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

Epidemiology and pathogenicity of Haemophilus parasuis in eastern China

Haemophilus parasuis (H. parasuis), the causative agent of Glässer's disease (polyserositis), exhibits considerable serotype diversity and is globally distributed. To investigate the epidemiological characteristics of H. parasuis in China, 810 suspected infection samples, including lung tissues and pleural effusions, were systematically analyzed. These samples were collected between 2022 and 2024 from six major pig-producing provinces in China. The analysis revealed a national infection rate of 52.10% (422/810), with Jiangxi Province exhibiting the highest prevalence (71.76%). Seasonal analysis indicated significantly higher incidence rates during winter (66.29%) and spring (60.45%) compared to summer (34.81%) and autumn (46.46%). Serotyping of 56 H. parasuis isolates demonstrated that serotype 5 (42.86%) and serotype 12 (19.64%) were predominant, while 10.71% of the strains were nontypeable. Notably, Jiangxi Province displayed a single-serotype profile, whereas other provinces exhibited multiple-serotype cocirculation patterns. Virulence gene analysis revealed the universal absence of HPM-1370 and the consistent presence of vta3 across all isolates. Serotypes 4 and 8 exclusively harbored the vta gene cluster, while serotype 5 retained other target genes despite lacking HPM-1370. The HPM-1371 gene was detected only in serotypes 5 and 14. Serotypes 1, 11, 12, and 13 exhibited a combination of conserved wza-vta1-vta2 -vta3 genes, although 36.37% (4/11) of serotype 12 isolates lacked the wza gene. Serotypes 2 and 7 carried only wza and vta3 genes. Animal challenge experiments demonstrated marked differences in strain pathogenicity: the H5-1 strain induced 100% mortality with acute septicemia, widespread alveolar destruction, and fibrinous exudation; the H12-1 strain caused 50% mortality accompanied by severe pleural adhesions and hemorrhagic lesions; and the H7-1 strain resulted in 33.33% mortality with localized pulmonary damage. These findings provide essential evidence for the development of targeted prevention and control strategies against H. parasuis infection.

Neuraminidase and nitric oxide dual response via PSA-PLGA nanoparticles: A novel approach for controlling Glaesserella parasuis inflammation and infection

The overexpression of virulence factors and the induced inflammatory cytokine storm, resulting in tissue damage, represent significant challenges in treating antibiotic-resistant bacterial infections with conventional antibiotics. Herein, we have developed a bifunctional nanoparticle loaded with antibiotics (PSA@PLGA-TD/SMT) designed for precise response to the virulence proteins of drug-resistant bacteria while protecting the host from excessive inflammatory damage. This is achieved by modifying polylactic-co-glycolic acid (PLGA) nanoparticles with polysialic acid (PSA). Tildipirosin (TD), through hydrophobic interactions, is encapsulated within the core, while S-Methylisothiourea (SMT) is electrostatically adsorbed onto the shell layer. In vitro, the polysialic acid in the nanoparticles interacts with the neuraminidase overexpressed on the surface of Glaesserella parasuis (Gps), triggering disintegration and subsequent release of TD, which effectively kills the bacteria. Additionally, SMT functions to suppress the secretion of inducible nitric oxide synthase (NOS2), promoting the phenotypic transformation of macrophages and reducing the expression of pro-inflammatory factors. Ultimately, PSA@PLGA-TD/SMT achieves an extended drug circulation time through its prolonged retention effect, effectively alleviating discomfort caused by resistant bacteria. Therefore, the construction of nanocarriers for precise targeting of drug-resistant pathogens, delivering antibiotics, and preventing host-induced inflammatory damage presents an effective treatment strategy. This approach not only addresses the direct issue of bacterial eradication but also mitigates the collateral damage typically associated with severe infections, offering a more comprehensive and nuanced approach to managing drug-resistant bacterial infections.

The forecasting power of the mucin-microbiome interplay in livestock respiratory diseases

Complex respiratory diseases are a significant challenge for the livestock industry worldwide. These diseases considerably impact animal health and welfare and cause severe economic losses. One of the first lines of pathogen defense combines the respiratory tract mucus, a highly viscous material primarily composed of mucins, and a thriving multi-kingdom microbial ecosystem. The microbiome-mucin interplay protects from unwanted substances and organisms, but its dysfunction may enable pathogenic infections and the onset of respiratory disease. Emerging evidence also shows that noncoding regulatory RNAs might modulate the structure and function of the microbiome-mucin relationship. This opinion paper unearths the current understanding of the triangular relationship between mucins, the microbiome, and noncoding RNAs in the context of respiratory infections in animals of veterinary interest. There is a need to look at these molecular underpinnings that dictate distinct health and disease outcomes to implement effective prevention, surveillance, and timely intervention strategies tailored to the different epidemiological contexts.

Genomic insight into the diversity of Glaesserella parasuis isolates from 19 countries

Glaesserella parasuis is a commensal bacterial organism found in the upper respiratory tract of healthy pigs and the etiological agent of Glässer's disease, which causes severe economic losses in the swine industry. This study aimed to better understand the epidemiological characteristics of this opportunistic pathogen. We investigated the prevalence and distribution of sequence types (STs), serovars, antimicrobial resistance genes (ARGs), and potential virulence factors (VFs) in 764 G. parasuis isolates collected from diseased and healthy pigs from 19 countries, including China. Multilocus sequence typing showed a high degree of variation with 334 STs, of which 93 were not previously recognized. Phylogenetic analysis revealed two major clades distinguished by isolation year, source, country, and serovar. The dominant serovars of G. parasuis were serovars 4 (19.50%), 7 (15.97%), 5/12 (13.87%), and 13 (12.30%). Serovar 7 gradually became one of the dominant serovars in G. parasuis with more VFs and fewer ARGs. Serovars 4 and 5/12 were the most frequent serovars in diseased pigs, whereas serovars 2, 8, and 11 were predominant in healthy pigs. Serovars 7 and 13 possessed more VFs than the other serovars. This study provides novel insights into the global prevalence and epidemiology of G. parasuis and valuable clues for further investigation into the pathogenicity of G. parasuis, which will facilitate the development of effective vaccines.IMPORTANCEGlaesserella parasuis is a clinically important gram-negative opportunistic pathogen, which causes serious financial losses in swine industry on a global scale. No vaccine is known that provides cross-protection against all 15 serovars; furthermore, the correlation between serovar and virulence is largely unknown. This study provides a large number of sequenced strains in 19 countries and compares the genomic diversity of G. parasuis between diseased and healthy pigs. We found a slight change in the dominant serovar of G. parasuis in the world, with serovar 7 gradually emerging as one of the predominant serovars. The observed higher average number of VFs in this particular serovar strain challenges the previously held notion that serovar 7 is non-virulent, indicating a more complex virulence landscape than previously understood. Our analysis indicating that six ARGs [tet(B), sul2, aph(3')-Ia, aph (6)-Id, blaROB-1, and aph(3'')-Ib] are likely to be transmitted horizontally in their entirety. By analyzing VFs, we provided an improved understanding of the virulence of G. parasuis, and these key findings suggest that vaccine development will be challenging.

A comparative study of the substrate preference of the sialidases, CpNanI, HpNanH, and BbSia2 towards 2-Aminobenzamide-labeled 3'-Sialyllactose, 6'-Sialyllactose, and Sialyllacto-N-tetraose-b

Sialidases catalyze the removal of terminal sialic acids from sialylated biomolecules, and their substrate preference is frequently indicated in terms of the glycosidic linkages cleaved (α2-3, α2-6, and α2-8) without mention of the remaining sub-terminal reducing-end saccharide moieties. Many human gut commensal and pathogenic bacteria secrete sialidases to forage for sialic acids, which are then utilized as an energy source or assimilated into membrane/capsular structural components. Infant gut commensals similarly utilize sialylated human milk oligosaccharides containing different glycosidic linkages. Here, we have studied the preference of the bacterial sialidases, BbSia2 from Bifidobacterium bifidum, CpNanI from Clostridium perfringens, and HpNanH from Glaesserella parasuis, for the glycosidic linkages, Siaα2-3Gal, Siaα2-6Gal, and Siaα2-6GlcNAc, by employing 2-Aminobenzamide-labeled human milk oligosaccharides, 3'-Sialyllactose (3'-SL), 6'-Sialyllactose (6'-SL), and Sialyllacto-N-tetraose-b (LSTb), respectively, as proxies for these glycosidic linkages. BbSia2, CpNanI, and HpNanH hydrolyzed these three oligosaccharides with the glycosidic linkage preferences, 3'-SL (Siaα2-3Gal) ≥ LSTb (Siaα2-6GlcNAc) ≥ 6'-SL (Siaα2-6Gal), 3'-SL (Siaα2-3Gal) ≥ 6'-SL (Siaα2-6Gal) > LSTb (Siaα2-6GlcNAc), and 3'-SL (Siaα2-3Gal) ≥ 6'-SL (Siaα2-6Gal) > LSTb (Siaα2-6GlcNAc), respectively. Our finding suggests that sub-terminal reducing-end saccharide moieties can profoundly influence the substrate preference of sialidases, and advocates for the characterization and indication of the substrate preference of sialidases in terms of both the glycosidic linkage and the sub-terminal reducing-end saccharide moiety.

Impact of bronchoalveolar lavage from influenza A virus diseased pigs on neutrophil functions and growth of co-infecting pathogenic bacteria

Introduction

Influenza A viruses (IAVs) infect the respiratory tract of mainly humans, poultry, and pigs. Co-infections with pathogenic lung bacteria are a common event and contribute to the severity of disease progression. Neutrophils are a major cell type of the innate immune system and are rapidly recruited to the site of infection. They have several effector functions to fight invading pathogens such as the secretion of reactive oxygen species (ROS) or the release of neutrophil extracellular traps (NETs). NETs are known to promote the growth of Pasteurellaceae bacteria, especially if degraded by nucleases.

Methods

In this study, bronchoalveolar lavage fluid (BALF) from 45 field-infected pigs was analyzed for 1) NET markers, 2) influence on growth of lung bacteria, and 3) impact on neutrophil functions. BALF samples from 21 IAV-positive pigs and 24 lung diseased but IAV-negative pigs were compared.

Results

Here, we show that neutrophils in the lungs of IAV-positive pigs release vesicular NETs. Several NET markers were increased in the BALF of IAV-positive pigs compared with the BALF from IAV-negative pigs. The amount of NET markers positively correlated with the viral load of the IAV infection. Interestingly, the BALF of IAV-positive pigs enhanced the growth of bacteria belonging to the family of Pasteurellaceae as potential coinfecting bacteria. These effects were weaker with the BALF derived from IAV-negative pigs with other lung infections. The intensity of oxidative burst in neutrophils was significantly decreased by BALF from IAVpositive pigs, indicating impaired antimicrobial activity of neutrophils. Finally, the lung milieu reflected by IAV-positive BALF does not enable neutrophils to kill Actinobacillus pleuropneumoniae but rather enhances its growth.

Discussion

In summary, our data show that an IAV infection is affecting neutrophil functions, in particular the release of NETs and ROS. Furthermore, IAV infection seems to provide growth-enhancing factors for especially coinfecting Pasteurellaceae and reduces the killing efficiency of neutrophils.

Research progress of nanoparticle targeting delivery systems in bacterial infections

Bacterial infection is a huge threat to the health of human beings and animals. The abuse of antibiotics have led to the occurrence of bacterial multidrug resistance, which have become a difficult problem in the treatment of clinical infections. Given the outstanding advantages of nanodrug delivery systems in cancer treatment, many scholars have begun to pay attention to their application in bacterial infections. However, due to the similarity of the microenvironment between bacterial infection lesions and cancer sites, the targeting and accuracy of traditional microenvironment-responsive nanocarriers are questionable. Therefore, finding new specific targets has become a new development direction of nanocarriers in bacterial prevention and treatment. This article reviews the infectious microenvironment induced by bacteria and a series of virulence factors of common pathogenic bacteria and their physiological functions, which may be used as potential targets to improve the targeting accuracy of nanocarriers in lesions.

Molecular characterization of Glaesserella parasuis strains circulating in North American swine production systems

BACKGROUND

Glaesserella parasuis is the causative agent of Glässer's disease in pigs. Serotyping is the most common method used to type G. parasuis isolates. However, the high number of non-typables (NT) and low discriminatory power make serotyping problematic. In this study, 218 field clinical isolates and 15 G. parasuis reference strains were whole-genome sequenced (WGS). Multilocus sequence types (MLST), serotypes, core-genome phylogeny, antimicrobial resistance (AMR) genes, and putative virulence gene information was extracted.

RESULTS

In silico WGS serotyping identified 11 of 15 serotypes. The most frequently detected serotypes were 7, 13, 4, and 2. MLST identified 72 sequence types (STs), of which 66 were novel. The most predominant ST was ST454. Core-genome phylogeny depicted 3 primary lineages (LI, LII, and LIII), with LIIIA sublineage isolates lacking all vtaA genes, based on the structure of the phylogenetic tree and the number of virulence genes. At least one group 1 vtaA virulence genes were observed in most isolates (97.2%), except for serotype 8 (ST299 and ST406), 15 (ST408 and ST552) and NT (ST448). A few group 1 vtaA genes were significantly associated with certain serotypes or STs. The putative virulence gene lsgB, was detected in 8.3% of the isolates which were predominantly of serotype 5/12. While most isolates carried the bcr, ksgA, and bacA genes, the following antimicrobial resistant genes were detected in lower frequency;  blaZ (6.9%), tetM (3.7%), spc (3.7%), tetB (2.8%), bla-ROB-1 (1.8%), ermA (1.8%), strA (1.4%), qnrB (0.5%), and aph3''Ia (0.5%).   CONCLUSION: This study showed the use of WGS to type G. parasuis isolates and can be considered an alternative to the more labor-intensive and traditional serotyping and standard MLST. Core-genome phylogeny provided the best strain discrimination. These findings will lead to a better understanding of the molecular epidemiology and virulence in G. parasuis that can be applied to the future development of diagnostic tools, autogenous vaccines, evaluation of antibiotic use, prevention, and disease control.

Development of a new multiplex quantitative PCR for the detection of Glaesserella parasuis, Mycoplasma hyorhinis, and Mycoplasma hyosynoviae

Glaesserella parasuis, Mycoplasma hyorhinis, and Mycoplasma hyosynoviae are important porcine pathogens responsible for polyserositis, polyarthritis, meningitis, pneumonia, and septicemia causing significant economic losses in the swine industry. A new multiplex quantitative polymerase chain reaction (qPCR) was designed on one hand for the detection of G. parasuis and the virulence marker vtaA to distinguish between highly virulent and non-virulent strains. On the other hand, fluorescent probes were established for the detection and identification of both M. hyorhinis and M. hyosynoviae targeting 16S ribosomal RNA genes. The development of the qPCR was based on reference strains of 15 known serovars of G. parasuis, as well as on the type strains M. hyorhinis ATCC 17981T and M. hyosynoviae NCTC 10167T . The new qPCR was further evaluated using 21 G. parasuis, 26 M. hyorhinis, and 3 M. hyosynoviae field isolates. Moreover, a pilot study including different clinical specimens of 42 diseased pigs was performed. The specificity of the assay was 100% without cross-reactivity or detection of other bacterial swine pathogens. The sensitivity of the new qPCR was demonstrated to be between 11-180 genome equivalents (GE) of DNA for M. hyosynoviae and M. hyorhinis, and 140-1200 GE for G. parasuis and vtaA. The cut-off threshold cycle was found to be at 35. The developed sensitive and specific qPCR assay has the potential to become a useful molecular tool, which could be implemented in veterinary diagnostic laboratories for the detection and identification of G. parasuis, its virulence marker vtaA, M. hyorhinis, and M. hyosynoviae.

Molecular characterization of Glaesserella parasuis strains isolated from North America, Europe and Asia by serotyping PCR and LS-PCR

Glaesserella parasuis strains were characterized by serotyping PCR, vtaA virulence marker Leader Sequence (LS)-PCR, clinical significance, and geographic region. Overall, the serovars 4, 5/12, 7, 1, and 13 were the most commonly detected. Serovars of greatest clinical relevance were systemic isolates that had a higher probability of being serovar 5/12, 13, or 7. In comparison, pulmonary isolates had a higher likelihood of being serovars 2, 4, 7, or 14. Serovars 5/12 and 13 have previously been considered disease-associated, but this study agrees with other recent studies showing that serovar 7 is indeed associated with systemic G. parasuis disease. Serovar 4 strains illustrated how isolates can have varying degrees of virulence and be obtained from pulmonary, systemic, or nasal sites. Serovars 8, 9, 15, and 10 were predominantly obtained from nasal samples, which indicates a limited clinical significance of these serovars. Additionally, most internal G. parasuis isolates were classified as virulent by LS-PCR and were disease-associated isolates, including serovars 1, 2, 4, 5/12, 7, 13, and 14. Isolates from the nasal cavity, including serovars 6, 9, 10, 11, and 15, were classified as non-virulent by LS-PCR. In conclusion, the distribution of G. parasuis serovars remains constant, with few serovars representing most of the strains isolated from affected pigs. Moreover, it was confirmed that the LS-PCR can be used for G. parasuis virulence prediction of field strains worldwide.

Sialidase of Glaesserella parasuis Augments Inflammatory Response via Desialylation and Abrogation of Negative Regulation of Siglec-5

Siglecs are sialic acid-binding immunoglobulin-like lectins that play an important role in tissue homeostasis, immune response, and pathogen infection. Bacterial sialidases act on natural ligands of Siglecs, interfering with the Siglec-mediated immune response. Glaesserella parasuis is a porcine bacterial pathogen that secretes sialidase. However, little is known about the sialidase of G. parasuis and its impact on immune regulation. Here, we used wild-type G. parasuis, a sialidase-deficient mutant, and complementary strains to investigate the role of sialidase in porcine alveolar macrophage infection. Sialidase induced the release of proinflammatory cytokines, such as interleukin-1α (IL-1α), IL-6, and tumor necrosis factor alpha, from porcine alveolar macrophages. Moreover, sialidase desialylated the surface of porcine alveolar macrophages and altered the expression of Siglecs (the expression of Siglec-5 was reduced). Furthermore, sialidase led to a reduction in endogenous SH2 domain-containing protein tyrosine phosphatase (SHP-2) recruitment to Siglec-5 and simultaneously activated the inflammatory response via the mitogen-activated protein kinase and nuclear factor kappa light chain enhancer of activated B cell signaling pathways. This desialylation occurred before the release of proinflammatory cytokines, suggesting that the sialidase-induced inflammatory response was followed by reduced recruitment of SHP-2 to Siglec-5. Thus, this study is the first to demonstrate the role of sialidase in the inflammatory response of G. parasuis. This role resulted from the abrogation of negative regulation of Siglec-5 on proinflammatory cytokine release. This study helps to understand the molecular mechanism underlying the inflammatory response induced by sialidase secreted by G. parasuis and the acute inflammation caused by G. parasuis.

Sialylated Lipooligosaccharide Contributes to Glaesserella parasuis Penetration of Porcine Respiratory Epithelial Barrier

Pathogens utilize various mechanisms to escape host immunological surveillance, break down different tissue barriers, and cause infection. Sialylation is an important surface modification of bacterial outer membrane components, especially the lipooligosaccharide of Gram-negative bacteria. It is widely involved in multiple microbe-host interactions, such as bacterial virulence regulation, host recognition, and immune evasion. There are some sialylation modifications on the lipooligosaccharide structure of Glaesserella parasuis (G. parasuis) virulent strains. However, the role of lipooligosaccharide sialylation modification in the process of G. parasuis infection and penetration of the porcine respiratory epithelial barrier is still unclear. In this study, we investigated the role and mechanism of lsgB-mediated lipooligosaccharide sialylation in G. parasuis invasion of the host respiratory epithelial barrier. Specifically, G. parasuis lsgB-mediated lipooligosaccharide sialylation and sialylated-lipooligosaccharide interacted with Siglec1 on porcine alveolar macrophages 3D4/21 and triggered the subsequent generation of TGFβ1 through Siglec1/Dap12/Syk/p38 signaling cascade. TGFβ1 decreased the tracheal epithelial tight junctions and the expression of extracellular adhesion molecule fibronectin, thus assisting G. parasuis invasion and entry to the respiratory epithelial barrier. Characterizing the potential effects and mechanisms of lipooligosaccharide sialylation-mediated TGFβ1 production would further expand our current knowledge on the pathogenesis of G. parasuis which will contribute to better prevention and control of G. parasuis infection in piglets.

Antibacterial effect of Blumea balsamifera DC. essential oil against Haemophilus parasuis

Haemophilus parasuis (H. parasuis), the cause of the Glasser's disease, is a potentially pathogenic gram-negative organism that colonizes the upper respiratory tract of pigs. The extraction of Blumea balsamifera DC., as a traditional Chinese herb, has shown great bacteriostatic effect against several common bacteria. To study the antibacterial effect on H. parasuis in vitro, this study evaluated the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of Blumea balsamifera DC. essential oil (BBO) as well as morphological changes in H. parasuis treated with it. Furthermore, changes in expression of total protein and key virulence factors were also assessed. Results showed that the MIC and MBC were 0.625 and 1.25 μg/mL, respectively. As the concentration of BBO increased, the growth curve inhibition became stronger. H. parasuis cells were damaged severely after treatment with BBO for 4 h, demonstrating plasmolysis and enlarged vacuoles, along with broken cell walls and membranes. Total protein and virulence factor expression in H. parasuis was significantly downregulated by BBO. Taken together, these results indicated a substantial antibacterial effect of BBO on H. parasuis.

Whole-genome sequence analyses of Glaesserella parasuis isolates reveals extensive genomic variation and diverse antibiotic resistance determinants

Background

Glaesserella parasuis (G. parasuis) is a respiratory pathogen of swine and the etiological agent of Glässer’s disease. The structural organization of genetic information, antibiotic resistance genes, potential pathogenicity, and evolutionary relationships among global G. parasuis strains remain unclear. The aim of this study was to better understand patterns of genetic variation, antibiotic resistance factors, and virulence mechanisms of this pathogen.

Methods

The whole-genome sequence of a ST328 isolate from diseased swine in China was determined using Pacbio RS II and Illumina MiSeq platforms and compared with 54 isolates from China sequenced in this study and 39 strains from China and eigtht other countries sequenced by previously. Patterns of genetic variation, antibiotic resistance, and virulence mechanisms were investigated in relation to the phylogeny of the isolates. Electrotransformation experiments were performed to confirm the ability of pYL1—a plasmid observed in ST328—to confer antibiotic resistance.

Results

The ST328 genome contained a novel Tn6678 transposon harbouring a unique resistance determinant. It also contained a small broad-host-range plasmid pYL1 carrying aac(6’)-Ie-aph(2”)-Ia and bla_ROB-1; when transferred to Staphylococcus aureus RN4220 by electroporation, this plasmid was highly stable under kanamycin selection. Most (85.13–91.74%) of the genetic variation between G. parasuis isolates was observed in the accessory genomes. Phylogenetic analysis revealed two major subgroups distinguished by country of origin, serotype, and multilocus sequence type (MLST). Novel virulence factors (gigP, malQ, and gmhA) and drug resistance genes (norA, bacA, ksgA, and bcr) in G. parasuis were identified. Resistance determinants (sul2, aph(3”)-Ib, norA, bacA, ksgA, and bcr) were widespread across isolates, regardless of serovar, isolation source, or geographical location.

Conclusions

Our comparative genomic analysis of worldwide G. parasuis isolates provides valuable insight into the emergence and transmission of G. parasuis in the swine industry. The result suggests the importance of transposon-related and/or plasmid-related gene variations in the evolution of G. parasuis.

Haemophilus parasuis VtaA2 is involved in adhesion to extracellular proteins

Haemophilus parasuis is part of the microbiota of the upper respiratory tract in swine. However, virulent strains can cause a systemic disease known as Glässer’s disease. Several virulence factors have been described in H. parasuis including the virulence-associated trimeric autotransporters (VtaAs). VtaA2 is up-regulated during infection and is only found in virulent strains. In order to determine its biological function, the vtaA2 gene was cloned with its native promotor region in pACYC184, and the transformed Escherichia coli was used to perform functional in vitro assays. VtaA2 was found to have a role in attachment to plastic, mucin, BSA, fibronectin and collagen. As other VtaAs from H. parasuis, the passenger domain of VtaA2 contains collagen domains. In order to examine the contribution of the collagen repeats to VtaA2 function, a recombinant vtaA2 without the central collagen domains was obtained and named vtaA2OL. VtaA2OL showed similar capacity than VtaA2 to adhere to plastic, mucin, BSA, fibronectin and plasma but a reduced capacity to adhere to collagen, suggesting that the collagen domains of VtaA2 are involved in collagen attachment. No function in cell adhesion and invasion to epithelial alveolar cell line A549 or unspecific binding to primary alveolar macrophages was found. Likewise VtaA2 had no role in serum or phagocytosis resistance. We propose that VtaA2 mediates adherence to the host by binding to the mucin, found in the upper respiratory tract mucus, and to the extracellular matrix proteins, present in the connective tissue of systemic sites, such as the serosa.

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.

TLR2, Siglec-3 and CD163 expressions on porcine peripheral blood monocytes are increased during sepsis caused by Haemophilus parasuis

TLRs, Siglecs and CD163 are cell surface receptors that play an important role in immune response and sepsis. The objective of this study was to assess changes in the expression levels of several of these receptors (TLR2, TLR4, CD163, Siglec-1, Siglec-3, Siglec-5 and Siglec-10) on the surface of peripheral blood mononuclear cells from pigs with sepsis caused by Haemophilus parasuis. Flow cytometry was employed to analyze samples from an experimental infection and from cell cultures. A significant increase in CD163, TLR2 and Siglec-3 expression during infection was seen. However, in vitro exposure of peripheral blood monocytes to bacteria or sera from infected pigs did not increase the expression of these receptors. These changes may be due to recruitment of monocytes into the blood compartment in response to H. parasuis-induced sepsis.

Characterization of serotypes and virulence genes of Haemophilus parasuis isolates from Central Vietnam

Haemophilus parasuis is a commensal Gram-negative bacterial pathogen in the upper respiratory tract of pigs, which causes Glässer's disease. More than 15 serotypes of H. parasuis have been identified with apparent differences in virulence. In this research, we surveyed the prevalence and distribution of serotypes and known virulence genes of the H. parasuis isolates collected from sick and healthy pigs in Quang Binh and Thua Thien Hue provinces in Central Vietnam. By using bacterial isolation and polymerase chain reaction (PCR), 56 out of 814 (6.9%) samples were positive for H. parasuis. The most prevalent serotypes were serotype 5 (15/56, 26.8%), followed by serotype 2 (13/56, 23.2%) and serotype 4 (10/56, 17.9%). The vta1 was the most frequently detected virulence gene which was present in 62.5% of the strains, followed by vta3 (42.9%), vta2 (39.3%), HPM-1371 (35.7%), capD (30.4%), HPM-1372 (12.5%), lsgB and HPM-1373 (both shared 8.9%). Strong correlations between some serotypes and known virulence genes were observed, in which virulence genes HPM-1371, HPM-1372, vta3, vta2 and capD were mainly clustered in serotypes 5/12, and vta2 clustered in serotype 2. This study presents the first baseline information on the epidemiological characteristics of H. parasuis isolates from Central Vietnam.

Comparative genomic and methylome analysis of non-virulent D74 and virulent Nagasaki Haemophilus parasuis isolates

Haemophilus parasuis is a respiratory pathogen of swine and the etiological agent of Glässer's disease. H. parasuis isolates can exhibit different virulence capabilities ranging from lethal systemic disease to subclinical carriage. To identify genomic differences between phenotypically distinct strains, we obtained the closed whole-genome sequence annotation and genome-wide methylation patterns for the highly virulent Nagasaki strain and for the non-virulent D74 strain. Evaluation of the virulence-associated genes contained within the genomes of D74 and Nagasaki led to the discovery of a large number of toxin-antitoxin (TA) systems within both genomes. Five predicted hemolysins were identified as unique to Nagasaki and seven putative contact-dependent growth inhibition toxin proteins were identified only in strain D74. Assessment of all potential vtaA genes revealed thirteen present in the Nagasaki genome and three in the D74 genome. Subsequent evaluation of the predicted protein structure revealed that none of the D74 VtaA proteins contain a collagen triple helix repeat domain. Additionally, the predicted protein sequence for two D74 VtaA proteins is substantially longer than any predicted Nagasaki VtaA proteins. Fifteen methylation sequence motifs were identified in D74 and fourteen methylation sequence motifs were identified in Nagasaki using SMRT sequencing analysis. Only one of the methylation sequence motifs was observed in both strains indicative of the diversity between D74 and Nagasaki. Subsequent analysis also revealed diversity in the restriction-modification systems harbored by D74 and Nagasaki. The collective information reported in this study will aid in the development of vaccines and intervention strategies to decrease the prevalence and disease burden caused by H. parasuis.

Investigation of putative invasion determinants of Actinobacillus species using comparative genomics

Actinobacillus spp. are Gram-negative bacteria associated with mucosal membranes. While some are commensals, others can cause important human and animal diseases. A. pleuropneumoniae causes severe fibrinous hemorrhagic pneumonia in swine but not systemic disease whereas other species invade resulting in septicemia and death. To understand the invasive phenotype of Actinobacillus spp., complete genomes of eight isolates were obtained and pseudogenomes of five isolates were assembled and annotated. Phylogenetically, A. suis isolates clustered by surface antigen type and were more closely related to the invasive A. ureae, A. equuli equuli, and A. capsulatus than to the other swine pathogen, A. pleuropneumoniae. Using the LS-BSR pipeline, 251 putative virulence genes associated with serum resistance and invasion were detected. To our knowledge, this is the first genome-wide study of the genus Actinobacillus and should contribute to a better understanding of host tropism and mechanisms of invasion of pathogenic Actinobacillus and related genera.

Development and antigenic characterization of three recombinant proteins with potential for Glässer's disease prevention

Haemophilus parasuis is the causative agent of Glässer's disease, which causes high morbidity and mortality in piglets, leading to severe economic losses. The lack of a commercial vaccine against a broad spectrum of strains has limited the disease control. H. parasuis outer membrane proteins (OMPs) are potentially essential components for vaccine formulation. In this study, seven putative OMPs were selected from the annotated H. parasuis serovar 5 genome; they were predicted by bioinformatics and annotated as potential virulence-related factors. These proteins were cloned, expressed, and purified as His-tagged proteins. Antigenicity of the candidate proteins was assessed using Western blotting with convalescent sera against H. parasuis serovar 5. The immunogenicity of the seven OMPs was assessed in a guinea pig model. Except VacJ, all the other six recombinant proteins elicited a detectable antibody response. Antisera against four of the selected proteins effectively killed the bacteria in vitro. Three proteins (Omp26, VacJ, and HAPS_0742) were found to confer significant protection against challenge with a lethal dose of H. parasuis in a guinea pig model. The results suggest that these three proteins have a strong potential to be vaccine candidates against Glässer's disease.

Novel blaROB-1-bearing plasmid conferring resistance to β-lactams in Haemophilus parasuis isolates from healthy weaning pigs

Haemophilus parasuis, the causative agent of Glässer's disease, is one of the early colonizers of the nasal mucosa of piglets. It is prevalent in swine herds, and lesions associated with disease are fibrinous polyserositis and bronchopneumonia. Antibiotics are commonly used in disease control, and resistance to several antibiotics has been described in H. parasuis. Prediction of H. parasuis virulence is currently limited by our scarce understanding of its pathogenicity. Some genes have been associated with H. parasuis virulence, such as lsgB and group 1 vtaA, while biofilm growth has been associated with nonvirulent strains. In this study, 86 H. parasuis nasal isolates from farms that had not had a case of disease for more than 10 years were obtained by sampling piglets at weaning. Isolates were studied by enterobacterial repetitive intergenic consensus PCR and determination of the presence of lsgB and group 1 vtaA, biofilm formation, inflammatory cell response, and resistance to antibiotics. As part of the diversity encountered, a novel 2,661-bp plasmid, named pJMA-1, bearing the blaROB-1 β-lactamase was detected in eight colonizing strains. pJMA-1 was shown to share a backbone with other small plasmids described in the Pasteurellaceae, to be 100% stable, and to have a lower biological cost than the previously described plasmid pB1000. pJMA-1 was also found in nine H. parasuis nasal strains from a separate collection, but it was not detected in isolates from the lesions of animals with Glässer's disease or in nontypeable Haemophilus influenzae isolates. Altogether, we show that commensal H. parasuis isolates represent a reservoir of β-lactam resistance genes which can be transferred to pathogens or other bacteria.

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 intracellular Scots pine shoot symbiont Methylobacterium extorquens DSM13060 aggregates around the host nucleus and encodes eukaryote-like proteins

Endophytes are microbes that inhabit plant tissues without any apparent signs of infection, often fundamentally altering plant phenotypes. While endophytes are typically studied in plant roots, where they colonize the apoplast or dead cells, Methylobacterium extorquens strain DSM13060 is a facultatively intracellular symbiont of the meristematic cells of Scots pine (Pinus sylvestris L.) shoot tips. The bacterium promotes host growth and development without the production of known plant growth-stimulating factors. Our objective was to examine intracellular colonization by M. extorquens DSM13060 of Scots pine and sequence its genome to identify novel molecular mechanisms potentially involved in intracellular colonization and plant growth promotion. Reporter construct analysis of known growth promotion genes demonstrated that these were only weakly active inside the plant or not expressed at all. We found that bacterial cells accumulate near the nucleus in intact, living pine cells, pointing to host nuclear processes as the target of the symbiont’s activity. Genome analysis identified a set of eukaryote-like functions that are common as effectors in intracellular bacterial pathogens, supporting the notion of intracellular bacterial activity. These include ankyrin repeats, transcription factors, and host-defense silencing functions and may be secreted by a recently imported type IV secretion system. Potential factors involved in host growth include three copies of phospholipase A2, an enzyme that is rare in bacteria but implicated in a range of plant cellular processes, and proteins putatively involved in gibberellin biosynthesis. Our results describe a novel endophytic niche and create a foundation for postgenomic studies of a symbiosis with potential applications in forestry and agriculture.

All multicellular eukaryotes host communities of essential microbes, but most of these interactions are still poorly understood. In plants, bacterial endophytes are found inside all tissues. M. extorquens DSM13060 occupies an unusual niche inside cells of the dividing shoot tissues of a pine and stimulates seedling growth without producing cytokinin, auxin, or other plant hormones commonly synthesized by plant-associated bacteria. Here, we tracked the bacteria using a fluorescent tag and confocal laser scanning microscopy and found that they localize near the nucleus of the plant cell. This prompted us to sequence the genome and identify proteins that may affect host growth by targeting processes in the host cytoplasm and nucleus. We found many novel genes whose products may modulate plant processes from within the plant cell. Our results open up new avenues to better understand how bacteria assist in plant growth, with broad implications for plant science, forestry, and agriculture.

Molecular characterization of porcine Siglec-10 and analysis of its expression in blood and tissues

Siglecs are sialic acid binding Ig-like proteins involved in the control of leukocyte responses. In this study we describe the characterization of a porcine orthologue of Siglec-10. A cDNA clone was obtained from a porcine library which encodes a protein with sequence homology to human Siglec-10. This cDNA codes for a type I transmembrane protein containing four Ig-like domains, a transmembrane region, and a cytoplasmic tail with three tyrosine-based motifs, including a membrane-proximal Grb2-binding motif, and two ITIM motifs. When expressed on transfected cells, porcine Siglec-10 was able to bind red blood cells in a sialic acid-dependent manner. Monoclonal antibodies were developed against this protein and used to examine its cell and tissue distribution in the pig. Siglec-10 was found to be expressed on blood B cells and B cell areas of the spleen and lymph nodes. A weak expression was also detected on monocytes.

The use of genome wide association methods to investigate pathogenicity, population structure and serovar in Haemophilus parasuis

Background

Haemophilus parasuis is the etiologic agent of Glässer’s disease in pigs and causes devastating losses to the farming industry. Whilst some hyper-virulent isolates have been described, the relationship between genetics and disease outcome has been only partially established. In particular, there is weak correlation between serovar and disease phenotype. We sequenced the genomes of 212 isolates of H. parasuis and have used this to describe the pan-genome and to correlate this with clinical and carrier status, as well as with serotype.

Results

Recombination and population structure analyses identified five groups with very high rates of recombination, separated into two clades of H. parasuis with no signs of recombination between them. We used genome-wide association methods including discriminant analysis of principal components (DAPC) and generalised linear modelling (glm) to look for genetic determinants of this population partition, serovar and pathogenicity. We were able to identify genes from the accessory genome that were significantly associated with phenotypes such as potential serovar specific genes including capsule genes, and 48 putative virulence factors that were significantly different between the clinical and non-clinical isolates. We also show that the presence of many previously suggested virulence factors is not an appropriate marker of virulence.

Conclusions

These genes will inform the generation of new molecular diagnostics and vaccines, and refinement of existing typing schemes and show the importance of the accessory genome of a diverse species when investigating the relationship between genotypes and phenotypes.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1179) contains supplementary material, which is available to authorized users.

Biofilm formation by virulent and non-virulent strains of Haemophilus parasuis

Haemophilus parasuis is a commensal bacterium of the upper respiratory tract of healthy pigs. It is also the etiological agent of Glässer’s disease, a systemic disease characterized by polyarthritis, fibrinous polyserositis and meningitis, which causes high morbidity and mortality in piglets. The aim of this study was to evaluate biofilm formation by well-characterized virulent and non-virulent strains of H. parasuis. We observed that non-virulent strains isolated from the nasal cavities of healthy pigs formed significantly (p < 0.05) more biofilms than virulent strains isolated from lesions of pigs with Glässer’s disease. These differences were observed when biofilms were formed in microtiter plates under static conditions or formed in the presence of shear force in a drip-flow apparatus or a microfluidic system. Confocal laser scanning microscopy using different fluorescent probes on a representative subset of strains indicated that the biofilm matrix contains poly-N-acetylglucosamine, proteins and eDNA. The biofilm matrix was highly sensitive to degradation by proteinase K. Comparison of transcriptional profiles of biofilm and planktonic cells of the non-virulent H. parasuis F9 strain revealed a significant number of up-regulated membrane-related genes in biofilms, and genes previously identified in Actinobacillus pleuropneumoniae biofilms. Our data indicate that non-virulent strains of H. parasuis have the ability to form robust biofilms in contrast to virulent, systemic strains. Biofilm formation might therefore allow the non-virulent strains to colonize and persist in the upper respiratory tract of pigs. Conversely, the planktonic state of the virulent strains might allow them to disseminate within the host.

Comparative proteomic analysis of the membrane proteins of two Haemophilus parasuis strains to identify proteins that may help in habitat adaptation and pathogenesis

Background

Haemophilus parasuis is the causative agent of Glässer’s disease characterized by polyserositis, arthritis, and meningitis in pig, leading to serious economic loss. Despite many years of study, virulence factors and the mechanisms of the entire infection process remain largely unclear. So two-dimensional gel electrophoresis and mass spectrometry were used to search for distinctions at the membrane protein expression level between two H. parasuis isolates aimed at uncovering some proteins potentially involved in habitat adaption and pathogenesis.

Results

A comparative proteomic approach combining two-dimensional gel electrophoresis with mass spectrometry and tandem mass spectrometry was employed to explore the differences among membrane proteomes of a virulent Haemophilus parasuis strain isolated from the lung of a diseased pig and an avirulent strain isolated from the nasal swab of a healthy pig. Differentially expressed protein spots identified by mass spectrometry were annotated and analyzed by bioinformatic interpretation. The mRNA level was determined by quantitative real-time PCR. Proteins representing diverse functional activities were identified. Among them, the tonB-dependent siderophore receptor was a new discovery highlighted for its activity in iron uptake. In addition, periplasmic serine protease and putrescine/spermidine ABC transporter substrate-binding protein were given focus because of their virulence potential. This study revealed that the differentially expressed proteins were important in either the habitat adaption or pathogenesis of H. parasuis.

Conclusions

The outcome demonstrated the presence of some proteins which raise the speculation for their importance in helping in habitat adaption or pathogenesis within the host.

Genetic diversity of Haemophilus parasuis from sick and healthy pigs

A collection of 94 Haemophilus parasuis isolates was used for this study. It consisted of isolates from organs of pigs with Glässer's disease and pneumonia (n=54), from nasal swabs of healthy pigs in farms without Glässer's disease problems (n=25), and 15 reference strains. These isolates were typed using a new multilocus variable number of tandem repeats analysis (MLVA) protocol and investigated for the presence of nine putative virulence genes. The new MLVA protocol was highly discriminatory (54 types identified and discrimination index of 97.4%) and reproducible. Similar to previous investigations done with other methods, two major genetic clusters were identified by MLVA, which partially correlated with serotype and virulence gene distributions. Gene linkage analysis suggested that lateral gene transfer occurs within each of these clusters, but rarely between them. Although one single MLVA type included more than 20% of the clinical isolates, no significant correlation was detected between a specific MLVA type, the major genetic clusters, or the presence of any of the virulence genes investigated or the source of the isolates (clinical infection vs. healthy pig). The MLVA typing protocol described in this study is a promising new tool for future investigations into the epidemiology of Glässer's disease and could help us to better understand interacting microbial, host and environmental factors that lead to the development of H. parasuis disease.

Gene content and diversity of the loci encoding biosynthesis of capsular polysaccharides of the 15 serovar reference strains of Haemophilus parasuis

Haemophilus parasuis is the causative agent of Glässer's disease, a systemic disease of pigs, and is also associated with pneumonia. H. parasuis can be classified into 15 different serovars. Here we report, from the 15 serotyping reference strains, the DNA sequences of the loci containing genes for the biosynthesis of the group 1 capsular polysaccharides, which are potential virulence factors of this bacterium. We contend that these loci contain genes for polysaccharide capsule structures, and not a lipopolysaccharide O antigen, supported by the fact that they contain genes such as wza, wzb, and wzc, which are associated with the export of polysaccharide capsules in the current capsule classification system. A conserved region at the 3' end of the locus, containing the wza, ptp, wzs, and iscR genes, is consistent with the characteristic export region 1 of the model group 1 capsule locus. A potential serovar-specific region (region 2) has been found by comparing the predicted coding sequences (CDSs) in all 15 loci for synteny and homology. The region is unique to each reference strain with the exception of those in serovars 5 and 12, which are identical in terms of gene content. The identification and characterization of this locus among the 15 serovars is the first step in understanding the genetic, molecular, and structural bases of serovar specificity in this poorly studied but important pathogen and opens up the possibility of developing an improved molecular serotyping system, which would greatly assist diagnosis and control of Glässer's disease.

Changes in macrophage phenotype after infection of pigs with Haemophilus parasuis strains with different levels of virulence

Haemophilus parasuis is a colonizer of healthy piglets and the etiological agent of Glässer's disease. Differences in virulence among strains of H. parasuis have been widely observed. In order to explore the host-pathogen interaction, snatch-farrowed colostrum-deprived piglets were intranasally infected with 4 strains of H. parasuis: reference virulent strain Nagasaki, reference nonvirulent strain SW114, field strain IT29205 (from a systemic lesion and virulent in a previous challenge), and field strain F9 (from the nasal cavity of a healthy piglet). At different times after infection, two animals of each group were euthanized and alveolar macrophages were analyzed for the expression of CD163, CD172a, SLA I (swine histocompatibility leukocyte antigen I), SLA II, sialoadhesin (or CD169), and CD14. At 1 day postinfection (dpi), virulent strains induced reduced expression of CD163, SLA II, and CD172a on the surfaces of the macrophages, while nonvirulent strains induced increased expression of CD163, both compared to noninfected controls. At 2 dpi, the pattern switched into a strong expression of CD172a, CD163, and sialoadhesin by the virulent strains, which was followed by a steep increase in interleukin 8 (IL-8) and soluble CD163 in serum at 3 to 4 dpi. The early increase in surface expression of CD163 induced by nonvirulent strains went along with higher levels of IL-8 in serum than those induced by virulent strains in the first 2 days of infection. Alpha interferon (IFN-α) induction was observed only in animals infected with nonvirulent strains. Overall, these results are compatible with a delay in macrophage activation by virulent strains, which may be critical for disease production.

Differential interactions of virulent and non-virulent H. parasuis strains with naïve or swine influenza virus pre-infected dendritic cells

Pigs possess a microbiota in the upper respiratory tract that includes Haemophilus parasuis. Pigs are also considered the reservoir of influenza viruses and infection with this virus commonly results in increased impact of bacterial infections, including those by H. parasuis. However, the mechanisms involved in host innate responses towards H. parasuis and their implications in a co-infection with influenza virus are unknown. Therefore, the ability of a non-virulent H. parasuis serovar 3 (SW114) and a virulent serovar 5 (Nagasaki) strains to interact with porcine bone marrow dendritic cells (poBMDC) and their modulation in a co-infection with swine influenza virus (SwIV) H3N2 was examined. At 1 hour post infection (hpi), SW114 interaction with poBMDC was higher than that of Nagasaki, while at 8 hpi both strains showed similar levels of interaction. The co-infection with H3N2 SwIV and either SW114 or Nagasaki induced higher levels of IL-1β, TNF-α, IL-6, IL-12 and IL-10 compared to mock or H3N2 SwIV infection alone. Moreover, IL-12 and IFN-α secretion differentially increased in cells co-infected with H3N2 SwIV and Nagasaki. These results pave the way for understanding the differences in the interaction of non-virulent and virulent strains of H. parasuis with the swine immune system and their modulation in a viral co-infection.