Heterogeneity of lipopolysaccharides from Pseudomonas aeruginosa: analysis of lipopolysaccharide chain length

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.

Antibacterial Activity of Non-Cytotoxic, Amino Acid-Modified Polycationic Dendrimers against Pseudomonas aeruginosa and Other Non-Fermenting Gram-Negative Bacteria

Due to the rapid increase of antimicrobial resistance with ensuring therapeutic failures, the purpose of this study was to identify novel synthetic molecules as alternatives to conventional available, but presently ineffective antibiotics. Variously structured cationic dendrimers previously reported have provided promising outcomes. However, the problem of their cytotoxicity towards eukaryotic cells has not been completely overcome. We have now investigated the antibacterial activities of three not cytotoxic cationic dendrimers (G5Ds: G5H, G5K, and G5HK) against several multidrug-resistant (MDR) clinical strains. All G5Ds displayed remarkable activity against MDR non-fermenting Gram-negative species such as P. aeruginosa, S. maltophilia, and A. baumannii (MICs = 0.5-33.2 µM). In particular, very low MIC values (0.5-2.1 µM) were observed for G5K, which proved to be more active than the potent colistin (2.1 versus 3.19 µM) against P. aeruginosa. Concerning its mechanism of action, in time-killing and turbidimetric studies, G5K displayed a rapid non-lytic bactericidal activity. Considering the absence of cytotoxicity of these new compounds and their potency, comparable or even higher than that provided by the dendrimers previously reported, G5Ds may be proposed as promising novel antibacterial agents capable of overcoming the alarming resistance rates of several nosocomial non-fermenting Gram-negative pathogens.

The Role of Pseudomonas aeruginosa Lipopolysaccharide in Bacterial Pathogenesis and Physiology

The major constituent of the outer membrane of Gram-negative bacteria is lipopolysaccharide (LPS), which is comprised of lipid A, core oligosaccharide, and O antigen, which is a long polysaccharide chain extending into the extracellular environment. Due to the localization of LPS, it is a key molecule on the bacterial cell wall that is recognized by the host to deploy an immune defence in order to neutralize invading pathogens. However, LPS also promotes bacterial survival in a host environment by protecting the bacteria from these threats. This review explores the relationship between the different LPS glycoforms of the opportunistic pathogen Pseudomonas aeruginosa and the ability of this organism to cause persistent infections, especially in the genetic disease cystic fibrosis. We also discuss the role of LPS in facilitating biofilm formation, antibiotic resistance, and how LPS may be targeted by new antimicrobial therapies.

The O-specific polysaccharide lyase from the phage LKA1 tailspike reduces Pseudomonas virulence

Pseudomonas phage LKA1 of the subfamily Autographivirinae encodes a tailspike protein (LKA1gp49) which binds and cleaves B-band LPS (O-specific antigen, OSA) of Pseudomonas aeruginosa PAO1. The crystal structure of LKA1gp49 catalytic domain consists of a beta-helix, an insertion domain and a C-terminal discoidin-like domain. The putative substrate binding and processing site is located on the face of the beta-helix whereas the C-terminal domain is likely involved in carbohydrates binding. NMR spectroscopy and mass spectrometry analyses of degraded LPS (OSA) fragments show an O5 serotype-specific polysaccharide lyase specificity. LKA1gp49 reduces virulence in an in vivo Galleria mellonella infection model and sensitizes P. aeruginosa to serum complement activity. This enzyme causes biofilm degradation and does not affect the activity of ciprofloxacin and gentamicin. This is the first comprehensive report on LPS-degrading lyase derived from a Pseudomonas phage. Biological properties reveal a potential towards its applications in antimicrobial design and as a microbiological or biotechnological tool.

Review: Conserved and variable structural features in the lipopolysaccharide of Pseudomonas aeruginosa

The review is devoted to recent progress in the structural elucidation of the lipopolysaccharide of the bacterium Pseudomonas aeruginosa, including O-antigen biological repeats, core oligosaccharide, and lipid A. Data on biosynthesis, genetics and serology of the lipopolysaccharide isolated from various P. aeruginosa O-serogroups are discussed in relation to the chemical structures.

Discovery, characterization and in vivo activity of pyocin SD2, a protein antibiotic from Pseudomonas aeruginosa

Increasing rates of antibiotic resistance among Gram-negative pathogens such as Pseudomonas aeruginosa means alternative approaches to antibiotic development are urgently required. Pyocins, produced by P. aeruginosa for intraspecies competition, are highly potent protein antibiotics known to actively translocate across the outer membrane of P. aeruginosa. Understanding and exploiting the mechanisms by which pyocins target, penetrate and kill P. aeruginosa is a promising approach to antibiotic development. In this work we show the therapeutic potential of a newly identified tRNase pyocin, pyocin SD2, by demonstrating its activity in vivo in a murine model of P. aeruginosa lung infection. In addition, we propose a mechanism of cell targeting and translocation for pyocin SD2 across the P. aeruginosa outer membrane. Pyocin SD2 is concentrated at the cell surface, via binding to the common polysaccharide antigen (CPA) of P. aeruginosa lipopolysaccharide (LPS), from where it can efficiently locate its outer membrane receptor FpvAI. This strategy of utilizing both the CPA and a protein receptor for cell targeting is common among pyocins as we show that pyocins S2, S5 and SD3 also bind to the CPA. Additional data indicate a key role for an unstructured N-terminal region of pyocin SD2 in the subsequent translocation of the pyocin into the cell. These results greatly improve our understanding of how pyocins target and translocate across the outer membrane of P. aeruginosa. This knowledge could be useful for the development of novel anti-pseudomonal therapeutics and will also support the development of pyocin SD2 as a therapeutic in its own right.

Synthesis of bacterial polysaccharides via the Wzx/Wzy-dependent pathway

The surfaces of bacteria mediate a multitude of functions in the environment and in an infected host, including adhesion to both biotic and abiotic substrata, motility, immune system interaction and (or) activation, biofilm formation, and cell–cell communication, with many of these features directly influenced by cell-surface glycans. In both Gram-negative and Gram-positive bacteria, the majority of cell-surface polysaccharides are produced via the Wzx/Wzy-dependent assembly pathway; these glycans include heteropolymeric O-antigen, enterobacterial common antigen, exopolysaccharide, spore coat, and capsule in diverse bacteria. The key components of this assembly pathway are the integral inner membrane Wzx flippase, Wzy polymerase, and Wzz chain-length regulator proteins, which until recently have resisted detailed structural and functional characterization. In this review, we have provided a comprehensive synthesis of the latest structural and mechanistic data for each protein, as well as an examination of substrate specificity for each assembly step and complex formation between the constituent proteins. To complement the unprecedented explosion of genomic-sequencing data for bacteria, we have also highlighted both classical and state-of-the-art methods by which encoded Wzx, Wzy, and Wzz proteins can be reliably identified and annotated, using the model Gram-negative bacterium Pseudomonas aeruginosa as an example data set. Lastly, we outline future avenues of research, with the aim of stimulating researchers to take the next steps in investigating the function of, and interplay between, the constituents of this widespread assembly scheme.

Influence of Water Hardness on Silver Ion and Silver Nanoparticle Fate and Toxicity Toward Nitrosomonas europaea

This study investigated the influence of water hardness (Mg²⁺ and Ca²⁺) on the fate and toxicity of 20 nm citrate silver nanoparticles (AgNPs) and Ag⁺ toward Nitrosomonas europaea, a model ammonia-oxidizing bacterium. Nitrification inhibition of N. europaea by 1 ppm AgNPs and 0.5 ppm Ag⁺ was reduced from 80% and 83%, respectively, in the absence of Mg²⁺ to 2% and 33%, respectively, in the presence of 730 μM Mg²⁺. Introduction of Mg²⁺ resulted in the rapid aggregation of the AgNP suspensions and reduced the 3 h Ag⁺ dissolution rates from 30%, in the absence of Mg²⁺, to 9%, in the presence of 730 μM Mg²⁺. Reduced AgNP dissolution rates resulted in decreased concentrations of silver that were found adsorbed to N. europaea cells. Increasing AgNP concentrations in the presence of Mg²⁺ increased the observed inhibition of nitrification, but was always less than what was observed in the absence of Mg²⁺. The presence of Mg²⁺ also reduced the adsorption of Ag⁺ to cells, possibly due to multiple mechanisms, including a reduction in the negative surface charge of the N. europaea membrane and a competition between Mg²⁺ and Ag⁺ for membrane binding sites and transport into the cells. Ca²⁺ demonstrated similar protection mechanisms, as Ag⁺ toxicity was reduced and AgNP suspensions aggregated and decreased their dissolution rates. These results indicate that the toxicity of Ag⁺ and AgNPs to nitrifying bacteria in wastewater treatment would be less pronounced in systems with hard water.

A novel approach for purification and selective capture of membrane vesicles of the periodontopathic bacterium, Porphyromonas gingivalis: membrane vesicles bind to magnetic beads coated with epoxy groups in a noncovalent, species-specific manner

Membrane vesicles (MVs) of Porphyromonas gingivalis are regarded as an offensive weapon of the bacterium, leading to tissue deterioration in periodontal disease. Therefore, isolation of highly purified MVs is indispensable to better understand the pathophysiological role of MVs in the progression of periodontitis. MVs are generally isolated by a conventional method based on ultracentrifugation of the bacterial culture supernatant. However, the resulting MVs are often contaminated with co-precipitating bacterial appendages sheared from the live bacteria. Here, we report an intriguing property of P. gingivalis MVs–their ability to bind superparamagnetic beads coated with epoxy groups (SB-Epoxy). Analysis of fractions collected during the purification revealed that all MVs of five tested P. gingivalis stains bound to SB-Epoxy. In contrast, free fimbriae in the crude MV preparation did not bind to the SB-Epoxy. The SB-Epoxy-bound MVs were easily dissociated from the SB-Epoxy using a mild denaturation buffer. These results suggest that the surface chemistry conferred by epoxy on the beads is responsible for the binding, which is mediated by noncovalent bonds. Both the structural integrity and purity of the isolated MVs were confirmed by electron microscopy. The isolated MVs also caused cell detachment from culture dishes at a physiologically relevant concentration. Assays of competitive binding between the SB-Epoxy and mixtures of MVs from five bacterial species demonstrated that only P. gingivalis MVs could be selectively eliminated from the mixtures. We suggest that this novel approach enables efficient purification and selective elimination of P. gingivalis MVs.

Lectin-like bacteriocins from Pseudomonas spp. utilise D-rhamnose containing lipopolysaccharide as a cellular receptor

Lectin-like bacteriocins consist of tandem monocot mannose-binding domains and display a genus-specific killing activity. Here we show that pyocin L1, a novel member of this family from Pseudomonas aeruginosa, targets susceptible strains of this species through recognition of the common polysaccharide antigen (CPA) of P. aeruginosa lipopolysaccharide that is predominantly a homopolymer of d-rhamnose. Structural and biophysical analyses show that recognition of CPA occurs through the C-terminal carbohydrate-binding domain of pyocin L1 and that this interaction is a prerequisite for bactericidal activity. Further to this, we show that the previously described lectin-like bacteriocin putidacin L1 shows a similar carbohydrate-binding specificity, indicating that oligosaccharides containing d-rhamnose and not d-mannose, as was previously thought, are the physiologically relevant ligands for this group of bacteriocins. The widespread inclusion of d-rhamnose in the lipopolysaccharide of members of the genus Pseudomonas explains the unusual genus-specific activity of the lectin-like bacteriocins.

Due to rapidly increasing rates of antibiotic resistance observed among Gram-negative pathogens, such as Pseudomonas aeruginosa, there is an urgent requirement for novel approaches to the treatment of bacterial infections. Lectin-like bacteriocins are highly potent protein antibiotics that display an unusual ability to kill a select group of bacteria within a specific genus. In this work, we show how the lectin-like protein antibiotic, pyocin L1, can kill Pseudomonas aeruginosa with extraordinary potency through specific binding to the common polysaccharide antigen (CPA) of P. aeruginosa lipopolysaccharide. The CPA is predominantly a homopolymer of the sugar d-rhamnose that although generally rare in nature is found frequently as a component of the lipopolysaccharide of members of the genus Pseudomonas. The targeting of d-rhamnose containing polysaccharides by pyocin L1 and a related lectin-like protein antibiotic, putidacin L1, explains the unusual genus- specific killing activity of the lectin-like bacteriocins. As we learn more about the link between changes to the microbiome and a range of chronic diseases there is a growing realisation that the ability to target specific bacterial pathogens while maintaining the normal gut flora is a desirable property for next generation antibiotics.

Membrane vesicle release in bacteria, eukaryotes, and archaea: a conserved yet underappreciated aspect of microbial life

Interaction of microbes with their environment depends on features of the dynamic microbial surface throughout cell growth and division. Surface modifications, whether used to acquire nutrients, defend against other microbes, or resist the pressures of a host immune system, facilitate adaptation to unique surroundings. The release of bioactive membrane vesicles (MVs) from the cell surface is conserved across microbial life, in bacteria, archaea, fungi, and parasites. MV production occurs not only in vitro but also in vivo during infection, underscoring the influence of these surface organelles in microbial physiology and pathogenesis through delivery of enzymes, toxins, communication signals, and antigens recognized by the innate and adaptive immune systems. Derived from a variety of organisms that span kingdoms of life and called by several names (membrane vesicles, outer membrane vesicles [OMVs], exosomes, shedding microvesicles, etc.), the conserved functions and mechanistic strategies of MV release are similar, including the use of ESCRT proteins and ESCRT protein homologues to facilitate these processes in archaea and eukaryotic microbes. Although forms of MV release by different organisms share similar visual, mechanistic, and functional features, there has been little comparison across microbial life. This underappreciated conservation of vesicle release, and the resulting functional impact throughout the tree of life, explored in this review, stresses the importance of vesicle-mediated processes throughout biology.

Genetic and Functional Diversity of Pseudomonas aeruginosa Lipopolysaccharide

Lipopolysccharide (LPS) is an integral component of the Pseudomonas aeruginosa cell envelope, occupying the outer leaflet of the outer membrane in this Gram-negative opportunistic pathogen. It is important for bacterium-host interactions and has been shown to be a major virulence factor for this organism. Structurally, P. aeruginosa LPS is composed of three domains, namely, lipid A, core oligosaccharide, and the distal O antigen (O-Ag). Most P. aeruginosa strains produce two distinct forms of O-Ag, one a homopolymer of D-rhamnose that is a common polysaccharide antigen (CPA, formerly termed A band), and the other a heteropolymer of three to five distinct (and often unique dideoxy) sugars in its repeat units, known as O-specific antigen (OSA, formerly termed B band). Compositional differences in the O units among the OSA from different strains form the basis of the International Antigenic Typing Scheme for classification via serotyping of different strains of P. aeruginosa. The focus of this review is to provide state-of-the-art knowledge on the genetic and resultant functional diversity of LPS produced by P. aeruginosa. The underlying factors contributing to this diversity will be thoroughly discussed and presented in the context of its contributions to host-pathogen interactions and the control/prevention of infection.

Purification and characterization of lipopolysaccharides

Lipopolysaccharides are the major components on the surface of most Gram-negative bacteria, and recognized by immune cells as a pathogen-associated molecule. They can cause severe diseases like sepsis and therefore known as endotoxins. Lipopolysaccharide consists of lipid A, core oligosaccharide and O-antigen repeats. Lipid A is responsible for the major bioactivity of endotoxin. Because of their specific structure and amphipathic property, purification and analysis of lipopolysaccharides are difficult. In this chapter, we summarize the available approaches for extraction, purification and analysis of lipopolysaccharides.

Salt modulates bacterial hydrophobicity and charge properties influencing adhesion of Pseudomonas aeruginosa (PA01) in aqueous suspensions

The influence on cell hydrophobicity of differential extension with ionic strength of lipopolysaccharide molecules (LPS), which exist as charged and uncharged polymers at the surface of the gram-negative bacterium Pseudomonas aeruginosa (PA01), has been investigated. Attenuated total reflection infrared (ATR-IR) spectral absorptions from a single layer of cells adsorbed to ZnSe increased in intensity with increasing NaCl concentration up to 0.1 mol L(-1). Dynamic contact angle measurements (Wilhelmy plate tensiometry) made with a ZnSe plate having an adsorbed cell layer and the adherence of the cells to hexadecane suggest that PA01 cells were most hydrophobic in contact with 0.1 mol L(-1) NaCl solutions. These data indicate a charge screening induced compression of the charged LPS polymers decreasing the cell-surface approach distance and increasing the cell hydrophobicity due to the greater surface predominance of the uncharged LPS polymers. Interestingly, adsorbed cell layers in 0.3 mol L(-1) NaCl had a lower IR absorption intensity, and PA01 cells suspended in 0.3 mol L(-1) were found to be more hydrophilic, indicating that other factors influence the cell-surface approach distance and hydrophobicity. The examination of cell electrophoretic mobility variation with NaCl concentration suggests that the compression of charged polysaccharides increases the polysaccharide charge density and may also reduce the flow of liquid through the polysaccharide layer affecting the effective potential at the interface, the cell hydrophobicity, and the cell-surface approach distance.

Review: Lipopolysaccharide biosynthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa causes serious nosocomial infections, and an important virulence factor produced by this organism is lipopolysaccharide (LPS). This review summarizes knowledge about biosynthesis of all three structural domains of LPS - lipid A, core oligosaccharide, and O polysaccharides. In addition, based on similarities with other bacterial species, this review proposes new hypothetical pathways for unstudied steps in the biosynthesis of P. aeruginosa LPS. Lipid A biosynthesis is discussed in relation to Escherichia coli and Salmonella, and the biosyntheses of core sugar precursors and core oligosaccharide are summarised. Pseudomonas aeruginosa attaches a Common Polysaccharide Antigen and O-Specific Antigen polysaccharides to lipid A-core. Both forms of O polysaccharide are discussed with respect to their independent synthesis mechanisms. Recent advances in understanding O-polysaccharide biosynthesis since the last major review on this subject, published nearly a decade ago, are highlighted. Since P. aeruginosa O polysaccharides contain unusual sugars, sugar-nucleotide biosynthesis pathways are reviewed in detail. Knowledge derived from detailed studies in the O5, O6 and O11 serotypes is applied to predict biosynthesis pathways of sugars in poorly-studied serotypes, especially O1, O4, and O13/O14. Although further work is required, a full understanding of LPS biosynthesis in P. aeruginosa is almost within reach.

Genetic and biochemical analyses of the Pseudomonas aeruginosa Psl exopolysaccharide reveal overlapping roles for polysaccharide synthesis enzymes in Psl and LPS production

Exopolysaccharides contribute significantly to attachment and biofilm formation in the opportunisitc pathogen Pseudomonas aeruginosa. The Psl polysaccharide, which is synthesized by the polysaccharide synthesis locus (psl), is required for biofilm formation in non-mucoid strains that do not rely on alginate as the principal biofilm polysaccharide. In-frame deletion and complementation studies of individual psl genes revealed that 11 psl genes, pslACDEFGHIJKL, are required for Psl production and surface attachment. We also present the first structural analysis of the psl-dependent polysaccharide, which consists of a repeating pentasaccharide containing d-mannose, d-glucose and l-rhamnose: [See text]. In addition, we identified the sugar nucleotide precursors involved in Psl generation and demonstrated the requirement for GDP-d-mannose, UDP-d-glucose and dTDP-l-rhamnose in Psl production and surface attachment. Finally, genetic analyses revealed that wbpW restored Psl production in a pslB mutant and pslB promoted A-band LPS synthesis in a wbpW mutant, indicating functional redundancy and overlapping roles for these two enzymes. The structural and genetic data presented here provide a basis for further investigation of the Psl proteins and potential roles for Psl in the biology and pathogenesis of P. aeruginosa.

Mechanisms of Cation Exchange by Pseudomonas aeruginosa PAO1 and PAO1 wbpL, a Strain with a Truncated Lipopolysaccharide

The ability of bacterial cells to sequester cations is well recognized, despite the fact that the specific binding sites and mechanistic details of the process are not well understood. To address these questions, the cation-exchange behavior of Pseudomonas aeruginosa PAO1 cells with a truncated lipopolysaccharide (LPS) (PAO1 wbpL) and cells further modified by growth in a magnesium-deficient medium (PAO1 wbpL - Mg(2+)) were compared with that of wild-type P. aeruginosa PAO1 cells. P. aeruginosa PAO1 cells had a negative surface charge (zeta potential) between pH 11 and 2.2, due to carboxylate groups present in the B-band LPS. The net charge on PAO1 wbpL cells was increasingly positive below pH 3.5, due to the influence of NH(3)(+) groups in the core LPS. The zeta potentials of these cells were also measured in Na(+), Ca(2+), and La(3+) electrolytes. Cells in the La(3+) electrolyte had a positive zeta potential at all pH values tested. Growing P. aeruginosa PAO1 wbpL in magnesium-deficient medium (PAO1 wbpL - Mg(2+)) resulted in an increase in its zeta potential in the pH range from 3.0 to 6.5. In cation-exchange experiments carried out at neutral pH with either P. aeruginosa PAO1 or PAO1 wbpL, the concentration of bound Ca(2+) was found to decrease as the pH was reduced from 7.0 to 3.5. At pH 3.5, the bound Mg(2+) concentration decreased sharply, revealing the activity of surface sites for cation exchange and their pH dependence. Infrared spectroscopy of attached biofilms suggested that carboxylate and phosphomonoester functional groups within the core LPS are involved in cation exchange.

Importance of LPS structure on protein interactions with Pseudomonas aeruginosa

Atomic force microscopy (AFM) was used to quantify the adhesion forces between Pseudomonas aeruginosa PAO1 and AK1401, and a representative model protein, bovine serum albumin (BSA). The two bacteria strains differ in terms of the structure of their lipopolysaccharide (LPS) layers. While PAO1 is the wild-type expressing a complete LPS and two types of saccharide units in the O-antigen (A(+) B(+)), the mutant AK1401 expresses only a single unit of the A-band saccharide (A(+) B(-)). The mean adhesion force (F(adh)) between BSA and AK1401 was 1.12 nN, compared to 0.40 nN for F(adh) between BSA and PAO1. In order to better understand the fundamental forces that would control bacterial-protein interactions at equilibrium conditions, we calculated interfacial free energies using the van Oss-Chaudhury-Good (VCG) thermodynamic modeling approach. The hydrogen bond strength was also calculated using a Poisson statistical analysis. AK1401 has a higher ability to participate in hydrogen bonding with BSA than does PAO1, which may be because the short A-band and absence of B-band polymer allowed the core oligosaccharides and lipid A regions to be more exposed and to participate in hydrogen and chemical bonding. Interactions between PAO1 and BSA were weak due to the dominance of neutral and hydrophilic sugars of the A-band polymer. These results show that bacterial interactions with protein-coated surfaces will depend on the types of bonds that can form between bacterial surface macromolecules and the protein. We suggest that strategies to prevent bacterial colonization of biomaterials can focus on inhibiting these bonds.

Calcium and magnesium cations enhance the adhesion of motile and nonmotile pseudomonas aeruginosa on alginate films

We investigated the impact of calcium and magnesium ions on the deposition kinetics of flagellated and nonflagellated Pseudomonas aeruginosa onto an alginate conditioning film in a radial stagnation point flow system. The bacterial deposition/adhesion behavior was related to structural changes of the alginate film in the presence of the divalent cations. Our results showed that adhesion of nonmotile bacteria was governed by cation bridging interactions between high-affinity sites at the bacterial surface and either clean or alginate-conditioned substrate surfaces. For motile bacteria, the adhesion onto clean quartz was governed by electrostatic interactions while adhesion onto alginate-conditioned quartz was dependent on the structure and viscoelastic properties of the alginate film in the presence of calcium or magnesium. We demonstrate that bacterial adhesion behavior is governed both by the effects of divalent cations on the surface properties of the bacteria and the substrate and by the type of specific interactions occurring between these two surfaces.

Functional characterization of MigA and WapR: putative rhamnosyltransferases involved in outer core oligosaccharide biosynthesis of Pseudomonas aeruginosa

Pseudomonas aeruginosa lipopolysaccharide (LPS) contains two glycoforms of core oligosaccharide (OS); one form is capped with O antigen through an alpha-1,3-linked L-rhamnose (L-Rha), while the other is uncapped and contains an alpha-1,6-linked L-Rha. Two genes in strain PAO1, wapR (PA5000) and migA (PA0705), encode putative glycosyltransferases associated with core biosynthesis. We propose that WapR and MigA are the rhamnosyltransferases responsible for the two linkages of L-Rha to the core. Knockout mutants with mutations in both genes were generated. The wapR mutant produced LPS lacking O antigen, and addition of wapR in trans complemented this defect. The migA mutant produced LPS with a truncated outer core and showed no reactivity to outer core-specific monoclonal antibody (MAb) 5C101. Complementation of this mutant with migA restored reactivity of the LPS to MAb 5C101. Interestingly, LPS from the complemented migA strain was not reactive to MAb 18-19 (specific for the core-plus-one O repeat). This was due to overexpression of MigA in the complemented strain that caused an increase in the proportion of the uncapped core OS, thereby decreasing the amount of the core-plus-one O repeat, indicating that MigA has a regulatory role. The structures of LPS from both mutants were elucidated using nuclear magnetic resonance spectroscopy and mass spectrometry. The capped core of the wapR mutant was found to be truncated and lacked alpha-1,3-L-Rha. In contrast, uncapped core OS from the migA mutant lacked alpha-1,6-L-Rha. These results provide evidence that WapR is the alpha-1,3-rhamnosyltransferase, while MigA is the alpha-1,6-rhamnosyltransferase.

Quantifying the adhesion and interaction forces between Pseudomonas aeruginosa and natural organic matter

Atomic force microscopy (AFM) was used to characterize interactions between natural organic matter (NOM), and glass or bacteria. Poly(methacrylic acid) (PMA), soil humic Acid (SHA), and Suwannee River humic Acid (SRHA), were adsorbed to silica AFM probes. Adhesion forces (Fadh) for the interaction of organic-probes and glass slides correlated with organic molecular weight (MW), but not with radius of the organic aggregate (R), charge density (Q), or zeta potential (zeta). Two Pseudomonas aeruginosa strains with different lipopolysaccharides (LPS) were chosen: PAO1 (A+B+), whose LPS have common antigen (A-band) + O-antigen (B-band); and mutant AK1401 (A+B-). Fadh between bacteria and organics correlated with organic MW, R, and Q, but not zeta. PAO1 had lower Fadh with silica than NOM, which was attributed to negative charges from the B-band polymers causing electrostatic repulsion. AK1401 adhered stronger to silica than to the organics, perhaps because the absence of the B-band exposed underlying positively charged proteins. DLVO calculations could not explain the differences in the two bacteria or predict qualitative or quantitative trends in interaction forces in these systems. Molecular-level information from AFM studies can bring us closer to understanding the complex nature of bacterial-NOM interactions.

Atomic force microscopy study of the effect of lipopolysaccharides and extracellular polymers on adhesion of Pseudomonas aeruginosa

The roles of lipopolysaccharides (LPS) and extracellular polymers (ECP) on the adhesion of Pseudomonas aeruginosa PAO1 (expresses the A-band and B-band of O antigen) and AK1401 (expresses the A-band but not the B-band) to silicon were investigated with atomic force microscopy (AFM) and related to biopolymer physical properties. Measurement of macroscopic properties showed that strain AK1401 is more negatively charged and slightly more hydrophobic than strain PAO1 is. Microscopic AFM investigations of individual bacteria showed differences in how the biopolymers interacted with silicon. PAO1 showed larger decay lengths in AFM approach cycles, suggesting that the longer polymers on PAO1 caused greater steric repulsion with the AFM tip. For both bacterial strains, the long-range interactions we observed (hundreds of nanometers) were inconsistent with the small sizes of LPS, suggesting that they were also influenced by ECP, especially polysaccharides. The AFM retraction profiles provide information on the adhesion strength of the biopolymers to silicon (F(adh)). For AK1401, the adhesion forces were only slightly lower (F(adh) = 0.51 nN compared to 0.56 nN for PAO1), but the adhesion events were concentrated over shorter distances. More than 90% of adhesion events for AK1401 were at distances of <600 nm, while >50% of adhesion events for PAO1 were at distances of >600 nm. The sizes of the observed molecules suggest that the adhesion of P. aeruginosa to silicon was controlled by ECP, in addition to LPS. Steric and electrostatic forces each contributed to the interfacial interactions between P. aeruginosa and the silicon surface.

Molecular origin of two polysaccharides of Campylobacter jejuni 81116

The nature of the polysaccharide molecules of the human enteric pathogen Campylobacter jejuni has been the subject of debate. Previously, C. jejuni 81116 was shown to contain two different polysaccharides, one acidic (polysaccharide A) and the other neutral (polysaccharide B), occurring in a 3 : 1 ratio, respectively. The aim of this study was to determine the molecular origin of these polysaccharides. Using a combination of centrifugation, gel permeation chromatography, chemical assays, and (1)H-NMR analysis, polysaccharide B was shown to be derived from lipopolysaccharide and polysaccharide A from capsular polysaccharide. Thus, C. jejuni 81116 produces both lipopolysaccharide-like molecules and capsular polysaccharide.

Construction of a mini-Tn5-luxCDABE mutant library in Pseudomonas aeruginosa PAO1: a tool for identifying differentially regulated genes

Pseudomonas aeruginosa is a major cause of nosocomial (hospital-derived) infections, is the predominant pathogen in chronic cystic fibrosis lung infections, and remains difficult to treat due to its high intrinsic antibiotic resistance. The completion of the P. aeruginosa PAO1 genome sequence provides the opportunity for genome-wide studies to increase our understanding of the pathogenesis and biology of this important pathogen. In this report, we describe the construction of a mini-Tn5-luxCDABE mutant library and a high-throughput inverse PCR method to amplify DNA flanking the site of insertion for sequencing and insertion site mapping. In addition to producing polar knockout mutations in nonessential genes, the promoterless luxCDABE reporter present in the transposon serves as a real-time reporter of gene expression for the inactivated gene. A total of 2519 transposon insertion sites were mapped, 77% of which were nonredundant insertions. Of the insertions within an ORF, -55% of total and unique insertion sites were transcriptional luxCDABE fusions. A bias toward low insertion-site density in the genome region that surrounds the predicted terminus of replication was observed. To demonstrate the utility of chromosomal lux fusions, we performed extensive regulatory screens to identify genes that were differentially regulated under magnesium or phosphate limitation. This approach led to the discovery of many known and novel genes necessary for these environmental adaptations, including genes involved in resistance to cationic antimicrobial peptides. This dual-purpose mutant library allows for functional and regulation studies and will serve as a resource for the research community to further our understanding of P. aeruginosa biology.

The galU Gene of Pseudomonas aeruginosa is required for corneal infection and efficient systemic spread following pneumonia but not for infection confined to the lung

Acute pneumonias and corneal infections due to Pseudomonas aeruginosa are typically caused by lipopolysaccharide (LPS)-smooth strains. In cystic fibrosis patients, however, LPS-rough strains of P. aeruginosa, which lack O antigen, can survive in the lung and cause chronic infection. It is not clear whether an LPS-rough phenotype affects cytotoxicity related to the type III secretion system (TTSS). We previously reported that interruption of the galU gene in P. aeruginosa results in production of a rough LPS and truncated LPS core. Here we evaluated the role of the galU gene in the pathogenesis of murine lung and eye infections and in cytotoxicity due to the TTSS effector ExoU. We studied galU mutants of strain PAO1, of its cytotoxic variant expressing ExoU from a plasmid, and of the inherently cytotoxic strain PA103. The galU mutants were more serum sensitive than the parental strains but remained cytotoxic in vitro. In a corneal infection model, the galU mutants were significantly attenuated. In an acute pneumonia model, the 50% lethal doses of the galU mutants were higher than those of the corresponding wild-type strains, yet these mutants could cause mortality and severe pneumonia, as judged by histology, even with minimal systemic spread. These findings suggest that the galU gene is required for corneal infection and for efficient systemic spread following lung infection but is not required for infection confined to the lung. Host defenses in the lung appear to be insufficient to control infection with LPS-rough P. aeruginosa when local bacterial levels are high.

[Recovery method of serotypable character in non serotypable pseudomonas aeruginosa strains]

Serotyping is one of the most used techniques for typing Pseudomonas aeruginosa strains. During chronic infections, and especially in cystic fibrosis, the decrease of lipopolysaccharide production is responsible for difficulties in determining O antigens. The possibility of serotyping can be simply restored by using a primary culture broth containing amikacin (1/6 of the strain MIC for this antibiotic); this is due to the ability of this antibiotic to inhibit alginate production. This technique allowed us to determine the serotype of 108 non-serotypable strains of P. aeruginosa isolated in 14 different hospitals. Among these isolates, serotype O:1 and O:13, had a high prevalence; the origin is a deficiency in D-glucose and L-rhamnose, required for the synthesis of lipopolysaccharide. In contrast, these sugars are not present in lipopolysaccharide of O:12, and these strains are always serotypable. The main protein is Alg C; this bifunctional enzyme is required in the exopolysaccharide and lipopolysaccharide production, according stress conditions in the bacterial-cells' environment. Determination of the serotype, as Antibiogram, is essential for genotypic inquiries.

Cryo-transmission electron microscopy of frozen-hydrated sections of Escherichia coli and Pseudomonas aeruginosa

High-pressure freezing of Escherichia coli K-12 and Pseudomonas aeruginosa PAO1 in the presence of cryoprotectants provided consistent vitrification of cells so that frozen-hydrated sections could be cut, providing approximately 2-nm resolution of structure. The size and shape of the bacteria, as well as their surface and cytoplasmic constituents, were nicely preserved and compared well with other published high-resolution techniques. Cells possessed a rich cytoplasm containing a diffuse dispersion of ribosomes and genetic material. Close examination of cells revealed that the periplasmic space was compressed during cryosectioning, a finding which provided supporting evidence that this space is filled by a compressible gel. Since the outer membrane and peptidoglycan layer are bonded together via lipoproteins, the space between them (although still part of the periplasmic space) was not as compacted. Even when this cryosectioning compression was taken into account, there was still substantial variability in the width of the periplasmic space. It is possible that the protoplast has some capacity to float freely within the periplasm.

Fatty acids of lipid fractions in extracellular polymeric substances of activated sludge flocs

Phospholipid (PL), glycolipid (GL), and neutral lipid (NL) FA, and the lipopolysaccharide 2- and 3-hydroxy (LPS 2-OH and 3-OH) FA of activated sludges and extracted extracellular polymeric substances (EPS) were determined on samples collected from two wastewater treatment plants. EPS extracted from sludges by means of sonication and cation exchange contained proteins (43.4%), humic-like substances (11.5%), nucleic acids (10.9%), carbohydrates (9.9%), and lipid-bound FA (1.8%). The lipids associated with EPS were composed of GL, PL, NL, and LPS acids in proportions of 61, 21, 16, and 2%, respectively. The profiles of lipid-bound FA in activated sludges and EPS were similar (around 85 separate FA were identified). The FA signatures observed can be attributed to the likely presence of yeasts, fungi, sulfate-reducing bacteria, gram-positive and gram-negative bacteria, and, in lesser quantities, mycobacteria. Comparison of data from the dates of sampling (January and September) showed that there were more unsaturated PLFA in the EPS extracted from the activated sludges sampled in January. This observation could be partly related to microorganism adaptation to temperature variations. The comparison between two wastewater treatment plants showed that the FA profiles were similar, although differences in microbial community structure were also seen. Most of the FA in sludges had an even number of carbons.

Pseudomonas aeruginosa O-antigen chain length is determined before ligation to lipid A core

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that infects immunocompromised patients and trauma victims and causes fatal lung infections in people with cystic fibrosis. This microorganism produces a number of virulence factors, one of which is lipopolysaccharide (LPS), which has been shown to mediate many biological effects including resistance to serum killing and phagocytosis. These biological activities have been correlated to the length of the O-polysaccharide and its distribution on the outer membrane. Wzz is responsible for regulation of the size distribution of the O-antigen. Wzz has been found to participate solely in the Wzy-dependent pathway for LPS biosynthesis, which produces heteropolymeric O-polysaccharide such as the B-band LPS of P. aeruginosa. Our laboratory has previously reported characterization of a Wzz protein encoded in the B-band O-antigen biosynthesis cluster of PAO1. The availability of the genome sequence of P. aeruginosa PAO1 has made it possible to identify a second functional Wzz protein (PA0938, Wzz2). Gene replacement was used to generate an unmarked wzz2delta knock-out and a wzz2delta/wzz1::Gm double knock-out. As expected, the wzz2delta strain produced LPS with modal length imparted by Wzz1, and the wzz2delta/wzz1::Gm strain produced LPS O-antigen with a non-modal (random) length. Both wzz1 and wzz2 from P. aeruginosa PAO1 were cloned and expressed with an N-terminal His6 tag. His6-Wzz1 and His6-Wzz2 were purified to near homogeneity by immobilized metal affinity chromatography (IMAC). These preparations were used to develop specific polyclonal antibodies against each of the proteins. In vivo protein cross-linking followed by Western immunoblotting indicated that Wzz1 forms dimers whereas Wzz2 forms octamers. By generation of a wzz2delta/rmlC double mutant and analysis of the LPS, we have made the novel observation that polymerization of modal chain length-distributed O-antigen occurred before ligation to the lipid A core. We have shown an association between the Wzz proteins and O-antigen polymer chains using immunoprecipitation with anti-O5 O-antigen monoclonal antibody MF15-4. Both Wzz1 and Wzz2 could be co-precipitated with O5 polymer.

O-antigen structural variation: mechanisms and possible roles in animal/plant-microbe interactions

Current data from bacterial pathogens of animals and from bacterial symbionts of plants support some of the more general proposed functions for lipopolysaccharides (LPS) and underline the importance of LPS structural versatility and adaptability. Most of the structural heterogeneity of LPS molecules is found in the O-antigen polysaccharide. In this review, the role and mechanisms of this striking flexibility in molecular structure of the O-antigen in bacterial pathogens and symbionts are illustrated by some recent findings. The variation in O-antigen that gives rise to an enormous structural diversity of O-antigens lies in the sugar composition and the linkages between monosaccharides. The chemical composition and structure of the O-antigen is strain-specific (interstrain LPS heterogeneity) but can also vary within one bacterial strain (intrastrain LPS heterogeneity). Both LPS heterogeneities can be achieved through variations at different levels. First of all, O-polysaccharides can be modified non-stoichiometrically with sugar moieties, such as glucosyl and fucosyl residues. The addition of non-carbohydrate substituents, i.e. acetyl or methyl groups, to the O-antigen can also occur with regularity, but in most cases these modifications are again non-stoichiometric. Understanding LPS structural variation in bacterial pathogens is important because several studies have indicated that the composition or size of the O-antigen might be a reliable indicator of virulence potential and that these important features often differ within the same bacterial strain. In general, O-antigen modifications seem to play an important role at several (at least two) stages of the infection process, including the colonization (adherence) step and the ability to bypass or overcome host defense mechanisms. There are many reports of modifications of O-antigen in bacterial pathogens, resulting either from altered gene expression, from lysogenic conversion or from lateral gene transfer followed by recombination. In most cases, the mechanisms underlying these changes have not been resolved. However, in recent studies some progress in understanding has been made. Changes in O-antigen structure mediated by lateral gene transfer, O-antigen conversion and phase variation, including fucosylation, glucosylation, acetylation and changes in O-antigen size, will be discussed. In addition to the observed LPS heterogeneity in bacterial pathogens, the structure of LPS is also altered in bacterial symbionts in response to signals from the plant during symbiosis. It appears to be part of a molecular communication between bacterium and host plant. Experiments ex planta suggest that the bacterium in the rhizosphere prepares its LPS for its roles in symbiosis by refining the LPS structure in response to seed and root compounds and the lower pH at the root surface. Moreover, modifications in LPS induced by conditions associated with infection are another indication that specific structures are important. Also during the differentiation from bacterium to bacteroid, the LPS of Rhizobium undergoes changes in the composition of the O-antigen, presumably in response to the change of environment. Recent findings suggest that, during symbiotic bacteroid development, reduced oxygen tension induces structural modifications in LPS that cause a switch from predominantly hydrophilic to predominantly hydrophobic molecular forms. However, the genetic mechanisms by which the LPS epitope changes are regulated remain unclear. Finally, the possible roles of O-antigen variations in symbiosis will be discussed.

Epithelial cell contact-induced alterations in Salmonella enterica serovar Typhi lipopolysaccharide are critical for bacterial internalization

The invasion of Pseudomonas aeruginosa and Salmonella enterica serovar Typhi into epithelial cells depends on the cystic fibrosis transmembrane conductance regulator (CFTR) protein as an epithelial receptor. In the case of P. aeruginosa, the bacterial ligand for CFTR is the outer core oligosaccharide portion of the lipopolysaccharide (LPS). To determine whether serovar Typhi LPS is also a bacterial ligand mediating internalization, we used both P. aeruginosa and serovar Typhi LPS as a competitive inhibitor of serovar Typhi invasion into the epithelial cell line T84. P. aeruginosa LPS containing a complete core efficiently inhibited serovar Typhi invasion. However, neither killed wild-type Typhi cells nor purified LPS were effective inhibitors. LPS from mutant Typhi strains defective in O side-chain synthesis, but with an apparently normal core, was capable of inhibiting invasion, but LPS obtained from a deeper rough mutant strain with alterations in fast-migrating core oligosaccharide failed to inhibit invasion. Lastly, exposure of wild-type serovar Typhi to T84 cultures before heat killing resulted in a structural alteration in its LPS that allowed the heat-killed cells to inhibit invasion of wild-type serovar Typhi. These data indicate that the serovar Typhi LPS core, like the P. aeruginosa LPS core, is a ligand mediating internalization of bacteria by epithelial cells, and that exposure of this ligand on wild-type Typhi is induced by the bacteria's interaction with host cells.

Involvement of the rml locus in core oligosaccharide and O polysaccharide assembly in Pseudomonas aeruginosa

L-Rhamnose (L-Rha) is a component of the lipopolysaccharide (LPS) core, several O antigen polysaccharides, and the cell surface surfactant rhamnolipid of Pseudomonas aeruginosa. In this study, four contiguous genes (rmlBDAC) responsible for the synthesis of dTDP-L-Rha in P. aeruginosa have been cloned and characterized. Non-polar chromosomal rmlC mutants were generated in P. aeruginosa strains PAO1 (serotype O5) and PAK (serotype O6) and LPS extracted from the mutants was analysed by SDS-PAGE and Western immunoblotting. rmlC mutants of both serotype O5 and serotype O6 synthesized a truncated core region which was unable to act as an attachment point for either A-band or B-band O antigen. A rmd rmlC PAO1 double mutant (deficient in biosynthesis of both D-Rha and L-Rha) was constructed to facilitate structural analysis of the mutant core region. This strain has an incomplete core oligosaccharide region and does not produce A-band O antigen. These results provide the genetic and structural evidence that L-Rha is the receptor on the P. aeruginosa LPS core for the attachment of O polysaccharides. This is the first report of a genetically defined mutation that affects the synthesis of a single sugar in the core oligosaccharide region of P. aeruginosa LPS, and provides further insight into the mechanisms of LPS biosynthesis and assembly in this bacterium.

Genetics of O-antigen biosynthesis in Pseudomonas aeruginosa

Pathogenic bacteria produce an elaborate assortment of extracellular and cell-associated bacterial products that enable colonization and establishment of infection within a host. Lipopolysaccharide (LPS) molecules are cell surface factors that are typically known for their protective role against serum-mediated lysis and their endotoxic properties. The most heterogeneous portion of LPS is the O antigen or O polysaccharide, and it is this region which confers serum resistance to the organism. Pseudomonas aeruginosa is capable of concomitantly synthesizing two types of LPS referred to as A band and B band. The A-band LPS contains a conserved O polysaccharide region composed of D-rhamnose (homopolymer), while the B-band O-antigen (heteropolymer) structure varies among the 20 O serotypes of P. aeruginosa. The genes coding for the enzymes that direct the synthesis of these two O antigens are organized into two separate clusters situated at different chromosomal locations. In this review, we summarize the organization of these two gene clusters to discuss how A-band and B-band O antigens are synthesized and assembled by dedicated enzymes. Examples of unique proteins required for both A-band and B-band O-antigen synthesis and for the synthesis of both LPS and alginate are discussed. The recent identification of additional genes within the P. aeruginosa genome that are homologous to those in the A-band and B-band gene clusters are intriguing since some are able to influence O-antigen synthesis. These studies demonstrate that P. aeruginosa represents a unique model system, allowing studies of heteropolymeric and homopolymeric O-antigen synthesis, as well as permitting an examination of the interrelationship of the synthesis of LPS molecules and other virulence determinants.

Metal binding by Pseudomonas aeruginosa PAO1 is influenced by growth of the cells as a biofilm

The metal-binding properties of Pseudomonas aeruginosa PAO1 biofilms were investigated using four metals (Cu, Fe, Au, and La). All but one of the metals (i.e., Cu) were bound by the biofilms in amounts that were significantly greater than those bound by planktonically grown cells of the same strain. Lanthanum precipitation appeared to be limited to the base of the biofilms and was not promoted by a shift in lipopolysaccharide production by the cells.Key words: metal binding, biofilms, Gram-negative bacteria, bioremediation.

Effect of O-side-chain-lipopolysaccharide chemistry on metal binding

Pseudomonas aeruginosa PAO1 produces two chemically distinct types of lipopolysaccharides (LPSs), termed A-band LPS and B-band LPS. The A-band O-side chain is electroneutral at physiological pH, while the B-band O-side chain contains numerous negatively charged sites due to the presence of uronic acid residues in the repeat unit structure. Strain PAO1 (A+ B+) and three isogenic LPS mutants (A+ B-, A- B+, and A- B-) were studied to determine the contribution of the O-side-chain portion of LPS to metal binding by the surfaces of gram-negative cells. Transmission electron microscopy with energy-dispersive X-ray spectroscopy was used to locate and analyze sites of metal deposition, while atomic absorption spectrophotometry and inductively coupled plasma-mass spectrometry were used to perform bulk quantitation of bound metal. The results indicated that cells of all of the strains caused the precipitation of gold as intracellular, elemental crystals with a d-spacing of 2.43 A. This type of precipitation has not been reported previously for gram-negative cells and suggests that in the organisms studied gold binding is not a surface-mediated event. All four strains bound similar amounts of copper (0.213 to 0.222 micromol/mg [dry weight] of cells) at the cell surface, suggesting that the major surface metal-binding sites reside in portions of the LPS which are common to all strains (perhaps the phosphoryl groups in the core-lipid A region). However, significant differences were observed in the abilities of strains dps89 (A- B+) and AK1401 (A+ B-) to bind iron and lanthanum, respectively. Strain dps89 caused the precipitation of iron (1.623 micromol/mg [dry weight] of cells) as an amorphous mineral phase (possibly iron hydroxide) on the cell surface, while strain AK1401 nucleated precipitation of lanthanum (0.229 micromol/mg [dry weight] of cells) as apiculate, surface-associated crystals. Neither iron nor lanthanum precipitates were observed on the cells of other strains, which suggests that the combination of A-band LPS and B-band LPS produced by a cell may result in a cell surface which promotes the formation of metal-rich precipitates. We therefore propose that the negatively charged sites located in the O-side chains are not directly responsible for the binding of metallic ions; however, the B-band LPS molecule as a whole may contribute to overall cell surface properties which favor the precipitation of distinct metal-rich mineral phases.

Phenotypic changes in the lipopolysaccharide of Pseudomonas aeruginosa and Escherichia coli grown in milk-based enteral nutrition solutions

Previous studies have shown enteral nutritional solutions (ENS) contaminated with large numbers of microorganisms from the environment or gastrointestinal (GI) tract of patients have caused respiratory infections, acute and chronic enteritis, and septicemia. The introduction of "closed" enteral feeding systems has been used to prevent contaminating organisms from entering enteral feeding systems in large numbers. However, there is some discussion as to whether this has been an effective measure in reducing ENS-related infections because there is anecdotal evidence to suggest that disease processes resulting from enteral feeding are still commonplace in the hospital and home. This is because there is very little information about the growth of microorganisms in ENS and whether growth in ENS may affect the virulence and pathogenicity of microorganisms. This study shows that Escherichia coli and Pseudomonas aeruginosa may grow at 25 degrees C from either high or low initial numbers to up to 9.2 log colony-forming units per mL in a range of milk-based ENS. However, these organisms did not grow in the fruit-based ENS. The effect on the lipopolysaccharide (LPS) of culturing E. coli and P. aeruginosa in milk-based ENS as opposed to standard laboratory media was examined using polyacrylamide gel electrophoresis. We found that there were significant qualitative changes in the phenotype of O-polysaccharide side chains of the LPS from these organisms. O-polysaccharide is known to mediate in the complement, antibiotic and bile resistance, and affect adherence. Therefore, changes in the virulence and pathogenicity of these microorganisms when cultured in ENS may be indicated. Thus, the study provides further evidence for reevaluating the microbiologic and immunologic effects of enteral feeding, especially on the microbial flora of the GI tract.

Pseudomonas aeruginosa antigens as potential vaccines

Pseudomonas aeruginosa is one of the most important opportunistic bacterial pathogens in humans and animals. This organism is ubiquitous and has high intrinsic resistance to antibiotics due to the low permeability of the outer membrane and the presence of numerous multiple drug efflux pumps. Various cell-associated and secreted antigens of P. aeruginosa have been the subject of vaccine development. Among pseudomonas antigens, the mucoid substance, which is an extracellular slime consisting predominantly of alginate, was found to be heterogenous in terms of size and immunogenicity. High molecular mass alginate components (30-300 kDa) appear to contain conserved epitopes while lower molecular mass alginate components (10-30 kDa) possess conserved epitopes in addition to unique epitopes. Surface-exposed antigens including O-antigens (O-specific polysaccharide of LPS) or H-antigens (flagellar antigens) have been used for serotyping due to their highly immunogenic nature. Chemical structures of repeating units of O-specific polysaccharides have been elucidated and these data allowed the identification of 31 chemotypes of P. aeruginosa. Conserved epitopes among all serotypes of P. aeruginosa are located in the core oligosaccharide and the lipid A region of LPS and immunogens containing these epitopes induce cross-protective immunity in mice against different P. aeruginosa immunotypes. To examine the protective properties of OM proteins, a vaccine containing P. aeruginosa OM proteins of molecular masses ranging from 20 to 100 kDa has been used in pre-clinical and clinical trials. This vaccine was efficacious in animal models against P. aeruginosa challenge and induced high levels of specific antibodies in human volunteers. Plasma from human volunteers containing anti-P. aeruginosa antibodies provided passive protection and helped the recovery of 87% of patients with severe forms of P. aeruginosa infection. Vaccines prepared from P. aeruginosa ribosomes induced protective immunity in mice, but the efficacy of ribosomal vaccines in humans is not yet known. A number of recent studies indicated the potential of some P. aeruginosa antigens that deserve attention as new vaccine candidates. The outer core of LPS was implicated to be a ligand for binding of P. aeruginosa to airway and ocular epithelial cells of animals. However, heterogeneity exists in this outer core region among different serotypes. Epitopes in the inner core are highly conserved and it has been demonstrated to be surface-accessible, and not masked by O-specific polysaccharide. The use of an in vivo selection/expression technology (IVET) by a group of researchers identified a number of P. aeruginosa proteins that are expressed in vivo and essential for virulence. Two of these in vivo-expressed proteins are FptA (ferripyochelin receptor protein) and a homologue of an LPS biosynthetic enzyme. Our laboratory has identified a highly conserved protein, WbpM, and P. aeruginosa with a deficiency in this protein produces only rough LPS and became serum sensitive. Results from these studies have provided the foundation for a variety of vaccine formulations.

Interactions between biofilms and the environment

The surfaces of bacteria are highly interactive with their environment. Whether the bacterium is Gram-negative or Gram-positive, most surfaces are charged at neutral pH because of the ionization of the reactive chemical groups which stud them. Since prokaryotes have a high surface area-to-volume ratio, this can have surprising ramifications. For example, many bacteria can concentrate dilute environmental metals on their surfaces and initiate the development of fine-grained minerals. In natural environments, it is not unusual to find such bacteria closely associated with the minerals which they have helped develop. Bacteria can be free-living (planktonic), but in most natural ecosystems they prefer to grow on interfaces as biofilms; supposedly to take advantage of the nutrient concentrative effect of the interface, although there must also be gained some protective value against predators and toxic agents. Using a Pseudomonas aeruginosa model system, we have determined that lipopolysaccharide is important in the initial attachment of this Gram-negative bacterium to interfaces and that this surface moiety subtly changes during biofilm formation. Using this same model system, we have also discovered that there is a natural tendency for Gram-negative bacteria to concentrate and package periplasmic components into membrane vesicles which bleb-off the surface. Since some of these components (e.g., peptidoglycan hydrolases) can degrade other surrounding cells, the vesicles could be predatory; i.e., a natural system by which neighboring bacteria are targeted and lysed, thereby liberating additional nutrients to the microbial community. This obviously would be of benefit to vesicle-producing bacteria living in biofilms containing mixed microbial populations.

O-antigenic determinants in Salmonella species of serogroup C1 are expressed in distinct immunochemical populations of chains

The O-antigenic specificities found among Salmonellae of serogroup C1 are O:6(1),7, O:6(2),7, O:6(1),6(2),7 and O:6,7,14, as defined by classical serology. Factor O:7 is the group-wide determinant while factors O:6(1), O:6(2) and O:14 are found in some strains but not others. Strains of the O:6(2),7 specificity are subject to lysogenic conversion by phages 6(1) and 14 to the O:6(1),7 and O:6,7,14 specificities, respectively. To further delineate antigenic complexity and serological relationships among strains of this serogroup monoclonal antibodies (mAbs) were generated against the O:6(1),6(2),7 polysaccharide of Salmonella thompson. Five mAbs of either the O:6(1) or O:6(2) specificities did not bind O:6,7,14 strains or LPS, showing that the O:6 determinant in these strains is neither O:6(1) nor O:6(2). Thus antigenic conversion of O:6(2),7 strains by phage 14 is accompanied by addition of O:14 as well as loss of O:6(2). Three mAbs which demonstrated group-wide reactivity, and were thus specific for O:7, recognized clearly by separable epitopes hereby defined as sub-specificities, O:7(1), O:7(2) and O:7(3). Immunoblotting of mAbs against electrophoretically resolved LPS showed that factors O:6(1) and O:6(2) are expressed only in LPS molecules of high molecular mass whereas O:7(2) and O:7(3) are expressed only in relatively low-molecular-mass chains. These results are consistent with the expression of different antigenic determinants in structurally distinct subpopulations of O chains. The implications of the existence of distinct subpopulation of chains is that the published structure of the O:6,7 repeat unit is not fully representative of the O-antigenic structure of this group.

Pseudomonas aeruginosa lipopolysaccharides and pathogenesis

Pseudomonas aeruginosa lipopolysaccharide (LPS) plays a key role in pathogenesis. In acute infections, a smooth LPS protects the organism from complement-mediated killing and, during chronic lung infections, an altered rough LPS helps the organism evade host defense mechanisms.

Immunochemical characterization of O polysaccharides composing the alpha-D-rhamnose backbone of lipopolysaccharide of Pseudomonas syringae and classification of bacteria into serogroups O1 and O2 with monoclonal antibodies

Murine monoclonal antibodies (MAbs) reacting with Pseudomonas syringae lipopolysaccharide (LPS) O polysaccharides (OPS) composed of tetra- and tri-alpha-D-rhamnose repeats in the backbone [3)D-Rha(alpha1-3)D-Rha(alpha1-2)D-Rha(alpha1-2)D-Rha(alpha1] and [3)D-Rha(alpha1-3)D-Rha(alpha1-2)D-Rha(alpha1] were generated and used for immunochemical analysis and for serological classification of the bacteria. A total of 195 of 358 P. syringae strains tested representing 21 pathovars were shown to share a common epitope, 1a, and were classified into serogroup O1. All strains with pathovars aptata, glycinea, japonica, phaseolicola, and pisi, most of the strains with pathovars atrofaciens and striafaciens, and half of the strains with pathovar syringae were classified into serotypes O1a', O1b, O1c, and O1d within serogroup O1. Serogroup-specific epitope 1a was inferred to be related to the (alpha1-2)D-Rha(alpha1-3) site of the OPS backbone. The serotype-specific epitopes 1b, 1c, 1d, and 1a' were inferred as relating to the immunodominant lateral (alpha1-3)D-Rha, (beta1-4)D-GlcNAc, and (alpha1-4)D-Fuc substituents and backbone-located site (alpha1-3)D-Rha(alpha1-2), respectively, of OPSs that share the common tetra-D-rhamnose repeats in the backbone. A total of 7.3% of the strains studied, all with pathovars morsprunorum and lapsa, were classified as serotypes O2a and O2d within serogroup 02. Serotype-specific epitope 2a was inferred as being related to the backbone-located site D-Rha(alpha1-3)D-Rha and epitope 2d to the immunodominant lateral (alpha1-4)D-Fuc residue of OPS consisting of tri-D-rhamnose repeats in the backbone. Epitope 2d alternated with 2a within the same LPS molecule and did not cross-react with epitope 1d. Serotypes O2a and O2d were observed in some strains correlating with the coexpression of the two chemotypes of OPS by the same strain. The serogroup O1-specific MAb Ps1a reacted weakly but definitely with all strains from serogroup 02. We propose serological formulas for serogroups O1 and 02 as well as for individual strains within these serogroups.

Pseudomonas aeruginosa PAO1 ceases to express serotype-specific lipopolysaccharide at 45 degrees C

Most Pseudomonas aeruginosa strains are able to produce two distinct lipopolysaccharide (LPS) O-polysaccharide types, A-band (common-antigen) and B-band (serotype-specific) LPSs. The relative expression levels of these two LPS types in P. aeruginosa PAO1 (O5 serotype) at various growth temperatures were investigated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining or Western blotting (immunoblotting) with monoclonal antibodies specific for each O polysaccharide. A-band and B-band LPSs were expressed concurrently when the cells grew at 15, 25, and 35 degrees C; however, growth at 45 degrees C resulted in a surface deficiency in B-band LPS as determined by immunoblotting and agglutination with B-band-specific monoclonal antibody. Transfer of these cells (expressing A-band LPS but deficient in B-band LPS) [A+B-]) to a lower temperature (at which the division time was comparable) resulted in a rapid resumption of normal A-band and B-band expression. B-band LPS was detectable by immunoblotting before measurable growth of the culture had occurred.