Biosynthesis of a Novel 3-Deoxy-D-manno-oct-2-ulosonic Acid-containing Outer Core Oligosaccharide in the Lipopolysaccharide of Klebsiella pneumoniae

Development of phage resistance in multidrug-resistant Klebsiella pneumoniae is associated with reduced virulence: a case report of a personalised phage therapy

OBJECTIVES

Phage-resistant bacteria often emerge rapidly when performing phage therapy. However, the relationship between the emergence of phage-resistant bacteria and improvements in clinical symptoms is still poorly understood.

METHODS

An inpatient developed a pulmonary infection caused by multidrug-resistant Klebsiella pneumoniae. He received a first course of treatment with a single nebulized phage (ΦKp_GWPB35) targeted at his bacterial isolate of Kp7450. After 14 days, he received a second course of treatment with a phage cocktail (ΦKp_GWPB35+ΦKp_GWPA139). Antibiotic treatment was continued throughout the course of phage therapy. Whole-genome analysis was used to identify mutations in phage-resistant strains. Mutated genes associated with resistance were further analysed by generating knockouts of Kp7450 and by measuring phage adsorption rates of bacteria treated with proteinase K and periodate. Bacterial virulence was evaluated in mouse and zebrafish infection models.

RESULTS

Phage-resistant Klebsiella pneumoniae strains emerged after the second phage treatment. Comparative genomic analyses revealed that fabF was deleted in phage-resistant strains. The fabF knockout strain (Kp7450ΔfabF) resulted in an altered structure of lipopolysaccharide (LPS), which was identified as the host receptor for the therapeutic phages. Virulence evaluations in mice and zebrafish models showed that LPS was the main determinant of virulence in Kp7450 and alteration of LPS structure in Kp7450ΔfabF, and the bacteriophage-resistant strains reduced their virulence at cost.

DISCUSSION

This study may shed light on the mechanism by which some patients experience clinical improvement in their symptoms post phage therapy, despite the incomplete elimination of pathogenic bacteria.

A Bioactive Synthetic Outer-Core Oligosaccharide Derived from a Klebsiella pneumonia Lipopolysaccharide for Bacteria Recognition

There is an urgent need for new treatment options for carbapenem-resistant Klebsiella pneumoniae (K. pneumoniae), which is a common cause of life-threatening hospital- and community-acquired infections. Prophylactic or therapeutic vaccination may offer an approach to control these infections, however, none has yet been approved for human use. Here, we report the chemical synthesis of an outer core tetra- and pentasaccharide derived from the lipopolysaccharide of K. pneumoniae. The oligosaccharides were equipped with an aminopentyl linker, which facilitated conjugation to the carrier proteins CRM197 and BSA. Mice immunized with the glycoconjugate vaccine candidates elicited antibodies that recognized isolated LPS as well as various strains of K. pneumoniae. The successful preparation of the oligosaccharides relied on the selection of monosaccharide building blocks equipped with orthogonal hydroxyl and amino protecting groups. It allowed the differentiation of three types of amines of the target compounds and the installation of a crowded 4,5-branched Kdo moiety.

Mechanistic Insights into the Capsule-Targeting Depolymerase from a Klebsiella pneumoniae Bacteriophage

The production of capsular polysaccharides by Klebsiella pneumoniae protects the bacterial cell from harmful environmental factors such as antimicrobial compounds and infection by bacteriophages (phages). To bypass this protective barrier, some phages encode polysaccharide-degrading enzymes referred to as depolymerases to provide access to cell surface receptors. Here, we characterized the phage RAD2, which infects K. pneumoniae strains that produce the widespread, hypervirulence-associated K2-type capsular polysaccharide. Using transposon-directed insertion sequencing, we have shown that the production of capsule is an absolute requirement for efficient RAD2 infection by serving as a first-stage receptor. We have identified the depolymerase responsible for recognition and degradation of the capsule, determined that the depolymerase forms globular appendages on the phage virion tail tip, and present the cryo-electron microscopy structure of the RAD2 capsule depolymerase at 2.7-Å resolution. A putative active site for the enzyme was identified, comprising clustered negatively charged residues that could facilitate the hydrolysis of target polysaccharides. Enzymatic assays coupled with mass spectrometric analyses of digested oligosaccharide products provided further mechanistic insight into the hydrolase activity of the enzyme, which, when incubated with K. pneumoniae, removes the capsule and sensitizes the cells to serum-induced killing. Overall, these findings expand our understanding of how phages target the Klebsiella capsule for infection, providing a framework for the use of depolymerases as antivirulence agents against this medically important pathogen. IMPORTANCE Klebsiella pneumoniae is a medically important pathogen that produces a thick protective capsule that is essential for pathogenicity. Phages are natural predators of bacteria, and many encode diverse "capsule depolymerases" which specifically degrade the capsule of their hosts, an exploitable trait for potential therapies. We have determined the first structure of a depolymerase that targets the clinically relevant K2 capsule and have identified its putative active site, providing hints to its mechanism of action. We also show that Klebsiella cells treated with a recombinant form of the depolymerase are stripped of capsule, inhibiting their ability to grow in the presence of serum, demonstrating the anti-infective potential of these robust and readily producible enzymes against encapsulated bacterial pathogens such as K. pneumoniae.

Phage Resistance in Multidrug-Resistant Klebsiella pneumoniae ST258 Evolves via Diverse Mutations That Culminate in Impaired Adsorption

The therapeutic use of bacteriophage (phage) is garnering renewed interest in the setting of difficult-to-treat infections. Phage resistance is one major limitation of phage therapy; therefore, developing effective strategies to avert or lessen its impact is critical. Characterization of in vitro phage resistance may be an important first step in evaluating the relative likelihood with which phage-resistant populations emerge, the most likely phenotypes of resistant mutants, and the effect of certain phage cocktail combinations in increasing or decreasing the genetic barrier to resistance. If this information confers predictive power in vivo, then routine studies of phage-resistant mutants and their in vitro evolution should be a valuable means for improving the safety and efficacy of phage therapy in humans.

The evolution of phage resistance poses an inevitable threat to the efficacy of phage therapy. The strategic selection of phage combinations that impose high genetic barriers to resistance and/or high compensatory fitness costs may mitigate this threat. However, for such a strategy to be effective, the evolution of phage resistance must be sufficiently constrained to be consistent. In this study, we isolated lytic phages capable of infecting a modified Klebsiella pneumoniae clinical isolate and characterized a total of 57 phage-resistant mutants that evolved from their prolonged coculture in vitro. Single- and double-phage-resistant mutants were isolated from independently evolved replicate cocultures grown in broth or on plates. Among resistant isolates evolved against the same phage under the same conditions, mutations conferring resistance occurred in different genes, yet in each case, the putative functions of these genes clustered around the synthesis or assembly of specific cell surface structures. All resistant mutants demonstrated impaired phage adsorption, providing a strong indication that these cell surface structures functioned as phage receptors. Combinations of phages targeting different host receptors reduced the incidence of resistance, while, conversely, one three-phage cocktail containing two phages targeting the same receptor increased the incidence of resistance (relative to its two-phage, nonredundant receptor-targeting counterpart). Together, these data suggest that laboratory characterization of phage-resistant mutants is a useful tool to help optimize therapeutic phage selection and cocktail design.

Identification and Characterization of Two Klebsiella pneumoniae lpxL Lipid A Late Acyltransferases and Their Role in Virulence

Klebsiella pneumoniae causes a wide range of infections, from urinary tract infections to pneumonia. The lipopolysaccharide is a virulence factor of this pathogen, although there are gaps in our understanding of its biosynthesis. Here we report on the characterization of K. pneumoniaelpxL, which encodes one of the enzymes responsible for the late secondary acylation of immature lipid A molecules. Analysis of the available K. pneumoniae genomes revealed that this pathogen's genome encodes two orthologues of Escherichia coli LpxL. Using genetic methods and mass spectrometry, we demonstrate that LpxL1 catalyzes the addition of laureate and LpxL2 catalyzes the addition of myristate. Both enzymes acylated E. coli lipid A, whereas only LpxL2 mediated K. pneumoniae lipid A acylation. We show that LpxL1 is negatively regulated by the two-component system PhoPQ. The lipid A produced by the lpxL2 mutant lacked the 2-hydroxymyristate, palmitate, and 4-aminoarabinose decorations found in the lipid A synthesized by the wild type. The lack of 2-hydroxymyristate was expected since LpxO modifies the myristate transferred by LpxL2 to the lipid A. The absence of the other two decorations is most likely caused by the downregulation of phoPQ and pmrAB expression. LpxL2-dependent lipid A acylation protects Klebsiella from polymyxins, mediates resistance to phagocytosis, limits the activation of inflammatory responses by macrophages, and is required for pathogen survival in the wax moth (Galleria mellonella). Our findings indicate that the LpxL2 contribution to virulence is dependent on LpxO-mediated hydroxylation of the LpxL2-transferred myristate. Our studies suggest that LpxL2 might be a candidate target in the development of anti-K. pneumoniae drugs.

Review: Lipopolysaccharide inner core oligosaccharide structure and outer membrane stability in human pathogens belonging to the Enterobacteriaceae

In the Enterobacteriaceae, the outer membrane is primarily comprised of lipopolysaccharides. The lipopolysaccharide molecule is important in mediating interactions between the bacterium and its environment and those regions of the molecule extending further away from the cell surface show a higher amount of structural diversity. The hydrophobic lipid A is highly conserved, due to its important role in the structural integrity of the outer membrane. Attached to the lipid A region is the core oligosaccharide. The inner core oligosaccharide (lipid A proximal) backbone is also well conserved. However, non-stoichiometric substitutions of the basic inner core structure lead to structural variation and microheterogeneity. These include the addition of negatively charged groups (phosphate or galacturonic acid), ethanolamine derivatives, and glycose residues (Kdo, rhamnose, galactose, glucosamine, N-acetylglucosamine, heptose, Ko). The genetics and biosynthesis of these substitutions is beginning to be elucidated. Modification of heptose residues with negatively charged molecules (such as phosphate in Escherichia coli and Salmonella and galacturonic acid in Klebsiella pneumoniae ) has been shown to be involved in maintaining membrane stability. However, the biological role(s) of the remaining substitutions is unknown.

Biochemical Characterization of Bifunctional 3-Deoxy-β-d-manno-oct-2-ulosonic Acid (β-Kdo) Transferase KpsC from Escherichia coli Involved in Capsule Biosynthesis

3-Deoxy-d-manno-oct-2-ulosonic acid (Kdo) is an essential component of bacterial lipopolysaccharides, where it provides the linkage between lipid and carbohydrate moieties. In all known LPS structures, Kdo residues possess α-anomeric configurations, and the corresponding inverting α-Kdo transferases are well characterized. Recently, it has been shown that a large group of capsular polysaccharides from Gram-negative bacteria, produced by ATP-binding cassette transporter-dependent pathways, are also attached to a lipid anchor through a conserved Kdo oligosaccharide. In the study reported here, the structure of this Kdo linker was determined by NMR spectroscopy, revealing alternating β-(2→4)- and β-(2→7)-linked Kdo residues. KpsC contains two retaining β-Kdo glycosyltransferase domains belonging to family GT99 that are responsible for polymerizing the β-Kdo linker on its glycolipid acceptor. Full-length Escherichia coli KpsC was expressed and purified, together with the isolated N-terminal domain and a mutant protein (KpsC D160A) containing a catalytically inactivated N-terminal domain. The Kdo transferase activities of these proteins were determined in vitro using synthetic acceptors, and the reaction products were characterized using TLC, mass spectrometry, and NMR spectroscopy. The N- and C-terminal domains were found to catalyze formation of β-(2→4) and β-(2→7) linkages, respectively. Based on phylogenetic analyses, we propose the linkage specificities of the glycosyltransferase domains are conserved in KpsC homologs from other bacterial species.

Bacterial β-Kdo glycosyltransferases represent a new glycosyltransferase family (GT99)

Kdo (3-deoxy-d-manno-oct-2-ulosonic acid) is an eight-carbon sugar mostly confined to Gram-negative bacteria. It is often involved in attaching surface polysaccharides to their lipid anchors. α-Kdo provides a bridge between lipid A and the core oligosaccharide in all bacterial LPSs, whereas an oligosaccharide of β-Kdo residues links "group 2" capsular polysaccharides to (lyso)phosphatidylglycerol. β-Kdo is also found in a small number of other bacterial polysaccharides. The structure and function of the prototypical cytidine monophosphate-Kdo-dependent α-Kdo glycosyltransferase from LPS assembly is well characterized. In contrast, the β-Kdo counterparts were not identified as glycosyltransferase enzymes by bioinformatics tools and were not represented among the 98 currently recognized glycosyltransferase families in the Carbohydrate-Active Enzymes database. We report the crystallographic structure and function of a prototype β-Kdo GT from WbbB, a modular protein participating in LPS O-antigen synthesis in Raoultella terrigena The β-Kdo GT has dual Rossmann-fold motifs typical of GT-B enzymes, but extensive deletions, insertions, and rearrangements result in a unique architecture that makes it a prototype for a new GT family (GT99). The cytidine monophosphate-binding site in the C-terminal α/β domain closely resembles the corresponding site in bacterial sialyltransferases, suggesting an evolutionary connection that is not immediately evident from the overall fold or sequence similarities.

KpsC and KpsS are retaining 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) transferases involved in synthesis of bacterial capsules

Capsular polysaccharides (CPSs) are high-molecular-mass cell-surface polysaccharides, that act as important virulence factors for many pathogenic bacteria. Several clinically important Gram-negative pathogens share similar systems for CPS biosynthesis and export; examples include Escherichia coli, Campylobacter jejuni, Haemophilus influenzae, Neisseria meningitidis, and Pasteurella multocida. Each CPS contains a serotype-specific repeat-unit structure, but the glycans all possess a lipid moiety at their reducing termini. In E. coli and N. meningitidis, the predominant lipid is a lysophosphatidylglycerol moiety that is attached to the repeat-unit domain of the CPS via multiple residues of 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo), referred to as a poly-Kdo linker. The Kdo residues are β-linked, suggesting that they are synthesized by retaining glycosyltransferases. To date, the only characterized Kdo transferases are the inverting enzymes that catalyze the α-linkages found in lipopolysaccharide. Here, we identify two conserved proteins from CPS assembly systems, KpsC and KpsS, as the β-Kdo-transferases and demonstrate in vitro reconstitution of poly-Kdo linker assembly on a fluorescent phosphatidylglycerol acceptor. KpsS adds the first Kdo residue, and this reaction product is then extended by KpsC. Cross-complementation experiments demonstrate that the E. coli and N. meningitidis protein homologs are functionally conserved.

Three enzymatic steps required for the galactosamine incorporation into core lipopolysaccharide

The core lipopolysaccharides (LPS) of Proteus mirabilis as well as those of Klebsiella pneumoniae and Serratia marcescens are characterized by the presence of a hexosamine-galacturonic acid disaccharide (αHexN-(1,4)-αGalA) attached by an α1,3 linkage to L-glycero-D-manno-heptopyranose II (L-glycero-α-D-manno-heptosepyranose II). In K. pneumoniae, S. marcescens, and some P. mirabilis strains, HexN is D-glucosamine, whereas in other P. mirabilis strains, it corresponds to D-galactosamine. Previously, we have shown that two enzymes are required for the incorporation of D-glucosamine into the core LPS of K. pneumoniae; the WabH enzyme catalyzes the incorporation of GlcNAc from UDP-GlcNAc to outer core LPS, and WabN catalyzes the deacetylation of the incorporated GlcNAc. Here we report the presence of two different HexNAc transferases depending on the nature of the HexN in P. mirabilis core LPS. In vivo and in vitro assays using LPS truncated at the level of galacturonic acid as acceptor show that these two enzymes differ in their specificity for the transfer of GlcNAc or GalNAc. By contrast, only one WabN homologue was found in the studied P. mirabilis strains. Similar assays suggest that the P. mirabilis WabN homologue is able to deacetylate both GlcNAc and GalNAc. We conclude that incorporation of d-galactosamine requires three enzymes: Gne epimerase for the generation of UDP-GalNAc from UDP-GlcNAc, N-acetylgalactosaminyltransferase (WabP), and LPS:HexNAc deacetylase.

Structural investigation of bacterial lipopolysaccharides by mass spectrometry and tandem mass spectrometry

Mass spectrometric studies are now playing a leading role in the elucidation of lipopolysaccharide (LPS) structures through the characterization of antigenic polysaccharides, core oligosaccharides and lipid A components including LPS genetic modifications. The conventional MS and MS/MS analyses together with CID fragmentation provide additional structural information complementary to the previous analytical experiments, and thus contribute to an integrated strategy for the simultaneous characterization and correct sequencing of the carbohydrate moiety.

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.

The structures of core regions from enterobacterial lipopolysaccharides – an update

To the major virulence factors of Gram-negative bacteria belong the lipopolysaccharides (endotoxins), which are very well characterized for their immunological, pharmacological and pathophysiological effects displayed in eucaryotic cells and organisms. In general, these amphiphilic lipopolysaccharides comprise three regions, which can be differentiated by their structures, function, genetics and biosynthesis: lipid A, the core region and a polysaccharide portion, which may be the O-specific polysaccharide, Enterobacterial Common Antigen (ECA) or a capsular polysaccharide. In the past, much emphasis has been laid on the elucidation of the structure-function relation. The lipid A was proven to represent the toxic principle of endotoxic active lipopolysaccharides, however, its toxicity depends not only on its structure but also on that of the core region, which is covalently linked to lipid A. Thus, and since the core region possesses immunogenic properties, complete structural analyses of lipopolysaccharides core regions and of structure-function relation are highly important for a better understanding of lipopolysaccharides action. To date, quite a number of core structures from lipopolysaccharides of various Gram-negative bacteria have been published and summarized in several overviews. This short review adds to this knowledge those structures of enterobacterial lipopolysaccharides that were published between January 2002 and October 2006.

A second galacturonic acid transferase is required for core lipopolysaccharide biosynthesis and complete capsule association with the cell surface in Klebsiella pneumoniae

The core lipopolysaccharide (LPS) of Klebsiella pneumoniae contains two galacturonic acid (GalA) residues, but only one GalA transferase (WabG) has been identified. Data from chemical and structural analysis of LPS isolated from a wabO mutant show the absence of the inner core beta-GalA residue linked to L-glycero-D-manno-heptose III (L,D-Hep III). An in vitro assay demonstrates that the purified WabO is able to catalyze the transfer of GalA from UDP-GalA to the acceptor LPS isolated from the wabO mutant, but not to LPS isolated from waaQ mutant (deficient in l,d-Hep III). The absence of this inner core beta-GalA residue results in a decrease in virulence in a capsule-dependent experimental mouse pneumonia model. In addition, this mutation leads to a strong reduction in cell-bound capsule. Interestingly, a K66 Klebsiella strain (natural isolate) without a functional wabO gene shows reduced levels of cell-bound capsule in comparison to those of other K66 strains. Thus, the WabO enzyme plays an important role in core LPS biosynthesis and determines the level of cell-bound capsule in Klebsiella pneumoniae.

The ionic interaction of Klebsiella pneumoniae K2 capsule and core lipopolysaccharide

The complete structures of LPS core types 1 and 2 fromKlebsiella pneumoniaehave been described by other authors. They are characterized by a lack of phosphoryl residues, but they contain galacturonic acid (GalA) residues, which contribute to the necessary negative charges. The presence of a capsule was determined in core-LPS non-polar mutants from strains 52145 (O1 : K2), DL1 (O1 : K1) and C3 (O8 : K66). O-antigen ligase (waaL) mutants produced a capsule. Core mutants containing the GalA residues were capsulated, while those lacking the residues were non capsulated. Since the proteins involved in the transfer of GalA (WabG) and glucosamine residues (WabH) are known, the chemical basis of the capsular-K2–cell-surface association was studied. Phenol/water extracts fromK. pneumoniae52145ΔwabH waaLand 52145ΔwaaLmutants, but not those from fromK. pneumoniae52145ΔwabG waaLmutant, contained both LPS and capsular polysaccharide, even after hydrophobic chromatography. The two polysaccharides were dissociated by gel-filtration chromatography, eluting with detergent and metal-ion chelators. From these results, it is concluded that the K2 capsular polysaccharide is associated by an ionic interaction to the LPS through the negative charge provided by the carboxyl groups of the GalA residues.

The Incorporation of Glucosamine into Enterobacterial Core Lipopolysaccharide

The core lipopolysaccharide (LPS) of Klebsiella pneumoniae is characterized by the presence of disaccharide alphaGlcN-(1,4)-alphaGalA attached by an alpha1,3 linkage to l-glycero-d-manno-heptopyranose II (ld-HeppII). Previously it has been shown that the WabH enzyme catalyzes the incorporation of GlcNAc from UDP-GlcNAc to outer core LPS. The presence of GlcNAc instead of GlcN and the lack of UDP-GlcN in bacteria indicate that an additional enzymatic step is required. In this work we identified a new gene (wabN) in the K. pneumoniae core LPS biosynthetic cluster. Chemical and structural analysis of K. pneumoniae non-polar wabN mutants showed truncated core LPS with GlcNAc instead of GlcN. In vitro assays using LPS truncated at the level of d-galacturonic acid (GalA) and cell-free extract containing WabH and WabN together led to the incorporation of GlcN, whereas none of them alone were able to do it. This result suggests that the later enzyme (WabN) catalyzes the deacetylation of the core LPS containing the GlcNAc residue. Thus, the incorporation of the GlcN residue to core LPS in K. pneumoniae requires two distinct enzymatic steps. WabN homologues are found in Serratia marcescens and some Proteus strains that show the same disaccharide alphaGlcN-(1,4)-alphaGalA attached by an alpha1,3 linkage to ld-HeppII.

A second outer-core region in Klebsiella pneumoniae lipopolysaccharide

Up to now only one major type of core oligosaccharide has been found in the lipopolysaccharide of all Klebsiella pneumoniae strains analyzed. Applying a different screening approach, we identified a novel Klebsiella pneumoniae core (type 2). Both Klebsiella core types share the same inner core and the outer-core-proximal disaccharide, GlcN-(1,4)-GalA, but they differ in the GlcN substituents. In core type 2, the GlcpN residue is substituted at the O-4 position by the disaccharide beta-Glcp(1-6)-alpha-Glcp(1, while in core type 1 the GlcpN residue is substituted at the O-6 position by either the disaccharide alpha-Hep(1-4)-alpha-Kdo(2 or a Kdo residue (Kdo is 3-deoxy-D-manno-octulosonic acid). This difference correlates with the presence of a three-gene region in the corresponding core biosynthetic clusters. Engineering of both core types by interchanging this specific region allowed studying the effect on virulence. The replacement of Klebsiella core type 1 in a highly type 2 virulent strain (52145) induces lower virulence than core type 2 in a murine infection model.

Characterization of Gla(KP), a UDP-galacturonic acid C4-epimerase from Klebsiella pneumoniae with extended substrate specificity

In Escherichia coli and Salmonella enterica, the core oligosaccharide backbone of the lipopolysaccharide is modified by phosphoryl groups. The negative charges provided by these residues are important in maintaining the barrier function of the outer membrane. In contrast, Klebsiella pneumoniae lacks phosphoryl groups in its core oligosaccharide but instead contains galacturonic acid residues that are proposed to serve a similar function in outer membrane stability. Gla(KP) is a UDP-galacturonic acid C4-epimerase that provides UDP-galacturonic acid for core synthesis, and the enzyme was biochemically characterized because of its potentially important role in outer membrane stability. High-performance anion-exchange chromatography was used to demonstrate the UDP-galacturonic acid C4-epimerase activity of Gla(KP), and capillary electrophoresis was used for activity assays. The reaction equilibrium favors UDP-galacturonic acid over UDP-glucuronic acid in a ratio of 1.4:1, with the K(m) for UDP-glucuronic acid of 13.0 microM. Gla(KP) exists as a dimer in its native form. NAD+/NADH is tightly bound by the enzyme and addition of supplementary NAD+ is not required for activity of the purified enzyme. Divalent cations have an unexpected inhibitory effect on enzyme activity. Gla(KP) was found to have a broad substrate specificity in vitro; it is capable of interconverting UDP-glucose/UDP-galactose and UDP-N-acetylglucosamine/UDP-N-acetylgalactosamine, albeit at much lower activity. The epimerase GalE interconverts UDP-glucose/UDP-galactose. Multicopy plasmid-encoded gla(KP) partially complemented a galE mutation in S. enterica and in K. pneumoniae; however, chromosomal gla(KP) could not substitute for galE in a K. pneumoniae galE mutant in vivo.

The C-terminal domain of the nucleotide-binding domain protein Wzt determines substrate specificity in the ATP-binding cassette transporter for the lipopolysaccharide O-antigens in Escherichia coli serotypes O8 and O9a

The polymannan O-antigenic polysaccharides (O-PSs) of Escherichia coli O8 and O9a are synthesized via an ATP-binding cassette (ABC) transporter-dependent pathway. The group 2 capsular polysaccharides of E. coli serve as prototypes for polysaccharide synthesis and export via this pathway. Here, we show that there are some fundamental differences between the ABC transporter-dependent pathway for O-PS biosynthesis and the capsular polysaccharide paradigm. In the capsule system, mutants lacking the ABC transporter are viable, and membranes isolated from these strains are no longer able to synthesize polymer using an endogenous acceptor. In contrast, E. coli strains carrying mutations in the membrane component (Wzm) and/or the nucleotide-binding component (Wzt) of the O8 and O9a polymannan transporters are nonviable under conditions permissive to O-PS biosynthesis and take on an aberrant elongated cell morphology. Whereas the ABC transporters for capsular polysaccharides with different structures are functionally interchangeable, the O8 and O9a exporters are specific for their cognate polymannan substrates. The E. coli O8 and O9a Wzt proteins contain a C-terminal domain not present in the corresponding nucleotide-binding protein (KpsT) from the capsule exporter. Whereas the Wzm components are functionally interchangeable, albeit with reduced efficiency, the Wzt components are not, indicating a specific role for Wzt in substrate specificity. Chimeric Wzt proteins were constructed in order to localize the region involved in substrate specificity to the C-terminal domain.

The role of galacturonic acid in outer membrane stability in Klebsiella pneumoniae

In most members of the Enterobacteriaceae, including Escherichia coli and Salmonella, the lipopolysaccharide core oligosaccharide backbone is modified by phosphoryl groups. The negative charges provided by these residues are important in maintaining the barrier function of the outer membrane. Mutants lacking the core heptose region and the phosphate residues display pleiotrophic defects collectively known as the deep-rough phenotype, characterized by changes in outer membrane structure and function. Klebsiella pneumoniae lacks phosphoryl residues in its core, but instead contains galacturonic acid. The goal of this study was to determine the contribution of galacturonic acid as a critical source of negative charge. A mutant was created lacking all galacturonic acid by targeting UDP-galacturonic acid precursor synthesis through a mutation in gla(KP). Gla(KP) is a K. pneumoniae UDP-galacturonic acid C4 epimerase providing UDP-galacturonic acid for core synthesis. The gla(KP) gene was inactivated and the structure of the mutant lipopolysaccharide was determined by mass spectrometry. The mutant displayed characteristics of a deep-rough phenotype, exhibiting a hypersensitivity to hydrophobic compounds and polymyxin B, an altered outer membrane profile, and the release of the periplasmic enzyme beta-lactamase. These results indicate that the negative charge provided by the carboxyl groups of galacturonic acid do play an equivalent role to the core oligosaccharide phosphate residues in establishing outer membrane integrity in E. coli and Salmonella.