gene revealed that the genes are arranged in two operons: orf1 tolQ tolR tolA tolB and oprL orf2. We were also able to find a transcript that was initiated at the orf1 promoter and covered the two operons in a single mRNA. On the basis of the OprL protein level, we surmised that this transcript contributed only about 10 to 15% of the total OprL protein. Primer extension analysis identified the oprL orf2 operon promoter within the tolB gene, and the -10 and -35 regions exhibited some similarity to those of sigma(70)-recognized promoters. The transcription start point of orf1 was located 91 bp upstream of the orf1 start codon, and the -10/-35 region also exhibited sigma(70) -10/-35 recognition sequences. The expression from both promoters in rich and minimal media was constitutive and was very little influenced by the growth phase or iron-deficient conditions. In addition, analyses of the beta-galactosidase activities of different translational fusion constructs revealed that translation of tolA and orf2 genes was dependent on the translation of their corresponding upstream genes (tolR and oprL, respectively).

Genetic analysis of functions involved in the late stages of biofilm development in Burkholderia cepacia H111

Burkholderia cepacia and Pseudomonas aeruginosa often co-exist as mixed biofilms in the lungs of patients suffering from cystic fibrosis (CF). Here, we report the isolation of 13 random mini-Tn5 insertion mutants of B. cepacia H111 that are defective in biofilm formation on a polystyrene surface. We show that the screening procedure used in this study is biased towards mutants defective in the late stages of biofilm development. A detailed quantitative analysis of the biofilm structures formed by wild-type and mutant strains revealed that the isolated mutants are impaired in their abilities to develop a typical three-dimensional biofilm structure. Molecular investigations showed that the genes required for biofilm maturation fall into several classes: (i). genes encoding for surface proteins; (ii). genes involved in the biogenesis and maintenance of an integral outer membrane; and (iii). genes encoding regulatory factors. It is shown that three of the regulatory mutants produce greatly reduced amounts of N-octanoylhomoserine lactone (C8-HSL). This compound serves as the major signal molecule of the cep quorum-sensing system. As this density-dependent regulatory system is involved in the regulation of biofilm maturation, we investigated the interplay between the three regulatory genes and the quorum-sensing cascade. The results of these investigations show that the identified genes encode for regulatory elements that are positioned upstream of the cep system, indicating that the quorum-sensing system of B. cepacia is a major checkpoint for biofilm formation.

Structure of the periplasmic domain of Pseudomonas aeruginosa TolA: evidence for an evolutionary relationship with the TonB transporter protein

The crystal structure of the C-terminal domain III of Pseudomonas aeruginosa TolA has been determined at 1.9 A resolution. The fold is similar to that of the corresponding domain of Escherichia coli TolA, despite the limited amino acid sequence identity of the two proteins (20%). A pattern was discerned that conserves the fold of domain III within the wider TolA family and, moreover, reveals a relationship between TolA domain III and the C-terminal domain of the TonB transporter proteins. We propose that the TolA and TonB C-terminal domains have a common evolutionary origin and are related by means of domain swapping, with interesting mechanistic implications. We have also determined the overall shape of the didomain, domains II + III, of P.aeruginosa TolA by solution X-ray scattering. The molecule is monomeric-its elongated, stalk shape can accommodate the crystal structure of domain III at one end, and an elongated helical bundle within the portion corresponding to domain II. Based on these data, a model for the periplasmic domains of P.aeruginosa TolA is presented that may explain the inferred allosteric properties of members of the TolA family. The mechanisms of TolA-mediated entry of bateriophages in P.aeruginosa and E.coli are likely to be similar.