Incorporation of N-acetylneuraminic acid into Haemophilus somnus lipooligosaccharide (LOS): enhancement of resistance to serum and reduction of LOS antibody binding

Contribution of Hfq to gene regulation and virulence in Histophilus somni

Histophilus somni is one of the predominant bacterial pathogens responsible for bovine respiratory and systemic diseases in cattle. Despite the identification of numerous H. somni virulence factors, little is known about the regulation of such factors. The post-transcriptional regulatory protein Hfq may play a crucial role in regulation of components that affect bacterial virulence. The contribution of Hfq to H. somni phenotype and virulence was investigated following creation of an hfq deletion mutant of H. somni strain 2336 (designated H. somni 2336Δhfq). A comparative analysis of the mutant to the wild-type strain was carried out by examining protein and carbohydrate phenotype, RNA sequence, intracellular survival in bovine monocytes, serum susceptibility, and virulence studies in mouse and calf models. H. somni 2336Δhfq exhibited a truncated lipooligosaccharide (LOS) structure, with loss of sialylation. The mutant demonstrated increased susceptibility to intracellular and serum-mediated killing compared to the wild-type strain. Transcriptomic analysis displayed significant differential expression of 832 upregulated genes and 809 downregulated genes in H. somni 2336Δhfq compared to H. somni strain 2336, including significant downregulation of lsgB and licA, which contribute to LOS oligosaccharide synthesis and sialylation. A substantial number of differentially expressed genes were associated with polysaccharide synthesis and other proteins that could influence virulence. The H. somni 2336Δhfq mutant strain was attenuated in a mouse septicemia model and somewhat attenuated in a calf intrabronchial challenge model. H. somni was recovered less frequently from nasopharyngeal swabs, endotracheal aspirates, and lung tissues of calves challenged with H. somni 2336Δhfq compared to the wild-type strain, and the percentage of abnormal lung tissue in calves challenged with H. somni 2336Δhfq was lower than in calves challenged with the wild-type strain. In conclusion, our results support that Hfq accounts for the regulation of H. somni virulence factors.

Enhanced phagocytosis and complement-mediated killing of Mannheimia haemolytica serotype 1 following in-frame CMP-sialic acid synthetase (neuA) gene deletion

IMPORTANCE

The Gram-negative coccobacillus Mannheimia haemolytica is a natural inhabitant of the upper respiratory tract in ruminants and the most common bacterial agent involved in bovine respiratory disease complex development. Key virulence factors harbored by M. haemolytica are leukotoxin, lipopolysaccharide, capsule, adhesins, and neuraminidase which are involved in evading innate and adaptive immune responses. In this study, we have shown that CMP-sialic acid synthetase (neuA) is necessary for the incorporation of sialic acid onto the membrane, and inactivation of neuA results in increased phagocytosis and complement-mediated killing of M. haemolytica, thus demonstrating that sialylation contributes to the virulence of M. haemolytica.

The role of uspE in virulence and biofilm formation by Histophilus somni

UspE is a global regulator in Escherichia coli. To study the function of Histophilus somni uspE, strain 2336::TnuspE was identified from a bank of mutants generated with EZ::Tn5™<KAN-2> Tnp Transposome™ that were biofilm deficient. The 2336::TnuspE mutant was highly attenuated in mice, the electrophoretic profile of its lipooligosaccharide (LOS) indicated the LOS was truncated, and the mutant was significantly more serum-sensitive compared to the wildtype strain. In addition to forming a deficient biofilm, exopolysaccharide (EPS) production was also compromised, but the electrophoretic profile of outer membrane proteins was not altered. RNA sequence analysis revealed that the transcription levels of some stress response chaperones, transport proteins, and a large number of ribosomal protein genes in 2336::TnuspE were significantly differentially regulated compared to strain 2336. Therefore, uspE may differentially function in direct and indirect expression of H. somni genes, but its attenuation may be linked to poor biofilm formation and rapid clearance of the bacteria resulting from a compromised LOS structure. Our results support that uspE is a global stress regulatory gene in H. somni.

Structural and Biosynthetic Diversity of Nonulosonic Acids (NulOs) That Decorate Surface Structures in Bacteria

Nonulosonic acids (NulOs) are a diverse family of 9-carbon α-keto acid sugars that are involved in a wide range of functions across all branches of life. The family of NulOs includes the sialic acids as well as the prokaryote-specific NulOs. Select bacteria biosynthesize the sialic acid N-acetylneuraminic acid (Neu5Ac), and the ability to produce this sugar and its subsequent incorporation into cell-surface structures is implicated in a variety of bacteria-host interactions. Furthermore, scavenging of sialic acid from the environment for energy has been characterized across a diverse group of bacteria, mainly human commensals and pathogens. In addition to sialic acid, bacteria have the ability to biosynthesize prokaryote-specific NulOs, of which there are several known isomers characterized. These prokaryotic NulOs are similar in structure to Neu5Ac but little is known regarding their role in bacterial physiology. Here, we discuss the diversity in structure, the biosynthesis pathways, and the functions of bacteria-specific NulOs. These carbohydrates are phylogenetically widespread among bacteria, with numerous structurally unique modifications recognized. Despite the diversity in structure, the NulOs are involved in similar functions such as motility, biofilm formation, host colonization, and immune evasion.

The Role of luxS in Histophilus somni Virulence and Biofilm Formation

S-Ribosylhomocysteinase (LuxS) is required for the synthesis of the autoinducer-2 (AI-2) quorum-sensing signaling molecule in many Gram-negative bacteria. The bovine (and ovine) opportunistic pathogen Histophilus somni contains luxS and forms a biofilm containing an exopolysaccharide (EPS) in the matrix. Since biofilm formation is regulated by quorum sensing in many bacteria, the roles of luxS in H. somni virulence and biofilm formation were investigated. Although culture supernatants from H. somni were ineffective at inducing bioluminescence in the Vibrio harveyi reporter strain BB170, H. somni luxS complemented the biosynthesis of AI-2 in the luxS-deficient Escherichia coli strain DH5α. H. somni strain 2336 luxS was inactivated by transposon mutagenesis. RNA expression profiles revealed that many genes were significantly differentially expressed in the luxS mutant compared to that in the wild-type, whether the bacteria were grown planktonically or in a biofilm. Furthermore, the luxS mutant had a truncated and asialylated lipooligosaccharide (LOS) and was substantially more serum sensitive than the wild-type. Not surprisingly, the luxS mutant was attenuated in a mouse model for H. somni virulence, and some of the altered phenotypes were partially restored after the mutation was complemented with a functional luxS However, no major differences were observed between the wild-type and the luxS mutant in regard to outer membrane protein profiles, biofilm formation, EPS production, or intracellular survival. These results indicate that luxS plays a role in H. somni virulence in the context of LOS biosynthesis but not biofilm formation or other phenotypic properties examined.

The Glaesserella parasuis phosphoglucomutase is partially required for lipooligosaccharide synthesis

Lipooligosaccharides (LOSs) are virulence determinants of Glaesserella parasuis, a pathogen of the respiratory tract of pigs. We previously reported that disruption of the galU or galE gene in G. parasuis results in increased sensitivity to porcine serum, indicating that the galactose catabolism pathway is required for polysaccharide formation in G. parasuis. Here, we evaluated the role of the HAPS_0849 gene in LOS synthesis. The G. parasuis SC096 HAPS_0849 mutant produced a highly truncated LOS molecule, although a small fraction of intact LOS was still observed, and this mutant was found to be more sensitive to serum than the parental strain. HAPS_0849 was overexpressed and purified for biochemical assays, and this protein exhibited phosphoglucomutase (PGM) activity. Heterologous expression of a pgm gene from Escherichia coli in the HAPS_0849 mutant led to restoration of the wild-type LOS glycoform, further demonstrating the PGM function of HAPS_0849 in G. parasuis. The autoagglutination and biofilm formation ability of this strain were also investigated. Disruption of HAPS_0849 led to an increased tendency to autoagglutinate and form more biofilms, and these enhanced phenotypes were observed in the absence of glucose. In addition, LOSs from HAPS_0849, galU and lgtB mutants had similar truncated glycoforms, while LOSs from the galE and lex-1 mutants exhibited another type of defective LOS pattern. These findings imply that HAPS_0849 may function upstream of GalU in the generation of glucose 1-phosphate. In conclusion, our results preliminarily described the functions of HAPS_0849 in G. parasuis, and this gene was partially required for LOS synthesis.

Reprioritization of biofilm metabolism is associated with nutrient adaptation and long-term survival of Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHI) is a human-restricted pathogen with an essential requirement for heme-iron acquisition. We previously demonstrated that microevolution of NTHI promotes stationary phase survival in response to transient heme-iron restriction. In this study, we examine the metabolic contributions to biofilm formation using this evolved NTHI strain, RM33. Quantitative analyses identified 29 proteins, 55 transcripts, and 31 metabolites that significantly changed within in vitro biofilms formed by RM33. The synthesis of all enzymes within the tryptophan and glycogen pathways was significantly increased in biofilms formed by RM33 compared with the parental strain. In addition, increases were observed in metabolite transport, adhesin production, and DNA metabolism. Furthermore, we observed pyruvate as a pivotal point in the metabolic pathways associated with changes in cAMP phosphodiesterase activity during biofilm formation. Taken together, changes in central metabolism combined with increased stores of nutrients may serve to counterbalance nutrient sequestration.

Haemophilus parasuis α-2,3-sialyltransferase-mediated lipooligosaccharide sialylation contributes to bacterial pathogenicity

Bacterial lipooligosaccharide (LOS) is an important virulence-associated factor, and its sialylation largely confers its ability to mediate cell adhesion, invasion, inflammation, and immune evasion. Here, we investigated the function of the Haemophilus parasuis α-2,3-sialyltransferase gene, lsgB, which determines the terminal sialylation of LOS, by generating a lsgB deletion mutant as well as a complementation strain. Our data indicate a direct effect of lsgB on LOS sialylation and reveal important roles of lsgB in promoting the pathogenicity of H. parasuis, including adhesion to and invasion of porcine cells in vitro, bacterial load and survival in vivo, as well as a contribution to serum resistance. These observations highlight the function of lsgB in mediating LOS sialylation and more importantly its role in H. parasuis infection. These findings provide a more profound understanding of the pathogenic mechanism of this disease-causing bacterium.

Structural and functional characterization of a modified legionaminic acid involved in glycosylation of a bacterial lipopolysaccharide

Nonulosonic acids (NulOs) are a diverse family of α-keto acid carbohydrates present across all branches of life. Bacteria biosynthesize NulOs among which are several related prokaryotic-specific isomers and one of which, N-acetylneuraminic acid (sialic acid), is common among all vertebrates. Bacteria display various NulO carbohydrates on lipopolysaccharide (LPS), and the identities of these molecules tune host-pathogen recognition mechanisms. The opportunistic bacterial pathogen Vibrio vulnificus possesses the genes for NulO biosynthesis; however, the structures and functions of the V. vulnificus NulO glycan are unknown. Using genetic and chemical approaches, we show here that the major NulO produced by a clinical V. vulnificus strain CMCP6 is 5-N-acetyl-7-N-acetyl-d-alanyl-legionaminic acid (Leg5Ac7AcAla). The CMCP6 strain could catabolize modified legionaminic acid, whereas V. vulnificus strain YJ016 produced but did not catabolize a NulO without the N-acetyl-d-alanyl modification. In silico analysis suggested that Leg5Ac7AcAla biosynthesis follows a noncanonical pathway but appears to be present in several bacterial species. Leg5Ac7AcAla contributed to bacterial outer-membrane integrity, as mutant strains unable to produce or incorporate Leg5Ac7AcAla into the LPS have increased membrane permeability, sensitivity to bile salts and antimicrobial peptides, and defects in biofilm formation. Using the crustacean model, Artemia franciscana, we demonstrate that Leg5Ac7AcAla-deficient bacteria have decreased virulence potential compared with WT. Our data indicate that different V. vulnificus strains produce multiple NulOs and that the modified legionaminic acid Leg5Ac7AcAla plays a critical role in the physiology, survivability, and pathogenicity of V. vulnificus CMCP6.

Histophilus somni Survives in Bovine Macrophages by Interfering with Phagosome-Lysosome Fusion but Requires IbpA for Optimal Serum Resistance

Histophilus somni is capable of intracellular survival within professional phagocytic cells, but the mechanism of survival is not understood. The Fic motif within the direct repeat (DR1)/DR2 domains of the IbpA fibrillary network protein of H. somni is cytotoxic to epithelial and phagocytic cells, which may interfere with the bactericidal activity of these cells. To determine the contribution of IbpA and Fic to resistance to host defenses, H. somni strains and mutants that lacked all or a region of ibpA (including the DR1/DR2 regions) were tested for survival in bovine monocytic cells and for serum susceptibility. An H. somni mutant lacking IbpA, but not the DR1/DR2 region within ibpA, was more susceptible to killing by antiserum than the parent, indicating that the entire protein was associated with serum resistance. H. somni strains expressing IbpA replicated in bovine monocytes for at least 72 h and were toxic for these cells. Virulent strain 2336 mutants lacking the entire ibpA gene or both DR1 and DR2 were not toxic to the monocytes but still survived within the monocytes for at least 72 h. Monitoring of intracellular trafficking of H. somni with monoclonal antibodies to phagosomal markers indicated that the early phagosomal marker early endosome antigen 1 colocalized with all isolates tested, but only strains that could survive intracellularly did not colocalize with the late lysosomal marker lysosome-associated membrane protein 2 and prevented the acidification of phagosomes. These results indicated that virulent isolates of H. somni were capable of surviving within phagocytic cells through interference in phagosome-lysosome maturation. Therefore, H. somni may be considered a permissive intracellular pathogen.

A Predominant Clonal Thromboembolic Meningoencephalitis Group of Histophilus somni Assigned by Major Outer Membrane Protein Gene Sequencing and Pulsed-Field Gel Electrophoresis

Histophilus somni, a member of the family Pasteurellaceae, causes a variety of diseases, including thromboembolic meningoencephalitis (TEME) and respiratory diseases, which result in considerable economic losses to the cattle and sheep industries. In this study, 132 chronologically diverse isolates from cattle in Japan and 68 isolates from other countries comprising 49 from cattle and 19 from sheep were characterized using major outer membrane protein (MOMP) gene sequence and pulsed-field gel electrophoresis (PFGE) analyses. The H. somni isolates formed nine MOMP genetic clades (clade Ia, Ib, and II-VIII) and 10 PFGE clusters (HS1-HS10). Except for two (1.0%), all isolates fell into one of the nine MOMP genetic clades, while 62 (31.0%) isolates belonged to no PFGE cluster. MOMP genetic clade Ia and PFGE cluster HS1 were the major groups, and all HS1 isolates possessed the clade Ia MOMP gene. Isolates from TEME cases were significantly associated with these major groups (chi-square test, p < 0.0001), as 88.2% of the TEME isolates belonged to MOMP genetic clade Ia and PFGE cluster HS1, which formed the most predominant clonal group. After an inactivated vaccine using an HS1 strain with the clade Ia MOMP gene was introduced in Japan in late 1989, the number of TEME cases and isolates assigned into the clonal group decreased simultaneously. However, the proportions of clade Ia and cluster HS1 isolates from TEME cases remained high after 1990. These results suggest a close association of TEME with PFGE cluster HS1 and MOMP genetic clade Ia, and imply the presence of factors or characteristics commonly possessed by those strains that contribute to the development of TEME.

Exopolysaccharide Production and Biofilm Formation by Histophilus somni

The biofilm matrix of Histophilus somni is a complex architecture that differs substantially in structure between a pathogenic and commensal isolate examined. Overall, most pathogenic isolates produce more biofilm than commensal isolates. A major component of the biofilm is exopolysaccharide (EPS), which is also produced in greater quantity in the pathogenic isolate than in the commensal isolate studied. The EPS is composed of a D-mannan polymer, with occasional galactose residues present on side chains, similar in composition to that of yeast mannan. When grown in the presence of sialic acid, the biofilm EPS becomes sialylated and the amino sugars N-acetylglucosamine and N-acetylgalactosamine can be detected. In vitro biofilm formation follows a typical 4-stage growth curve, characterized by attachment, growth, maturation, and detachment. Following experimental challenge, formation of an H. somni biofilm has been demonstrated in cardiopulmonary tissue, often with Pasteurella multocida cohabitating the biofilm. A recently developed diagnostic test can detect antibodies to the EPS only in animals with systemic disease due to H. somni and is therefore capable of distinguishing between healthy animals colonized with H. somni and animals with systemic disease.

Presumable role of outer membrane proteins of Salmonella containing sialylated lipopolysaccharides serovar Ngozi, sv. Isaszeg and subspecies arizonae in determining susceptibility to human serum

Background

The O48 group comprises Salmonella bacteria containing sialic acid in the lipopolysaccharide (LPS). Bacteria with sialylated surface structures are described as pathogens that avoid immunological response of the host by making similar their surface antigens to the host’s tissues (molecular mimicry). It is known that the smooth-type LPS of Salmonella enterica and outer membrane proteins (OMP) PgtE, PagC and Rck mediate serum resistant phenotype by affecting complement system (C). The aim of this study was to investigate C3 component activation by Salmonella O48 LPS and OMP.

Findings

In the present study, we examined C3 component deposition on the three Salmonella O48 strains: S. enterica subspecies enterica serovar Ngozi, S. enterica subsp. enterica sv. Isaszeg, and S.enterica subsp. arizonae containing sialic acid in the O-specific part of LPS. The greatest C3 deposition occurred on Salmonella sv. Isaszeg cells (p < 0.005) as well as on their LPS (low content of sialic acid in LPS) (p < 0.05) after 45 min of incubation in 50% human serum. Weaker C3 deposition ratio on the Salmonella sv. Ngozi (high content of sialic acid in LPS) and Salmonella subsp. arizonae (high content of sialic acid in LPS) cells correlated with the lower C3 activation on their LPS. Immunoblotting revealed that OMP isolated from the tested strains also bound C3 protein fragments.

Conclusions

We suggest that activation of C3 serum protein is dependent on the sialic acid contents in the LPS as well as on the presence of OMP in the range of molecular masses of 35–48 kDa.

Map-based comparative genomic analysis of virulent haemophilus parasuis serovars 4 and 5

Haemophilus parasuis is a commensal bacterium of the upper respiratory tract of healthy pigs. However, in conjunction with viral infections in immunocompromised animals H. parasuis can transform into a pathogen that is responsible for causing Glasser's disease which is typically characterized by fibrinous polyserositis, polyarthritis, meningitis and sometimes acute pneumonia and septicemia in pigs. Haemophilus parasuis serovar 5 is highly virulent and more frequently isolated from respiratory and systemic infection in pigs. Recently a highly virulent H. parasuis serovar 4 was isolated from the tissues of diseased pigs. To understand the differences in virulence and virulence-associated genes between H. parasuis serovar 5 and highly virulent H. parasuis serovar 4 strains, a genomic library was generated by TruSeq preparation and sequenced on Illumina HiSeq 2000 obtaining 50 bp PE reads. A three-way comparative genomic analysis was conducted between two highly virulent H. parasuis serovar 4 strains and H. parasuis serovar 5. Haemophilus parasuis serovar 5 GenBank isolate SH0165 (GenBank accession number CP001321.1) was used as reference strain for assembly.

Results of these analysis revealed the highly virulent H. parasuis serovar 4 lacks genes encoding for, glycosyl transferases, polysaccharide biosynthesis protein capD, spore coat polysaccharide biosynthesis protein C, polysaccharide export protein and sialyltransferase which can modify the lipopolysaccharide forming a short-chain LPS lacking O-specific polysaccharide chains often referred to as lipooligosaccharide (LOS). In addition, it can modify the outer membrane protein (OMP) structure. The lack of sialyltransferase significantly reduced the amount of sialic acid incorporated into LOS, a major and essential component of the cell wall and an important virulence determinant. These molecules may be involved in various stages of pathogenesis through molecular mimicry and by causing host cell cytotoxicity, reduced inflammatory and immunological response to infection with this organism. The mechanism by which sialyation of LPS contributes to virulence is a key to understanding the pathogenesis of this highly virulent H. parasuis serovar 4. This analysis also revealed the presence of virulence associated genes similar to the MerR family transcriptional regulators, macrophage infectivity potentiator protein, hemolysin, opacity associated protein, toxin antitoxin system, and virulence associated protein D and colicins. Haemophilus parasuis serovar 4 variants also possess extensive metal ion uptake and regulation mechanism which controls various virulence and virulence associated genes. A combination of virulence associated factors and/or genes and proteins with overlapping functions may be responsible for the apparent enhanced virulence of this organism.

The extensive structural modification of LOS and OMP of variant H. parasuis serovar 4 strains appear to aid in nasal colonization, are associated with the organisms' ability to evade the host immune response and provide serum-resistance. In addition, the combination of capsule modification and phase variation due to LOS substitutions could help variant H. parasuis serovar 4 transform into a highly virulent pathogen. Based on these results, the variant H. parasuis serovar 4 strains harbor a diverse repertoire of virulence associated genes which have not been previously reported.

The Many Facets of Lipooligosaccharide as a Virulence Factor for Histophilus somni

The lipooligosaccharide (LOS) of Histophilus somni is a multifaceted molecule that provides critical protection to the bacterium against host defenses, may act as an adhesin, and like similar molecules of gram-negative bacteria, is an endotoxin that signals through toll-like receptor 4 and NF-κB to cause inflammation. The lipid A component is responsible for the endotoxic and apoptotic activity of the LOS. The H. somni LOS lacks O-side chains typically characteristic of gram-negative bacteria that have lipopolysaccharide, but has a complex, microheterogeneous outer core. The LOS of disease isolates is capable of undergoing structural and antigenic phase variation of its outer core due to slip-strand mispairing of glycosyltransferase genes that contain repetitive sequences of DNA base pairs. Such variation enables the bacteria to evade bactericidal antibodies made to oligosaccharide antigens. In addition, the LOS can be decorated with phase-variable phosphorylcholine (ChoP), which binds to platelet-activating factor receptor on host cells, thereby aiding in colonization of the upper respiratory tract. However, ChoP is likely not expressed when the bacteria are in systemic sites because ChoP also binds to C-reactive protein, resulting in activation of host complement and promoting bactericidal activity. The structure of some LOS outer core chains is identical to oligosaccharides on host glycosphingolipids of red blood cells, other cells, and merconium (lacto-N-neotetraose, lacto-N-biose, N-acetyllactosamine, etc.). Furthermore, terminal galactose residues on LOS and elsewhere are decorated with sialic acid, which blocks antibody binding, activation of complement, phagocytosis, and intracellular killing. Therefore, antigenic mimicry of host antigens is an important defense mechanism provided by the oligosaccharide component of the LOS to avoid innate and adaptive host defense mechanisms. However, some strains of H. somni isolated from the bovine genital tract, particularly the normal bovine prepuce, are incapable of LOS phase variation, sialylation of the LOS, and expression of ChoP. At least 1 such strain has been shown to be avirulent, underscoring the importance of the LOS as a virulence factor, although this strain is deficient in other factors as well. The structure and arrangement of the inner core glycoses (heptose and 3-deoxy-D-manno-2-octulosnic acid) is remarkably similar to the inner core oligosaccharide on some strains of Neisseria spp., and mutants that contain a truncated LOS oligosaccharide are considerably more serum-sensitive than the parent strain. Therefore, the LOS is a critical component that enables H. somni to resist host defenses and cause disease.

Mannose receptor and macrophage galactose-type lectin are involved in Bordetella pertussis mast cell interaction

Mast cells are crucial in the development of immunity against Bordetella pertussis, and the function of TLRs in this process has been investigated. Here, the interaction between mast cells and B. pertussis with an emphasis on the role of CLRs is examined. In this study, two CLRs, MGL and MR, were detected for the first time on the surface of mast cells. The involvement of MR and MGL in the stimulation of mast cells by heat-inactivated BP was investigated by the use of blocking antibodies and specific carbohydrate ligands. The cell wall LOS of BP was also isolated to explore its role in this interaction. Mast cells stimulated with heat-inactivated BP or BP LOS induced TNF-α, IL-6, and IFN-γ secretion, which was suppressed by blocking MR or MGL. Inhibition of CLRs signaling during BP stimulation affected the ability of mast cells to promote cytokine secretion in T cells but had no effect on the cell-surface expression of ICAM1. Blocking MR or MGL suppressed BP-induced NF-κB expression but not ERK phosphorylation. Syk was involved in the CLR-mediated activation of mast cells by BP. Bacterial recognition by immune cells has been predominantly attributed to TLRs; in this study, the novel role of CLRs in the BP-mast cell interaction is highlighted.

Effects of lipooligosaccharide inner core truncation on bile resistance and chick colonization by Campylobacter jejuni

Campylobacter jejuni is the most common bacterium that causes diarrhea worldwide, and chickens are considered the main reservoir of this pathogen. This study investigated the effects of serial truncation of lipooligosaccharide (LOS), a major component of the outer membrane of C. jejuni, on its bile resistance and intestinal colonization ability in chickens. Genes encoding manno-heptose synthetases or glycosyltransferases were inactivated to generate isogenic mutants. Serial truncation of the LOS core oligosaccharide caused a stepwise increase in susceptibilities of two C. jejuni strains, NCTC 11168 and 81-176, to bile acids. Inactivation of hldE, hldD, or waaC caused severe truncation of the core oligosaccharide, which greatly increased the susceptibility to bile acids. Both wild-type strains grew normally in chicken intestinal extracts, whereas the mutants with severe oligosaccharide truncation were not detected 12 h after inoculation. These mutants attained viable bacterial counts in the bile acid-free extracts 24 h after inoculation. The wild-type strain 11-164 was present in the cecal contents at >10(7) CFU/g on 5 days after challenge infection and after this time period, whereas its hldD mutant was present at <10(3) CFU/g throughout the experimental period. Trans-complementation of the hldD mutant with the wild-type hldD allele completely restored the in vivo colonization level to that of the wild-type strain. Mutants with a shorter LOS had higher hydrophobicities. Thus, the length of the LOS core oligosaccharide affected the surface hydrophobicity and bile resistance of C. jejuni as well as its ability to colonize chicken intestines.

Decoration of Histophilus somni lipooligosaccharide with N-acetyl-5-neuraminic acid enhances bacterial binding of complement factor H and resistance to killing by serum and polymorphonuclear leukocytes

The incorporation of N-acetyl-5-neuraminic acid (Neu5Ac), or sialic acid, onto surface components of some bacterial species may enhance their virulence. We have previously shown that Neu5Ac can be incorporated onto the lipooligosaccharide (LOS) of the bovine pathogen Histophilus somni, resulting in diminished antibody binding and enhanced serum resistance (Inzana et al., 2002. Infect. Immun. 70, 4870). In the present study, we assessed the effect of sialylation of H. somni LOS on the interaction with bovine innate host defenses. Incubation of non-sialylated H. somni with pre-colostral calf serum (PCS) resulted in dose-dependent, complement-mediated killing of the bacteria by the alternative pathway. However, sialylated H. somni was significantly more resistant to killing at any of the concentrations of PCS used. Sialylated H. somni LOS activated and consumed less complement than non-sialylated LOS, as determined by reduction in hemolysis of opsonized red blood cells, and by Western blotting of C(3) activation products. Sialylated H. somni bound more factor H and iC(3)b and less C(3) than non-sialylated bacteria, as determined by enzyme-linked immunosorbent assay, supporting the deficiencies observed in complement activation and consumption by sialylated LOS. Sialylation of H. somni LOS inhibited both polymorphonuclear leukocyte phagocytosis of (3)H-thymidine-labeled bacteria and intracellular killing of the bacteria, compared to non-sialylated bacteria. Furthermore, sialylated H. somni bound less non-specific antibodies in normal bovine sera than non-sialylated bacteria. Therefore, sialylation of H. somni LOS had profound effects on resistance of the bacteria to innate bovine host defenses, which should be taken into consideration during in vitro studies of H. somni.

The role of lipooligosaccharide phosphorylcholine in colonization and pathogenesis of Histophilus somni in cattle

Histophilus somni is a Gram-negative bacterium and member of the Pasteurellaceae that is responsible for respiratory disease and other systemic infections in cattle. One of the bacterium’s virulence factors is antigenic phase variation of its lipooligosaccharide (LOS). LOS antigenic variation may occur through variation in composition or structure of glycoses or their substitutions, such as phosphorylcholine (ChoP). However, the role of ChoP in the pathogenesis of H. somni disease has not been established. In Haemophilus influenzae ChoP on the LOS binds to platelet activating factor on epithelial cells, promoting bacterial colonization of the host upper respiratory tract. However, ChoP is not expressed in the blood as it also binds C-reactive protein, resulting in complement activation and killing of the bacteria. In order to simulate the susceptibility of calves with suppressed immunity due to stress or previous infection, calves were challenged with bovine herpes virus-1 or dexamethazone 3 days prior to challenge with H. somni. Following challenge, expression of ChoP on the LOS of 2 different H. somni strains was associated with colonization of the upper respiratory tract. In contrast, lack of ChoP expression was associated with bacteria recovered from systemic sites. Histopathology of cardiac tissue from myocarditis revealed lesions containing bacterial clusters that appeared similar to a biofilm. Furthermore, some respiratory cultures contained substantial numbers of Pasteurella multocida, which were not present on preculture screens. Subsequent biofilm experiments have shown that H. somni and P. multocida grow equally well together in a biofilm, suggesting a commensal relationship may exist between the two species. Our results also showed that ChoP contributed to, but was not required for, adhesion to respiratory epithelial cells. In conclusion, expression of ChoP on H. somni LOS contributed to colonization of the bacteria to the host upper respiratory tract, but phase variable loss of ChoP expression may help the bacteria survive systemically.

Distribution of genes involved in sialic acid utilization in strains of Haemophilus parasuis

Haemophilus parasuis is a porcine respiratory pathogen, well known as the aetiological agent of Glässer's disease. H. parasuis comprises strains of different virulence, but the virulence factors of this bacterium are not well defined. A neuraminidase activity has been previously detected in H. parasuis, but the role of sialylation in the virulence of this bacterium has not been studied. To explore the relationship between sialic acid (Neu5Ac) and virulence, we assessed the distribution of genes involved in sialic acid metabolism in 21 H. parasuis strains from different clinical origins (including nasal and systemic isolates). The neuraminidase gene nanH, together with CMP-Neu5Ac synthetase and sialyltransferase genes neuA, siaB and lsgB, were included in the study. Neuraminidase activity was found to be common in H. parasuis isolates, and the nanH gene from 12 isolates was expressed in Escherichia coli and further characterized. Sequence analysis showed that the NanH predicted protein contained the motifs characteristic of the catalytic site of sialidases. While an association between the presence of nanH and the different origins of the strains was not detected, the lsgB gene was predominantly present in the systemic isolates, and was not amplified from any of the nasal isolates tested. Analysis of the lipooligosaccharide (LOS) from reference strains Nagasaki (virulent, lsgB(+)) and SW114 (non-virulent, lsgB(-)) showed the presence of sialic acid in the LOS from the Nagasaki strain, supporting the role of sialylation in the virulence of this bacterial pathogen. Further studies are needed to clarify the role of sialic acid in the pathogenicity of H. parasuis.

Sialylation of lipooligosaccharides is dispensable for the virulence of Haemophilus ducreyi in humans

Sialylated glycoconjugates on the surfaces of mammalian cells play important roles in intercellular communication and self-recognition. The sialic acid preferentially expressed in human tissues is N-acetylneuraminic acid (Neu5Ac). In a process called molecular mimicry, many bacterial pathogens decorate their cell surface glycolipids with Neu5Ac. Incorporation of Neu5Ac into bacterial glycolipids promotes bacterial interactions with host cell receptors called Siglecs. These interactions affect bacterial adherence, resistance to serum killing and phagocytosis, and innate immune responses. Haemophilus ducreyi, the etiologic agent of chancroid, expresses lipooligosaccharides (LOS) that are highly sialylated. However, an H. ducreyi sialyltransferase (lst) mutant, whose LOS contain reduced levels of Neu5Ac, is fully virulent in human volunteers. Recently, a second sialyltransferase gene (Hd0053) was discovered in H. ducreyi, raising the possibility that Hd0053 compensated for the loss of lst during human infection. CMP-Neu5Ac is the obligate nucleotide sugar donor for all bacterial sialyltransferases; LOS derived from an H. ducreyi CMP-Neu5Ac synthetase (neuA) mutant has no detectable Neu5Ac. Here, we compared an H. ducreyi neuA mutant to its wild-type parent in several models of pathogenesis. In human inoculation experiments, the neuA mutant formed papules and pustules at rates that were no different than those of its parent. When grown in media with and without Neu5Ac supplementation, the neuA mutant and its parent had similar phenotypes in bactericidal, macrophage uptake, and dendritic cell activation assays. Although we cannot preclude a contribution of LOS sialylation to ulcerative disease, these data strongly suggest that sialylation of LOS is dispensable for H. ducreyi pathogenesis in humans.

Horizontal gene transfer in Histophilus somni and its role in the evolution of pathogenic strain 2336, as determined by comparative genomic analyses

BACKGROUND

Pneumonia and myocarditis are the most commonly reported diseases due to Histophilus somni, an opportunistic pathogen of the reproductive and respiratory tracts of cattle. Thus far only a few genes involved in metabolic and virulence functions have been identified and characterized in H. somni using traditional methods. Analyses of the genome sequences of several Pasteurellaceae species have provided insights into their biology and evolution. In view of the economic and ecological importance of H. somni, the genome sequence of pneumonia strain 2336 has been determined and compared to that of commensal strain 129Pt and other members of the Pasteurellaceae.

RESULTS

The chromosome of strain 2336 (2,263,857 bp) contained 1,980 protein coding genes, whereas the chromosome of strain 129Pt (2,007,700 bp) contained only 1,792 protein coding genes. Although the chromosomes of the two strains differ in size, their average GC content, gene density (total number of genes predicted on the chromosome), and percentage of sequence (number of genes) that encodes proteins were similar. The chromosomes of these strains also contained a number of discrete prophage regions and genomic islands. One of the genomic islands in strain 2336 contained genes putatively involved in copper, zinc, and tetracycline resistance. Using the genome sequence data and comparative analyses with other members of the Pasteurellaceae, several H. somni genes that may encode proteins involved in virulence (e.g., filamentous haemaggutinins, adhesins, and polysaccharide biosynthesis/modification enzymes) were identified. The two strains contained a total of 17 ORFs that encode putative glycosyltransferases and some of these ORFs had characteristic simple sequence repeats within them. Most of the genes/loci common to both the strains were located in different regions of the two chromosomes and occurred in opposite orientations, indicating genome rearrangement since their divergence from a common ancestor.

CONCLUSIONS

Since the genome of strain 129Pt was ~256,000 bp smaller than that of strain 2336, these genomes provide yet another paradigm for studying evolutionary gene loss and/or gain in regard to virulence repertoire and pathogenic ability. Analyses of the complete genome sequences revealed that bacteriophage- and transposon-mediated horizontal gene transfer had occurred at several loci in the chromosomes of strains 2336 and 129Pt. It appears that these mobile genetic elements have played a major role in creating genomic diversity and phenotypic variability among the two H. somni strains.

Identification, structure, and characterization of an exopolysaccharide produced by Histophilus somni during biofilm formation

Background

Histophilus somni, a gram-negative coccobacillus, is an obligate inhabitant of bovine and ovine mucosal surfaces, and an opportunistic pathogen responsible for respiratory disease and other systemic infections in cattle and sheep. Capsules are important virulence factors for many pathogenic bacteria, but a capsule has not been identified on H. somni. However, H. somni does form a biofilm in vitro and in vivo, and the biofilm matrix of most bacteria consists of a polysaccharide.

Results

Following incubation of H. somni under growth-restricting stress conditions, such as during anaerobiosis, stationary phase, or in hypertonic salt, a polysaccharide could be isolated from washed cells or culture supernatant. The polysaccharide was present in large amounts in broth culture sediment after H. somni was grown under low oxygen tension for 4-5 days (conditions favorable to biofilm formation), but not from planktonic cells during log phase growth. Immuno-transmission electron microscopy showed that the polysaccharide was not closely associated with the cell surface, and was of heterogeneous high molecular size by gel electrophoresis, indicating it was an exopolysaccharide (EPS). The EPS was a branched mannose polymer containing some galactose, as determined by structural analysis. The mannose-specific Moringa M lectin and antibodies to the EPS bound to the biofilm matrix, demonstrating that the EPS was a component of the biofilm. The addition of N-acetylneuraminic acid to the growth medium resulted in sialylation of the EPS, and increased biofilm formation. Real-time quantitative reverse transcription-polymerase chain reaction analyses indicated that genes previously identified in a putative polysaccharide locus were upregulated when the bacteria were grown under conditions favorable to a biofilm, compared to planktonic cells.

Conclusions

H. somni is capable of producing a branching, mannose-galactose EPS polymer under growth conditions favorable to the biofilm phase of growth, and the EPS is a component of the biofilm matrix. The EPS can be sialylated in strains with sialyltransferase activity, resulting in enhanced density of the biofilm, and suggesting that EPS and biofilm formation may be important to persistence in the bovine host. The EPS may be critical to virulence if the biofilm state is required for H. somni to persist in systemic sites.

PmST2: a novel Pasteurella multocida glycolipid α2-3-sialyltransferase

Pasteurella multocida (Pm) is a multi-species pathogen that causes diseases in animals and humans. Sialyltransferase activity has been detected in multiple Pm strains and sialylation has been shown to be important for the pathogenesis of Pm. Three putative sialyltransferase genes have been identified in Pm genomic strain Pm70. We have reported previously that a Pm0188 gene homolog in Pm strain P-1059 (ATCC 15742) encodes a multifunctional sialyltransferase (PmST1). We demonstrate here that while PmST1 prefers to use oligosaccharides as acceptors, PmST2 encoded by the Pm0508 gene homolog in the same Pm strain is a novel glycolipid α2-3-sialyltransferase that prefers to use lactosyl lipids as acceptor substrates. PmST2 and PmST1 thus complement each other for an efficient synthesis of α2-3-linked sialosides with or without lipid portion. In addition, β1-4-linked galactosyl lipids are better PmST2 substrates than β1-3-linked galactosyl lipids. PmST2 has been used successfully in the preparative scale synthesis of sialyllactosyl sphingosine (lyso-GM3), which is an important glycolipid and an intermediate for synthesizing more complex glycolipids such as gangliosides.

Molecular characterization of phosphorylcholine expression on the lipooligosaccharide of Histophilus somni

Histophilus somni (Haemophilus somnus) is an important pathogen of cattle that is responsible for respiratory disease, septicemia, and systemic diseases such as thrombotic meningoencephalitis, myocarditis, and abortion. A variety of virulence factors have been identified in H. somni, including compositional and antigenic variation of the lipooligosaccharide (LOS). Phosphorylcholine (ChoP) has been identified as one of the components of H. somni LOS that undergoes antigenic variation. In this study, five genes (lic1ABCD(Hs) and glpQ) with homology to genes responsible for ChoP expression in Haemophilus influenzae LOS were identified in the H. somni genome. An H. somni open reading frame (ORF) with homology to H. influenzae lic1A (lic1A(Hi)) contained a variable number of tandem repeats (VNTR). However, whereas the tetranucleotide repeat 5'-CAAT-3' is present in lic1A(Hi), the VNTR in H. somni lic1A (lic1A(Hs)) consisted of 5'-AACC-3'. Due to the propensity of VNTR to vary during replication and cause the ORF to shift in and out of frame with the upstream start codon, the VNTR were deleted from lic1A(Hs) to maintain the gene constitutively on. This construct was cloned into Escherichia coli, and functional enzyme assays confirmed that lic1A(Hs) encoded a choline kinase, and that the VNTR were not required for expression of a functional gene product. Variation in the number of VNTR in lic1A(Hs) correlated with antigenic variation of ChoP expression in H. somni strain 124P. However, antigenic variation of ChoP expression in strain 738 predominately occurred through variable extension/truncation of the LOS outer core. These results indicated that the lic1(Hs) genes controlled expression of ChoP on the LOS, but that in H. somni there are two potential mechanisms that account for antigenic variation of ChoP.

Sialic acid uptake is necessary for virulence of Pasteurella multocida in turkeys

Many pathogenic bacteria employ systems to incorporate sialic acid into their membranes as a means of protection against host defense mechanisms. In Pasteurella multocida, an opportunistic pathogen which causes diseases of economic importance in a wide range of animal species, sialic acid uptake plays a role in a mouse model of systemic pasteurellosis. To further investigate the importance of sialic acid uptake in pathogenesis, sialic acid uptake mutants of an avian strain of P. multocida P-1059 (A:3) were constructed, characterized, and an in-frame sialic acid uptake deletion mutant was assessed for virulence in turkeys. Inactivation of sialic acid uptake resulted in a high degree of attenuation when turkeys were challenged either intranasally or intravenously. Resistance of the sialic acid uptake mutant to killing by turkey serum complement was similar to that of the parent, suggesting other mechanisms are responsible for attenuation of virulence in turkeys.

Histophilus somni biofilm formation in cardiopulmonary tissue of the bovine host following respiratory challenge

Biofilms form in a variety of host sites following infection with many bacterial species. However, the study of biofilms in a host is hindered due to the lack of protocols for the proper experimental investigation of biofilms in vivo. Histophilus somni is an agent of respiratory and systemic diseases in bovines, and readily forms biofilms in vitro. In the present study the capability of H. somni to form biofilms in cardiopulmonary tissue following experimental respiratory infection in the bovine host was examined by light microscopy, transmission electron microscopy, immunoelectron microscopy of ultrathin cryosections, scanning electron microscopy of freeze-fractured samples, and fluorescent in situ hybridization. Biofilms were evident and most prominent in the myocardium, and were associated with a large amount of amorphous extracellular material. Furthermore, Pasteurella multocida was often cultured with H. somni from heart and lung samples. Transposon mutagenesis of H. somni strain 2336 resulted in the generation of mutants that expressed more or less biofilm than the parent strain. Six mutants deficient in biofilm formation had an insertion in the gene encoding for a homolog of filamentous haemagglutinin (FHA), predicted to be involved in attachment. Thus, this investigation demonstrated that H. somni is capable of forming a biofilm in its natural host, that such a biofilm may be capable of harboring other bovine respiratory disease pathogens, and that the genes responsible for biofilm formation can be identified by transposon mutagenesis.

Histophilus somni host-parasite relationships

Histophilus somni (Haemophilus somnus) is one of the key bacterial pathogens involved in the multifactorial etiology of the Bovine Respiratory Disease Complex. This Gram negative pleomorphic rod also causes bovine septicemia, thrombotic meningencephalitis, myocarditis, arthritis, abortion and infertility, as well as disease in sheep, bison and bighorn sheep. Virulence factors include lipooligosaccharide, immunoglobulin binding proteins (as a surface fibrillar network), a major outer membrane protein (MOMP), other outer membrane proteins (OMPs) and exopolysaccharide. Histamine production, biofilm formation and quorum sensing may also contribute to pathogenesis. Antibodies are very important in protection as shown in passive protection studies. The lack of long-term survival of the organism in macrophages, unlike facultative intracellular bacteria, also suggests that antibodies should be critical in protection. Of the immunoglobulin classes, IgG2 antibodies are most implicated in protection and IgE antibodies in immunopathogenesis. The immunodominant antigen recognized by IgE is the MOMP and by IgG2 is a 40 kDa OMP. Pathogenetic synergy of bovine respiratory syncytial virus (BRSV) and H. somni in calves can be attributed, in part at least, to the higher IgE anti-MOMP antibody responses in dually infected calves. Other antigens are probably involved in stimulating host defense or immunopathology as well.

Immune evasion by pathogens of bovine respiratory disease complex

Bovine respiratory tract disease is a multi-factorial disease complex involving several viruses and bacteria. Viruses that play prominent roles in causing the bovine respiratory disease complex include bovine herpesvirus-1, bovine respiratory syncytial virus, bovine viral diarrhea virus and parinfluenza-3 virus. Bacteria that play prominent roles in this disease complex are Mannheimia haemolytica and Mycoplasma bovis. Other bacteria that infect the bovine respiratory tract of cattle are Histophilus (Haemophilus) somni and Pasteurella multocida. Frequently, severe respiratory tract disease in cattle is associated with concurrent infections of these pathogens. Like other pathogens, the viral and bacterial pathogens of this disease complex have co-evolved with their hosts over millions of years. As much as the hosts have diversified and fine-tuned the components of their immune system, the pathogens have also evolved diverse and sophisticated strategies to evade the host immune responses. These pathogens have developed intricate mechanisms to thwart both the innate and adaptive arms of the immune responses of their hosts. This review presents an overview of the strategies by which the pathogens suppress host immune responses, as well as the strategies by which the pathogens modify themselves or their locations in the host to evade host immune responses. These immune evasion strategies likely contribute to the failure of currently-available vaccines to provide complete protection to cattle against these pathogens.

The Hd0053 gene of Haemophilus ducreyi encodes an alpha2,3-sialyltransferase

Haemophilus ducreyi is a Gram-negative bacterium that causes chancroid, a sexually transmitted genital ulcer disease. Different lipooligosaccharide (LOS) structures have been identified from H. ducreyi strain 35000, including those sialylated glycoforms. Surface LOS of H. ducreyi is considered an important virulence factor that is involved in ulcer formation, cell adhesion, and invasion of host tissue. Gene Hd0686 of H. ducreyi, designated lst (for lipooligosaccharide sialyltransferase), was identified to encode an alpha2,3-sialyltransferase that is important for the formation of sialylated LOS. Here, we show that Hd0053 of H. ducreyi genomic strain 35000HP, the third member of the glycosyltransferase family 80 (GT80), also encodes an alpha2,3-sialyltransferase that may be important for LOS sialylation.

Characterization and comparison of biofilm development by pathogenic and commensal isolates of Histophilus somni

Histophilus somni (Haemophilus somnus) is an obligate inhabitant of the mucosal surfaces of bovines and sheep and an opportunistic pathogen responsible for respiratory disease, meningoencephalitis, myocarditis, arthritis, and other systemic infections. The identification of an exopolysaccharide produced by H. somni prompted us to evaluate whether the bacterium was capable of forming a biofilm. After growth in polyvinyl chloride wells a biofilm was formed by all strains examined, although most isolates from systemic sites produced more biofilm than commensal isolates from the prepuce. Biofilms of pneumonia isolate strain 2336 and commensal isolate strain 129Pt were grown in flow cells, followed by analysis by confocal laser scanning microscopy and scanning electron microscopy. Both strains formed biofilms that went through stages of attachment, growth, maturation, and detachment. However, strain 2336 produced a mature biofilm that consisted of thick, homogenous mound-shaped microcolonies encased in an amorphous extracellular matrix with profound water channels. In contrast, strain 129Pt formed a biofilm of cell clusters that were tower-shaped or distinct filamentous structures intertwined with each other by strands of extracellular matrix. The biofilm of strain 2336 had a mass and thickness that was 5- to 10-fold greater than that of strain 129Pt and covered 75 to 82% of the surface area, whereas the biofilm of strain 129Pt covered 35 to 40% of the surface area. Since H. somni is an obligate inhabitant of the bovine and ovine host, the formation of a biofilm may be crucial to its persistence in vivo, and our in vitro evidence suggests that formation of a more robust biofilm may provide a selective advantage for strains that cause systemic disease.

Chloramphenicol is a substrate for a novel nitroreductase pathway in Haemophilus influenzae

The p-nitroaromatic antibiotic chloramphenicol has been used extensively to treat life-threatening infections due to Haemophilus influenzae and Neisseria meningitidis; its mechanism of action is the inhibition of protein synthesis. We found that during incubation with H. influenzae cells and lysates, chloramphenicol is converted to a 4-aminophenyl allylic alcohol that lacks antibacterial activity. The allylic alcohol moiety undergoes facile re-addition of water to restore the 1,3-diol, as well as further dehydration driven by the aromatic amine to form the iminoquinone. Several Neisseria species and most chloramphenicol-susceptible Haemophilus species, but not Escherichia coli or other gram-negative or gram-positive bacteria we examined, were also found to metabolize chloramphenicol. The products of chloramphenicol metabolism by species other than H. influenzae have not yet been characterized. The strains reducing the antibiotic were chloramphenicol susceptible, indicating that the pathway does not appear to mediate chloramphenicol resistance. The role of this novel nitroreductase pathway in the physiology of H. influenzae and Neisseria species is unknown. Further understanding of the H. influenzae chloramphenicol reduction pathway will contribute to our knowledge of the diversity of prokaryotic nitroreductase mechanisms.

Complexities of the pathogenesis ofMannheimia haemolyticaandHaemophilus somnusinfections: challenges and potential opportunities for prevention?

Progress in producing improved vaccines against bacterial diseases of cattle is limited by an incomplete understanding of the pathogenesis of these agents. Our group has been involved in investigations of two members of the family Pasteurellaceae,Mannheimia haemolyticaandHaemophilus somnus, which illustrate some of the complexities that must be confronted. Susceptibility toM. haemolyticais greatly increased during active viral respiratory infection, resulting in rapid onset of a severe and even lethal pleuropneumonia. Despite years of investigation, understanding of the mechanisms underlying this viral–bacterial synergism is incomplete. We have investigated the hypothesis that active viral infection increases the susceptibility of bovine leukocytes to theM. haemolyticaleukotoxin by increasing the expression of or activating the β2integrin CD11a/CD18 (LFA-1) on the leukocyte surface.In vitroexposure to proinflammatory cytokines (i.e. interleukin-1β, tumor necrosis factor-α and interferon-γ) increases LFA-1 expression on bovine leukocytes, which in turn correlates with increased binding and responsiveness to the leukotoxin. Alveolar macrophages and peripheral blood leukocytes from cattle with active bovine herpesvirus-1 (BVH-1) infection are more susceptible to the lethal effects of the leukotoxinex vivothan leukocytes from uninfected cattle. Likewise,in vitroincubation of bovine leukocytes with bovine herpesvirus 1 (BHV-1) potentiates LFA-1 expression and makes the cells more responsive to leukotoxin. A striking characteristic ofH. somnusinfection is its propensity to cause vasculitis. We have shown thatH. somnusand its lipo-oligosaccharide (LOS) trigger caspase activation and apoptosis in bovine endothelial cellsin vitro. This effect is associated with the production of reactive oxygen and nitrogen intermediates, and is amplified in the presence of platelets. The adverse effects ofH. somnusLOS are mediated in part by activation of endothelial cell purinergic receptors such as P2X7. Further dissection of the pathways that lead to endothelial cell damage in response toH. somnusmight help in the development of new preventive or therapeutic regimens. A more thorough understanding ofM. haemolyticaandH. somnusvirulence factors and their interactions with the host might identify new targets for prevention of bovine respiratory disease.

Expression of phosphorylcholine by Histophilus somni induces bovine platelet aggregation

Histophilus somni-induced platelet aggregation was inhibited by antagonists of the platelet-activating factor (PAF) receptor but not inhibitors of PAF synthesis. In addition, H. somni cells expressing phosphorylcholine (ChoP) induced aggregation, while ChoP(-) H. somni cells did not. This suggests that H. somni ChoP may induce platelet aggregation via interactions with the PAF receptor.

Sialic Acid metabolism and systemic pasteurellosis

Pasteurella multocida subsp. multocida is a commensal and opportunistic pathogen of food animals, wildlife, and pets and a zoonotic cause of human infection arising from contacts with these animals. Here, an investigation of multiple serotype A strains demonstrated the occurrence of membrane sialyltransferase. Although P. multocida lacks the genes for the two earliest steps in de novo sialic acid synthesis, adding sialic acid to the growth medium resulted in uptake, activation, and subsequent transfer of sialic acid to a membrane acceptor resembling lipooligosaccharide. Two candidate-activating enzymes with homology to Escherichia coli cytidine 5'-monophospho-N-acetylneuraminate synthetase were overproduced as histidine-tagged polypeptides. The synthetase encoded by pm0187 was at least 37 times more active than the pm1710 gene product, suggesting pm0187 encodes the primary sialic acid cytidylyltransferase in P. multocida. A sialate aldolase (pm1715) mutant unable to initiate dissimilation of internalized sialic acid was not attenuated in the CD-1 mouse model of systemic pasteurellosis, indicating that the nutritional function of sialate catabolism is not required for systemic disease. In contrast, the attenuation of a sialate uptake-deficient mutant supports the essential role in pathogenesis of a sialylation mechanism that is dependent on an environmental (host) supply of sialic acid. The combined results provide the first direct evidence of sialylation by a precursor scavenging mechanism in pasteurellae and of a potential tripartite ATP-independent periplasmic sialate transporter in any species.

Structural analysis of the oligosaccharide of Histophilus somni (Haemophilus somnus) strain 2336 and identification of several lipooligosaccharide biosynthesis gene homologues

The structure of the core oligosaccharide from a pneumonic Histophilus somni (Haemophilus somnus) strain 2336 was elucidated. The lipooligosaccharide (LOS) was subjected to a variety of degradative procedures. The structures of the purified products were established by monosaccharide and methylation analyses, NMR spectroscopy and mass spectrometry. The following structure for the core oligosaccharide was determined on the basis of the combined data from these experiments: [formula-see text]. The structural elucidation was intriguing as it suggested several differences in the LOS structures between strain 2336 and the related strain 738. Strain 738 originated following passaging of strain 2336 through a calf. The differences between the two structures are a different linkage between Gal II and GlcNAc (1-->4 here; 1-->3 in 738), the absence of phosphocholine (PCho) from 2336 and the presence of two phosphoethanolamine (PEtn) residues and Gal III (at the 2-position) of Hep II in 2336. Although pulse-field gel electrophoresis data following digest with only one restriction enzyme showed identical profiles suggesting that strains 738 and 2336 are the same strain, the structural data does suggest that, if strain 738 is indeed a phase variant of strain 2336, considerable variation occurred on calf passaging and could therefore be an intriguing example of how broadly this bacterium can adapt itself in the host.

Structure of theBordetella trematumLPS O-chain subunit

Analysis of the O-chain subunit of the lipopolysaccharide (LPS, endotoxin) isolated from Bordetella trematum, a recently identified human pathogen, was undertaken. The polysaccharide (PS) moiety was shown to contain only two O-chain subunits, which differed in the anomeric bond of their first sugar. A trisaccharide fragment resulting from the cleavage of a FucNAc glycosidic bond was isolated after treatment of the PS with anhydrous HF. Nitrous deamination of the LPS led to the release of the following heptasaccharide corresponding to two trisaccharide subunits linked to an anhydromannitol residue. beta-ManNAc3NAmA-(1-4)-beta-ManNAc3NAmA-(1-3)-alpha-FucNAc-(1-4)-beta-ManNAc3NAmA-(1-4)-beta-ManNAc3NAmA-(1-3)-beta-FucNAc-(1-6)-2,5-anhManol.

Inhibition of bovine macrophage and polymorphonuclear leukocyte superoxide anion production by Haemophilus somnus

Virulent strains of the bovine opportunistic pathogen Haemophilus somnus (Histophilus somni) cause multi-systemic diseases in cattle. One of the reported virulence factors that H. somnus may use to persist in the host is resistance to intracellular killing. We report here that H. somnus significantly (P < 0.001) inhibited production of superoxide anion (O2-) by bovine mammary and alveolar macrophages as well as by polymorphonuclear leukocytes. Inhibition of O2- was time- and dose-dependent and did not occur after incubation with Escherichia coli, H. influenzae, or Brucella abortus. Non-viable H. somnus, purified lipooligosaccharide, or cell-free supernatant from mid-log phase cultures did not inhibit O2- production, indicating that O2- inhibition required contact with live H. somnus. Furthermore, preincubation of phagocytic cells with cytochalasin B to prevent phagocytosis did not decrease the ability of H. somnus to inhibit O2- production. Some H. somnus isolates from the prepuce of healthy bulls were less capable or incapable of inhibiting macrophage O2- production compared to isolates tested from disease sites. Our results suggest that inhibition of O2- may be an important virulence factor exploited by pathogenic strains of H. somnus to resist killing by professional phagocytic cells.

Structural analysis of the lipooligosaccharide from the commensal Haemophilus somnus genome strain 129Pt

The structure for the carbohydrate moiety of the lipooligosaccharide (LOS) from the commensal Haemophilus somnus strain 129Pt was elucidated. The structure of the core oligosaccharide and O-deacylated LOS was established by monosaccharide and methylation analyses, NMR spectroscopy and mass spectrometry. The following structure for the major fully extended carbohydrate glycoform of the LOS was determined on the basis of the combined data from these experiments. [Carbohydrate structure: see text]. In the structure Kdo is 3-deoxy-D-manno-octulosonic acid, Hep is L-glycero-D-manno-heptose and PEtn is phosphoethanolamine. Minor amounts of glycoforms containing nonstoichiometric substituents glycine and phosphate at the distal heptose residue were also identified.

Linking Mass Spectrometry and Slab-Polyacrylamide Gel Electrophoresis by Passive Elution of Lipopolysaccharides from Reverse-Stained Gels: Analysis of Gel-Purified Lipopolysaccharides from Haemophilus influenzae Strain Rd

Haemophilus influenzae is an important cause of human disease, and its lipopolysaccharide (LPS) is known to be a major virulence factor. H. influenzae produces short-chain LPS of which the heterogeneity is often visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) using silver staining for detection. Individual bands have not previously been recovered by this method in quantities sufficient for mass spectrometry. In an attempt toward the development of sensitive mass spectrometrical strategies to be used in structural studies of H. influenzae LPS and LPS from other bacteria, we have applied here our previously described slab-PAGE-based micropurification method to obtain unmodified LPS fractions of high purity (>95%) from a crude LPS preparation of H. influenzae strain Rd. Two LPS-fractions were obtained which, after a procedure including mild acid hydrolysis, dephosphorylation, and permethylation of the resulting oligosaccharides, were subjected to tandem electrospray ionization mass spectrometry (ESI-MS/MS). The quantities of micropurified LPS fractions-the recovery of LPS in terms of total mass was 30%-were found sufficient to allow the characterization of LPS glycoforms. The ESI-MS spectra of the individual bands showed reduced heterogeneity. Furthermore, the integrity of the micropurified LPS was confirmed. The spectra-displayed molecular ions showed improved intensity, increased respective signal-to-noise ratios demonstrating the sensitivity of analysis. Consequently, both the direct determination of the molecular masses of the gel-separated LPS glycoforms and sequence analyses using ESI-MS/MS were possible.

Structural analysis of the lipooligosaccharide from the commensal Haemophilus somnus strain 1P

The structure of the lipooligosaccharide (LOS) from the commensal Haemophilus somnus strain 1P was elucidated. The structure of the O-deacylated LOS was established by monosaccharide analysis, NMR spectroscopy and mass spectrometry. The following structure for the O-deacylated LOS was determined on the basis of the combined data from these experiments. [chemical structure: see text] In the structure Kdo is 3-deoxy-D-manno-octulosonic acid, Hep is L-glycero-D-manno-heptose and lipid A-OH refers to O-deacylated Lipid A. The elucidation of this structure has increased our understanding of the relationship between the variability in LOS structure and the pathogenic potential of this organism. Specifically, the inability of this commensal strain to sialylate its LOS suggests that LOS sialylation could be a crucial virulence factor for H. somnus.