Insertions in the OCL1 locus of Acinetobacter baumannii lead to shortened lipooligosaccharides

Genomes of 82 Acinetobacter baumannii global clones 1 (GC1) and 2 (GC2) isolates were sequenced and different forms of the locus predicted to direct synthesis of the outer core (OC) of the lipooligosaccharide were identified. OCL1 was in all GC2 genomes, whereas GC1 isolates carried OCL1, OCL3 or a new locus, OCL5. Three mutants in which an insertion sequence (ISAba1 or ISAba23) interrupted OCL1 were identified. Isolates with OCL1 intact produced only lipooligosaccharide, while the mutants produced lipooligosaccharide of reduced molecular weight. Thus, the assignment of the OC locus as that responsible for the synthesis of the OC is correct.

Redefining the requisite lipopolysaccharide structure in Escherichia coli

Gram-negative bacteria possess an asymmetric lipid bilayer surrounding the cell wall, the outer membrane (OM). The OM inner leaflet is primarily composed of various glycerophospholipids, whereas the outer leaflet predominantly contains the unique amphiphilic macromolecule, lipopolysaccharide (LPS or endotoxin). The majority of all gram-negative bacteria elaborate LPS containing at least one 2-keto 3-deoxy-D-manno-octulosonate (Kdo) molecule. The minimal LPS structure required for growth of Escherichia coli has long been recognized as two Kdo residues attached to lipid A, inextricably linking viability to toxicity. Here we report the construction and characterization of the nonconditional E. coli K-12 suppressor strain KPM22 that lacks Kdo and is viable despite predominantly elaborating the endotoxically inactive LPS precursor lipid IV(A). Our results challenge the established E. coli Kdo2-lipid A dogma, indicating that the previously observed and well-documented dependence of cell viability on the synthesis of Kdo stems from a lethal pleiotropy precipitated after the depletion of the carbohydrate, rather than an inherent need for the Kdo molecule itself as an indispensable structural component of the OM LPS layer. Inclusion of the inner membrane LPS transporter MsbA on a multicopy plasmid partially suppresses the lethal deltaKdo phenotype directly in the auxotrophic parent strain, suggesting increased rates of nonglycosylated lipid A transport can, in part, compensate for Kdo depletion. The unprecedented nature of a lipid IV(A) OM redefines the requisite LPS structure for viability in E. coli.

A single nucleotide exchange in the wzy gene is responsible for the semirough O6 lipopolysaccharide phenotype and serum sensitivity of Escherichia coli strain Nissle 1917

Structural analysis of lipopolysaccharide (LPS) isolated from semirough, serum-sensitive Escherichia coli strain Nissle 1917 (DSM 6601, serotype O6:K5:H1) revealed that this strain's LPS contains a bisphosphorylated hexaacyl lipid A and a tetradecasaccharide consisting of one E. coli O6 antigen repeating unit attached to the R1-type core. Configuration of the GlcNAc glycosidic linkage between O-antigen oligosaccharide and core (beta) differs from that interlinking the repeating units in the E. coli O6 antigen polysaccharide (alpha). The wa(*) and wb(*) gene clusters of strain Nissle 1917, required for LPS core and O6 repeating unit biosyntheses, were subcloned and sequenced. The DNA sequence of the wa(*) determinant (11.8 kb) shows 97% identity to other R1 core type-specific wa(*) gene clusters. The DNA sequence of the wb(*) gene cluster (11 kb) exhibits no homology to known DNA sequences except manC and manB. Comparison of the genetic structures of the wb(*)(O6) (wb(*) from serotype O6) determinants of strain Nissle 1917 and of smooth and serum-resistant uropathogenic E. coli O6 strain 536 demonstrated that the putative open reading frame encoding the O-antigen polymerase Wzy of strain Nissle 1917 was truncated due to a point mutation. Complementation with a functional wzy copy of E. coli strain 536 confirmed that the semirough phenotype of strain Nissle 1917 is due to the nonfunctional wzy gene. Expression of a functional wzy gene in E. coli strain Nissle 1917 increased its ability to withstand antibacterial defense mechanisms of blood serum. These results underline the importance of LPS for serum resistance or sensitivity of E. coli.

Surfactant protein D binds selectively to Klebsiella pneumoniae lipopolysaccharides containing mannose-rich O-antigens

Surfactant protein D (SP-D) plays important roles in the regulation of innate immune responses in the lung. We have previously shown that SP-D can agglutinate and enhance the macrophage-dependent killing of specific unencapsulated phase variants of Klebsiella pneumoniae. In the present studies, we used 16 clinical isolates of Klebsiella representing four O-serotypes and examined the interaction of SP-D with their isolated LPSs. Although SP-D bound to the core oligosaccharide of rough LPS from all isolates, it selectively bound to smooth forms of LPS expressed by O-serotypes with mannose-rich repeating units in their O-polysaccharides. SP-D was more potent in agglutinating unencapsulated phase variants of O-serotypes expressing these SP-D "reactive" O-polysaccharides, and more effectively inhibited the adhesion of these serotypes to lung epithelial cells. This novel anti-adhesion activity required the multimerization of trimeric SP-D subunits (dodecamers). Klebsiella serotypes expressing "nonreactive" LPS O-Ags were isolated at a significantly higher frequency from patients with K. pneumoniae. Our findings suggest that SP-D plays important roles in the clearance of opportunistic Gram-negative bacteria and contributes to known serotypic differences in the pathogenicity of Klebsiella through specific interactions with O-polysaccharides.

Identification of Acinetobacter isolates from species belonging to the Acinetobacter calcoaceticus-Acinetobacter baumannii complex with monoclonal antibodies specific for O Antigens of their lipopolysaccharides

The unambiguous identification of Acinetobacter strains, particularly those belonging to the Acinetobacter calcoaceticus-Acinetobacter baumannii complex, is often hindered by their close geno- and phenotypic relationships. In this study, monoclonal antibodies (MAbs) against the O antigens of the lipopolysaccharides from strains belonging to the A. calcoaceticus-A. baumannii complex were generated after the immunization of mice with heat-killed bacteria and shown by enzyme immunoassays and Western blotting to be specific for their homologous antigens. Since the A. calcoaceticus-A. baumannii complex comprises the most clinically relevant species, the MAbs were subsequently tested in dot and Western blots with proteinase K-treated lysates from a large collection of Acinetobacter isolates (n = 631) to determine whether the antibodies could be used for the reliable identification of strains from this complex. Reactivity was observed with 273 of the 504 isolates (54%) from the A. calcoaceticus-A. baumannii complex which were included in this study. Isolates which reacted positively did so with only one antibody; no reactivity was observed with isolates not belonging to the A. calcoaceticus-A. baumannii complex (n = 127). To identify additional putative O serotypes, isolates from the A. calcoaceticus-A. baumannii complex which showed no MAb reactivity were subjected to a method that enables the detection of lipid A moieties in lipopolysaccharides with a specific MAb on Western blots following acidic treatment of the membrane. By this method, additional serotypes were indeed identified, thus indicating which strains to select for future immunizations. This study contributes to the completion of a serotype-based identification scheme for Acinetobacter species, in particular, those which are presently of the most clinical importance.

O-antigen diversity among Acinetobacter baumannii strains from the Czech Republic and Northwestern Europe, as determined by lipopolysaccharide-specific monoclonal antibodies

O-antigen-specific monoclonal antibodies (MAbs) are currently being generated to develop an O-serotyping scheme for the genus Acinetobacter and to provide potent tools to study the diversity of O-antigens among Acinetobacter strains. In this report, Acinetobacter baumannii strains from the Czech Republic and from two clonal groups identified in Northwestern Europe (termed clones I and II) were investigated for their reactivity with a panel of O-antigen-specific MAbs generated against Acinetobacter strains from various species. The bacteria were characterized for their ribotype, biotype, and antibiotic susceptibility and the presence of the 8.7-kb plasmid pAN1. By using the combination of these typing profiles, the Czech strains could be classified into four previously defined groups (A. Nemec, L. Janda, O. Melter, and L. Dijkshoorn, J. Med. Microbiol. 48:287-296, 1999): two relatively homogeneous groups of multiresistant strains (termed groups A and B), a heterogeneous group of other multiresistant strains, and a group of susceptible strains. O-antigen reactivity was observed primarily with MAbs generated against Acinetobacter calcoaceticus and Acinetobacter baumannii strains. A comparison of reaction patterns confirmed the previously hypothesized clonal relationship between group A and clone I strains, which are also similar in other properties. The results show that there is limited O-antigen variability among strains with similar geno- and phenotypic characteristics and are suggestive of a high prevalence of certain A. baumannii serotypes in the clinical environment. It is also shown that O-antigen-specific MAbs are useful for the follow-up of strains causing outbreaks in hospitals.

Detection and initial characterization of novel capsular polysaccharide among diverse Campylobacter jejuni strains using alcian blue dye

We have recently demonstrated that most strains of Campylobacter jejuni produce capsular polysaccharide (CPS), which can be detected by immunoblotting with homologous Penner antisera on polyvinylidene difluoride membranes (A. V. Karlyshev, D. Linton, N. A. Gregson, A. J. Lastovica, and B. W. Wren, Mol. Microbiol. 35:529-541, 2000). In this report, we describe a universal and rapid staining procedure using Alcian blue for C. jejuni CPS, which does not rely on the availability of antisera and identifies CPS in untypeable strains. Furthermore, Alcian blue staining identified CPS in its lipid-free form directly on Tricine gels, and we demonstrate that CPS is thermostable and is accumulated in the culture supernatant in a lipid-free form. The identification of a newly described CPS and its lipid-free form in C. jejuni should prove invaluable in studying the pathogenesis and epidemiology of this important pathogen.

Comparative functional characterization in vitro of heptosyltransferase I (WaaC) and II (WaaF) from Escherichia coli

Heptosyltransferase II, encoded by the waaF gene of Escherichia coli, is a glycosyltransferase involved in the synthesis of the inner core region of lipopolysaccharide. The gene was subcloned from plasmid pWSB33 [Brabetz, W., Müller-Loennies, S., Holst, O. & Brade, H. (1997) Eur. J. Biochem. 247, 716-724] into a shuttle vector for the expression in the gram-positive host Corynebacterium glutamicum. The in vitro activity of the enzyme was investigated in comparison to that of heptosyltransferase I (WaaC) using as a source for the sugar nucleotide donor, ADP-LglyceroDmanno-heptose, a low molecular mass filtrate from a DeltawaaCF E. coli strain. Synthetic lipid A analogues varying in the acylation or phosphorylation pattern or both were tested as acceptors for the subsequent transfer of 3-deoxy-Dmanno-oct-2-ulosonic acid (Kdo) and heptose by successive action of Kdo transferase (WaaA), heptosyltransferase I (WaaC) and heptosyltransferase II (WaaF). The reaction products were characterized after separation by TLC and blotting with monoclonal antibodies specific for the acceptor, the intermediates and the final products.

Chemical and antigenic structure of the O-polysaccharide of the lipopolysaccharides from two Acinetobacter haemolyticus strains differing only in the anomeric configuration of one glycosyl residue in their O-antigens

In a previous study [Pantophlet, R., Brade, L., Dijkshoorn, L., and Brade, H. (1998) J. Clin. Microbiol. 36, 1245-1250] the O-polysaccharide of the lipopolysaccharides (LPS) from Acinetobacter haemolyticus strains 57 and 61 exhibited indistinguishable banding-patterns following Western blot and immunostaining with homologous or heterologous rabbit antiserum. In this report, the molecular basis for the observed cross-reactivity was elucidated, by determining the chemical structure of the polysaccharides by compositional analysis and NMR spectroscopy. The structures are: [sequence: see text] for strain 61 [GulpNAcA, 2-acetamido-2-deoxy-gulopyranosyluronic acid; ManpNAcA, 2-acetamido-2-deoxy-mannopyranosyluronic acid; QuipN4N, 2,4-diamino-2,4,6-trideoxy-glucopyranose; acyl (S)-3-hydroxybutyryl], thus, differing only in the anomeric configuration of the QuipN4N residue. The antigenic structures were determined by generating murine monoclonal antibodies, which were characterized by Western blot using LPS as antigen, by ELISA using LPS and de-O-acylated LPS as solid-phase antigens, and by ELISA inhibition studies using LPS, polysaccharide, and de-O-acylated LPS as inhibitors. Of the four antibodies selected, two were specific for the respective LPS moieties and two were cross-reactive. All antibodies were found to require the presence of the O-acetyl group for reactivity.

Use of a murine O-antigen-specific monoclonal antibody to identify Acinetobacter strains of unnamed genomic species 13 Sensu Tjernberg and Ursing

A monoclonal antibody against the O-antigenic polysaccharide chain of the lipopolysaccharide (LPS) of Acinetobacter strains belonging to the unnamed genomic species 13 Sensu Tjernberg and Ursing (13TU) was obtained after immunization of BALB/c mice with heat-killed bacteria and was characterized by enzyme immunoassay and Western blot analysis, by use of LPS and proteinase K-treated bacterial lysates, analyses in which the antibody was shown to be highly specific for the homologous antigen. In addition, when tested in dot and Western blots, reactivity was observed with 9 of 18 Acinetobacter strains of genomic species 13TU which had been isolated in Germany and Denmark; no reactivity was observed with strains of other genomic species, including the closely related genomic groups 1 (A. calcoaceticus), 2 (A. baumannii), and 3 (unnamed), or with other gram-negative bacteria. The antibody described here represents a convenient reagent for the simple, economical, and accurate differentiation of clinical isolates of genomic species 13TU from other Acinetobacter strains. Although the antibody does not identify all isolates of this genomic group, it is evident that it will be a useful reagent in the development of a serotyping scheme for clinical laboratories.

Identification of Acinetobacter baumannii strains with monoclonal antibodies against the O antigens of their lipopolysaccharides

Despite the emergence of Acinetobacter baumannii strains as nosocomial pathogens, simple methods for their phenotypic identification are still unavailable. Murine monoclonal antibodies specific for the O-polysaccharide moiety of the lipopolysaccharide (LPS) of two A. baumannii strains were obtained after immunization with heat-killed bacteria. The monoclonal antibodies were characterized by enzyme immunoassay and by Western and dot blot analyses and were investigated for their potential use for the identification of A. baumannii strains. The antibodies reacted with 46 of the 80 A. baumannii clinical isolates that were investigated, and reactivity was observed with 11 of 14 strains which were isolated during outbreaks in different northwestern European cities; no reactivity was observed with Acinetobacter strains of other genomic species, including the closely related genomic species 1 (Acinetobacter calcoaceticus), 3, and 13 sensu Tjernberg and Ursing, or with other gram-negative bacterial strains. The results show that O-antigen-specific monoclonal antibodies such as the ones described are convenient reagents which can be used to identify Acinetobacter strains in clinical and research laboratories.