Influenza A virus-infected hosts boost an invasive type of Streptococcus pyogenes infection in mice

A mouse model of lethal synergism between influenza virus and Haemophilus influenzae

Secondary bacterial infections that follow infection with influenza virus result in considerable morbidity and mortality in young children, the elderly, and immunocompromised individuals and may also significantly increase mortality in normal healthy adults during influenza pandemics. We herein describe a mouse model for investigating the interaction between influenza virus and the bacterium Haemophilus influenzae. Sequential infection with sublethal doses of influenza and H. influenzae resulted in synergy between the two pathogens and caused mortality in immunocompetent adult wild-type mice. Lethality was dependent on the interval between administration of the bacteria and virus, and bacterial growth was prolonged in the lungs of dual-infected mice, although influenza virus titers were unaffected. Dual infection induced severe damage to the airway epithelium and confluent pneumonia, similar to that observed in victims of the 1918 global influenza pandemic. Increased bronchial epithelial cell death was observed as early as 1 day after bacterial inoculation in the dual-infected mice. Studies using knockout mice indicated that lethality occurs via a mechanism that is not dependent on Fas, CCR2, CXCR3, interleukin-6, tumor necrosis factor, or Toll-like receptor-4 and does not require T or B cells. This model suggests that infection with virulent strains of influenza may predispose even immunocompetent individuals to severe illness on secondary infection with H. influenzae by a mechanism that involves innate immunity, but does not require tumor necrosis factor, interleukin-6, or signaling via Toll-like receptor-4.

Enhancement of immune responses to influenza vaccine (H3N2) by ginsenoside Re

This study was designed to evaluate the adjuvant effect of ginsenoside Re isolated from the root of Panax ginseng on the immune responses elicited by split inactivated H3N2 influenza virus antigen in a mouse model. Forty-eight ICR mice were randomly distributed into six groups with 8 mice in each group. All animals were subcutaneously (s.c.) immunized twice on weeks 0 and 3 with 50 microg Re, inactivated H3N2 influenza virus antigen equivalent to 10 or 100 ng of hemogglutinin (HA) or inactivated H3N2 influenza virus antigen equivalent to 10 ng HA adjuvanted with Re (25, 50 or 100 microg). Two weeks after the boost, blood samples were collected for measurement of serum IgG, the IgG isotypes and HI titers. Splenocytes were separated for the detection of lymphocyte proliferation and production of IFN-gamma and IL-5 in vitro. Results showed that co-administration of Re significantly enhanced serum specific IgG, IgG1, IgG2a and IgG2b responses, HI titers, lymphocyte proliferation responses as well as IFN-gamma and IL-5 secretions, indicating that both Th1 and Th2 were activated. Considering the adjuvant effect demonstrated in this study, Re deserve further studies for improving the quality of vaccines where mixed Th1/Th2 immune responses are needed.

Intervention strategies for an influenza pandemic taking into account secondary bacterial infections

Influenza infections often predispose individuals to consecutive bacterial infections. Both during seasonal and pandemic influenza outbreaks, morbidity and mortality due to secondary bacterial infections can be substantial. With the help of a mathematical model, we investigate the potential impact of such bacterial infections during an influenza pandemic, and we analyze how antiviral and antibacterial treatment or prophylaxis affect morbidity and mortality. We consider different scenarios for the spread of bacteria, the emergence of antiviral resistance, and different levels of severity for influenza infections (1918-like and 2009-like). We find that while antibacterial intervention strategies are unlikely to play an important role in reducing the overall number of cases, such interventions can lead to a significant reduction in mortality and in the number of bacterial infections. Antibacterial interventions become even more important if one considers the--very likely--scenario that during a pandemic outbreak, influenza strains resistant to antivirals emerge. Overall, our study suggests that pandemic preparedness plans should consider intervention strategies based on antibacterial treatment or prophylaxis through drugs or vaccines as part of the overall control strategy. A major caveat for our results is the lack of data that would allow precise estimation of many of the model parameters. As our results show, this leads to very large uncertainty in model outcomes. As we discuss, precise assessment of the impact of antibacterial strategies during an influenza pandemic will require the collection of further data to better estimate key parameters, especially those related to the bacterial infections and the impact of antibacterial intervention strategies.

Poly(gamma-glutamic acid) nano-particles combined with mucosal influenza virus hemagglutinin vaccine protects against influenza virus infection in mice

Adding poly(gamma-glutamic acid) nano-particles (gamma-PGA-NPs), a safe, natural material, to subcutaneous immunization with influenza virus hemagglutinin (HA) vaccine increases the protective immune responses against influenza virus in mice. Here, we examined whether intranasal administration of the HA vaccine with gamma-PGA-NPs would induce protection from influenza virus challenge in mice. Intranasal immunization with the mixture of gamma-PGA-NPs and HA vaccine from an influenza virus strain A/PR/8/34 (H1N1) or A/New Caledonia/20/99 (H1N1) enhanced protection of mice from A/PR/8/34 infection. Intranasal immunization with A/New Caledonia/20/99 HA vaccine and gamma-PGA-NPs induced cell-mediated immune responses and neutralizing antibody production for both A/New Caledonia/20/99 and A/PR/8/34. These data suggest that gamma-PGA-NPs may have potential for clinical applications as a mucosal adjuvant.

Herpesviruses and periodontal disease: a cautionary tale

Periodontitis is an inflammatory disease of bacterial origin, characterized by an inconstant progression of lesions affecting the tooth supporting tissues. In spite of more than half a century of research efforts, the clinician still lacks any specific molecular or microbial diagnostic tool to predict the progression of periodontal lesions. Recently, several reports have proposed a role for some herpesviruses in the etiology of destructive phases of periodontitis. This paper critically analyzes these data in the light of consolidated knowledge that was developed in the characterization of virus-bacteria cooperative interactions, and proposes new topics of investigation to clarify the role of herpesviral infections in periodontitis and their potential predictive role as markers of progression.

Epidemiology

The common cold is the result of an upper respiratory tract infection causing an acute syndrome characterised by a combination of non-specific symptoms, including sore throat, cough, fever, rhinorrhoea, malaise, headache, and myalgia. Respiratory viruses, alone or in combination, are the most common cause. The course f illness can be complicated by bacterial agents, causing pharyngitis or sinusitis, but the are a rare cause of cold and flu-like illnesses (CFLIs). Our understanding of CFLI epidemiology has been enhanced by molecular detection methods, particularly polymerase chain reaction (PCR) testing. PCR has not only improved detection of previously known viruses, but within the last decade has resulted in the detection of many divergent novel respiratory virus species. Human rhinovirus (HRV) infections cause nearly all CFLIs and they can be responsible for asthma and chronic obstructive pulmonary disease exacerbations. HRVs are co-detected with other respiratory viruses in statistically significant patterns, with HRVs occurring in the lowest proportion of co-detections, compared to most other respiratory viruses. Some recently identified rhinoviruses may populate an entirely new putative HRV species; HRV C. Further work is required to confirm a causal role for these newly identified viruses in CFLIs. The burden of illness associated with CFLIs is poorly documented, but where data are available, the impact of CFLIs is considerable. Individual infections, although they do not commonly result in more severe respiratory tract illness, are associated with substantial direct and indirect resource use. The product of frequency and burden for CFLIs is likely to be greater in magnitude than for any other respiratory syndrome, but further work is required to document this. Our understanding of the viral causes of CLFIs, although incomplete, has improved in recent years. Documenting burden is also an important step in progress towards improved control and management of these illnesses.

Encephalitis lethargica and influenza. I. The role of the influenza virus in the influenza pandemic of 1918/1919

An investigation of the characteristics of influenza epidemics in the nineteenth and early twentieth centuries was undertaken, principally in order to analyze the role of the 1918/1919 influenza pandemic in the etiology of encephalitis lethargica. Expectations regarding a future influenza pandemic derive principally from experiences in the 1918 epidemic. It is proposed that this pandemic was atypical with respect to many of its features, and that these have not been appropriately regarded in mapping expectations and responses of a future pandemic. Both a longer historical viewpoint (incorporating knowledge from all major nineteenth and twentieth century epidemics) and closer examination of individual epidemics at the town level is essential for producing an accurate picture of the challenge.

Rhinovirus disrupts the barrier function of polarized airway epithelial cells

RATIONALE

Secondary bacterial infection following rhinovirus (RV) infection has been recognized in chronic obstructive pulmonary disease.

OBJECTIVES

We sought to understand mechanisms by which RV infection facilitates secondary bacterial infection.

METHODS

Primary human airway epithelial cells grown at air-liquid interface and human bronchial epithelial (16HBE14o-) cells grown as polarized monolayers were infected apically with RV. Transmigration of bacteria (nontypeable Haemophilus influenzae and others) was assessed by colony counting and transmission electron microscopy. Transepithelial resistance (R(T)) was measured by using a voltmeter. The distribution of zona occludins (ZO)-1 was determined by immunohistochemistry and immunoblotting.

MEASUREMENTS AND MAIN RESULTS

Epithelial cells infected with RV showed 2-log more bound bacteria than sham-infected cultures, and bacteria were recovered from the basolateral media of RV- but not sham-infected cells. Infection of polarized airway epithelial cell cultures with RV for 24 hours caused a significant decrease in R(T) without causing cell death or apoptosis. Ultraviolet-treated RV did not decrease R(T), suggesting a requirement for viral replication. Reduced R(T) was associated with increased paracellular permeability, as determined by flux of fluorescein isothiocyanate (FITC)-inulin. Neutralizing antibodies to tumor necrosis factor (TNF)-alpha, IFN-gamma and IL-1beta reversed corresponding cytokine-induced reductions in R(T) but not that induced by RV, indicating that the RV effect is independent of these proinflammatory cytokines. Confocal microscopy and immunoblotting revealed the loss of ZO-1 from tight junction complexes in RV-infected cells. Intranasal inoculation of mice with RV1B also caused the loss of ZO-1 from the bronchial epithelium tight junctions in vivo.

CONCLUSIONS

RV facilitates binding, translocation, and persistence of bacteria by disrupting airway epithelial barrier function.

Concurrent influenza A and group A beta-hemolytic streptococcal pharyngotonsillitis

OBJECTIVES

A concurrent group A beta-hemolytic Streptococcus (GABHS)-influenza virus pharyngotonsillitis (PT) is generally not considered in diagnoses, even though mixed bacterial-viral infections are common in other respiratory tract infections. This report describes our experience in diagnosing a potential mixed GABHS-influenza virus PT in children.

METHODS

Acute and convalescent antistreptolysin O (ASO) and anti-DNase B titers were obtained from 12 children with acute PT and clinical presentation that suggested viral infection, and in whom both rapid influenza A virus and rapid GABHS tests were positive.

RESULTS

The children did not receive any antimicrobial therapy, and all recovered from their acute PT within 2 to 5 days and were all asymptomatic upon their return visit 3 to 4 weeks later. GABHS was recovered from 2 of the children on their return visit. However, ASO and anti-DNase B titers were not elevated in these individuals. The ASO and anti-DNase B titers determined in the first serum samples were less than the age-adjusted normal values for all of the children. However, these titers rose by at least two-dilution (0.2 logarithm) in the convalescent sera as compared with the acute ones in 4 of the 12 children (33%). One of the 8 children who had no increase in ASO and anti-DNase B titers had an acute GABHS PT 5 months later. One-year follow-up of all of the children showed no anomalies.

CONCLUSIONS

This report is the first to describe a concomitant GABHS and influenza A virus PT, as evident by increased ASO and anti-DNase B titers in a third of the patients who had both of these organisms detected in their upper airways.

Migration, cell-cell interaction and adhesion in the immune system

Migration is an essential function of immune cells. It is necessary to lead immune cell precursors from their site of generation to the places of maturation or function. Cells of the adaptive immune system also need to interact physically with each other or with specialized antigen presenting cells in lymphatic tissues in order to become activated. Thereby a complex series of controlled migration events, adhesive interactions and signalling responses is induced. Finally cells must be able to leave the activating tissues and re-enter the bloodstream from which they extravasate into inflamed tissue sites. Cells of the innate immune system can function directly without the need for previous activation. However, these cells have to adapt their function to a panoply of pathogens and environmental niches which can be invaded. The current review highlights the central aspects of cellular dynamics underlying adaptive and innate cellular immunity. Thereby a focus will be put on recent results obtained by microscopic observation of live cells in vitro or by intravital 2-photon microscopy in live animals.

Filamentous influenza A virus infection predisposes mice to fatal septicemia following superinfection with Streptococcus pneumoniae serotype 3

Previous studies have demonstrated that animals exposed to Streptococcus pneumoniae while recovering from influenza A virus infection exhibit exacerbated disease symptoms. However, many of the current animal models exploring dual viral and bacterial synergistic exacerbations of respiratory disease have utilized mouse-adapted influenza virus and strains of Streptococcus pneumoniae that in themselves are highly lethal to mice. Here we describe a mouse model of bacterial superinfection in which a mild, self-limiting influenza virus infection is followed by mild, self-limiting superinfection with S. pneumoniae serotype 3. S. pneumoniae superinfection results in rapid dissemination of the bacterium from the respiratory tract and systemic spread to all major organs of the mice, resulting in fatal septicemia. This phenomenon in mice was observed in superinfected animals undergoing an active viral infection as well as in mice that had completely cleared the virus 7 to 8 days prior to superinfection. Neutrophils were the predominant cellular inflammatory infiltrate in the lungs of superinfected mice compared to singly infected animals. Among other cytokines and chemokines, the neutrophil activator granulocyte colony-stimulating factor (G-CSF) was found to be significantly overexpressed in the spleens, lungs, and brains of superinfected animals. High G-CSF protein levels were observed in sera and lung lavage fluid from superinfected animals, suggesting that G-CSF is a major contributor to synergistic exacerbation of disease leading to fatal septicemia.

RSV mediates Pseudomonas aeruginosa binding to cystic fibrosis and normal epithelial cells

Cystic fibrosis lung disease typically has a course of exacerbations and remissions, suggesting that external factors like viral infections can influence this course. Clinical data suggest synergism between respiratory syncytial virus (RSV) infections and Pseudomonas aeruginosa in cystic fibrosis (CF) lung disease. We studied the influence of RSV infection on adherence of P. aeruginosa to IB3-1, HEp-2, and A549 epithelial cell monolayers in vitro. RSV infection of epithelial cells as well as simultaneous addition of RSV and P. aeruginosa to noninfected cells both strongly enhanced the pseudomonal adherence to epithelial cells. The increased adherence varied from 1.2- to 8.2-fold in case of previous RSV infection, and from 1.7- to 16.1-fold in case of simultaneous addition. We observed direct binding of RSV to P. aeruginosa, and blocking of RSV with heparin eliminated the effect on increased adherence. This suggests that RSV possibly acts as a coupling agent between P. aeruginosa and epithelial cells. In conclusion, RSV enhances P. aeruginosa infection of respiratory epithelial cells. It suggests a role of specific viral-bacterial interactions in exacerbations of CF lung disease, which could have important implications on prevention and treatment.

Increased susceptibility to bacterial superinfection as a consequence of innate antiviral responses

The reason why severe localized or systemic virus infections enhance and aggravate bacterial superinfection is poorly understood. Here we show that virus-induced IFN type I caused apoptosis in bone marrow granulocytes, drastically reduced granulocyte infiltrates at the site of bacterial superinfection, caused up to 1,000-fold higher bacterial titers in solid organs, and increased disease susceptibility. The finding that the innate antiviral immune response reduces the antibacterial granulocyte defense offers an explanation for enhanced susceptibility to bacterial superinfection during viral disease.

Fatal group A streptococcal myopericarditis during influenza A infection

Influenza A infection can be a serious and life-threatening disease in young children. Even in those who die, however, the cause of death may not be obvious. In this illustrative report, a 7-year-old child with acute influenza A infection developed an unsuspected fatal secondary infection caused by group A streptococcus. The diagnosis of bacterial myopericarditis was made at autopsy.

Efficacy of quinolones against secondary pneumococcal pneumonia after influenza virus infection in mice

We established a mouse model of secondary pneumococcal pneumonia after influenza virus infection and investigated the efficacy of several quinolones against pneumonia in this model. Gatifloxacin exhibited the highest efficacy among the quinolones examined and is probably useful for the treatment of secondary bacterial pneumonia.

Rhinoviruses promote internalisation of Staphylococcus aureus into non-fully permissive cultured pneumocytes

Respiratory viruses, including rhinoviruses, frequently promote bacterial opportunistic infections, through mechanisms that still deserve to be investigated in detail. This work was aimed at understanding how a viral infection mostly affecting the upper respiratory tract, such as the common cold, can repeatedly promote opportunistic infections in the lower airways, a site where viral replication is limited. The adhesivity and invasivity of Staphylococcus aureus were evaluated, in permissive and non-permissive cells, infected with Rhinovirus-1b. The role of inflammatory cytokines, and of ICAM-1 overexpression in the Rhinovirus-S. aureus cooperation was evaluated. Rhinovirus-1b enhanced the efficiency of internalisation of S. aureus irrespective of cellular permissivity, even when very low viral multiplicities of infection were used. Experiments performed with UV inactivated and heat inactivated viral particles suggested that this enhancement does not depend upon viral replication, but requires viral adhesion. Experimental data suggest that Rhinovirus-1b can significantly increase the ability of S. aureus to internalise into pneumocytes with a mechanism that involves the virus induced release of IL-6 and IL-8, and the overexpression of ICAM-1. Overall data disclose a possible mechanism through which rhinoviruses can promote bacterial infections in the lower respiratory tract.

Direct binding of respiratory syncytial virus to pneumococci: a phenomenon that enhances both pneumococcal adherence to human epithelial cells and pneumococcal invasiveness in a murine model

In a previous study we showed that pneumococcal adherence to epithelial cells was enhanced by a preceding respiratory syncytial virus (RSV) infection. RSV-glycoproteins, expressed on the infected cell surface, may play a role in this enhanced pneumococcal binding, by acting as bacterial receptors. In the current study, it was attempted to analyze the capacity of pneumococci to interact directly with RSV virions. By flow-cytometry, a direct interaction between RSV and pneumococci could be detected. Heparin, an inhibitor of RSV infectivity that interacts with RSV protein-G, blocked RSV-pneumococcal binding, indicating that the latter interaction is indeed mediated by protein-G. RSV-pneumococcal complexes showed enhanced adherence to uninfected human epithelial cells, compared with pneumococcal adherence without bound RSV, and this enhancement was also blocked by heparin. In addition, the significance of these findings in vitro was explored in vivo in a murine model. Both mice that were pretreated with RSV at day 4 before pneumococcal challenge and mice infected with both agents simultaneously showed significantly higher levels of bacteraemia than controls. Simultaneous infection with both agents enhanced the development of pneumococcal bacteraemia most strongly. It was hypothesized that direct viral binding is another mechanism by which RSV can induce enhanced pneumococcal binding to epithelial cells, a phenomenon that is translated in vivo by a higher invasiveness of pneumococci when administered simultaneously with RSV to mice. Apparently, RSV acts in this process as a direct coupling particle between bacteria and uninfected epithelial cells, thereby increasing colonization by and enhancing invasiveness of pneumococci.

Co-infection between influenza virus and flagellated bacteria

Trypsin is required in the hemagglutinin (HA) cleavage to in vitro influenza viruses activation. This HA cleavage is necessary for virus cell entry by receptor-mediated endocytosis. Bacteria in the respiratory tract are potential sources of proteases that could contribute to the cleavage of influenza virus in vivo. From 47 samples collected from horses, pigs, and from humans, influenza presence was confirmed in 13 and these samples demonstrated co-infection of influenza with flagellated bacteria, Stenotrophomonas maltophilia from the beginning of the experiments. Despite treatment with antibiotics, the bacteria remained resistant in several of the co-infected samples (48.39%). These bacteria, considered opportunistic invaders from environmental sources, are associated with viral infections in upper respiratory tract of hosts. The protease (elastase), secreted by Stenotrophomonas maltophilia plays a role in the potentiation of influenza virus infection. Proteolytic activity was detected by casein agar test. Positive samples from animals and humans had either a potentiated influenza infectivity or cytopathic effect (CPE) in MDCK and NCI H292 cells, Stenotrophomonas maltophilia were always present. Virus and bacteria were observed ultrastructurally. These in vitro findings show that microbial proteases could contribute to respiratory complications by host protease activity increasing inflammation or destroying endogenous cell protease inhibitors.

Systemic immunization with streptococcal immunoglobulin-binding protein Sib35 induces protective immunity against group A Streptococcus challenge in mice

The streptococcal immunoglobulin (Ig)-binding protein Sib 35 binds to IgG, IgM and IgA in human, mouse and bovine. Since all group A Streptococcus pyogenes (GAS) strains examined express the sib 35 gene, we evaluated the Sib 35 as a vaccine candidate against GAS infections. We detected significantly higher anti-Sib 35 IgG antibody titers in sera from patients with GAS infections than from healthy volunteers. Immunization of mice with Sib 35 induced antigen-specific IgG antibodies in their sera, and rabbit Sib 35-specific antiserum showed opsonic activity. Immunization with Sib 35 enhanced survival rates in mice challenged with a GAS strain, while exhibiting no toxicity in hosts. We conclude that Sib 35 is a promising vaccine for prevention of GAS infections.

Novel mechanism of immunosuppression by influenza virus haemagglutinin: selective suppression of interleukin 12 p35 transcription in murine bone marrow-derived dendritic cells

Infection with influenza virus strongly predisposes an individual to bacterial superinfection, which is often the significant cause of morbidity and mortality during influenza epidemics. Little is known about the immunomodulating properties of the virus that lead to this phenomenon, but the effect of the viral components on the development of immune dendritic cells (DCs) may prove vital. In this study, activation of and cytokine secretion by bacterial lipopolysaccharide (LPS)-stimulated bone marrow-derived dendritic cells (BMDCs) following treatment with the influenza virus major antigen haemagglutinin (HA) were examined. HA selectively inhibits the release of LPS-induced interleukin 12 (IL12) p70, which is independent of IL10 secretion. Suppression occurs at the transcriptional level, with selective inhibition of p35- and not p40-subunit mRNA expression. The downregulation of IL12 p70 by influenza HA is a novel and unexplored pathway that may be relevant in the predisposition to bacterial superinfection associated with influenza virus infections.

Retrospective review of serious bacterial infections in infants who are 0 to 36 months of age and have influenza A infection

OBJECTIVE

Previous studies of febrile children who were 3 to 36 months of age and had clinically recognizable viral syndromes have shown low rates of concurrent bacteremia. We sought to determine the prevalence of serious bacterial infections (SBIs) among children with influenza A, a viral syndrome that can be established definitively by specific tests.

METHODS

We performed a retrospective cross-sectional study of patients who were 0 to 36 months of age and presented with fever to the emergency department (ED) over 4 consecutive influenza seasons. Chest radiographs and urine and cerebrospinal fluid cultures also were reviewed.

RESULTS

Of 705 included patients, 163 (23%) were influenza positive (IP) and 542 (77%) were influenza negative (IN). Only 1 IP patient was bacteremic (0.6%) versus 23 of the 542 IN control subjects (4.2%). Two (1.8%) of 110 IP cases had urinary tract infections versus 38 (9.9%) of the 382 IN control subjects. Thirteen (25.4%) of 51 IP patients had radiographic evidence of pneumonia versus 99 (41.9%) of 236 IN control subjects. There were no cases of meningitis in 41 cerebrospinal fluid samples obtained from IP patients versus 4 (2.2%) cases of culture-positive meningitis in 179 IN control subjects. A total of 16 (9.8%) SBIs were identified in the IP cases versus 153 (28.2%) in the IN control subjects.

CONCLUSIONS

Febrile children with influenza A had a lower prevalence of bacteremia, urinary tract infections, consolidative pneumonia, or any SBI compared with those without influenza A infection in this study.

Vaccination with formalin-inactivated influenza vaccine protects mice against lethal influenza Streptococcus pyogenes superinfection

Intranasal infection with non-lethal influenza A virus (IAV) followed by infection with group A streptococci (GAS) induces invasive, lethal GAS infections, including necrotizing fasciitis, in mice. We demonstrate that subcutaneous immunization of formalin-inactivated IAV vaccine or intranasal immunization of IAV vaccine and cholera toxin protected more than 75% of mice from death by lethal IAV-GAS superinfection. The increased survival rate correlates with increase in IAV neutralizing activity and the levels of serum anti-IAV IgG. Moreover, elimination of IAV from the lungs of vaccinated mice led to depletion of GAS associated with alveolar epithelial cells. These findings suggest that formalin-inactivated IAV vaccine may be useful for prevention of secondary bacterial infections following prior IAV exposure.

The Streptococcus pyogenes capsule is required for adhesion of bacteria to virus-infected alveolar epithelial cells and lethal bacterial-viral superinfection

An apparent worldwide resurgence of invasive group A Streptococcus (GAS) infections remains unexplained. However, we recently demonstrated in mice that when an otherwise nonlethal intranasal GAS infection is preceded by a nonlethal influenza A virus (IAV) infection, induction of lethal invasive GAS infections is often the result. In the present study, we established several isogenic mutants from a GAS isolate and evaluated several virulence factors as candidates responsible for the induction of invasive GAS infections. Disruption of the synthesis of the capsule, Mga, streptolysin O, streptolysin S, or streptococcal pyrogenic exotoxin B of GAS significantly reduced mortality among mice superinfected with IAV and a mutant. In addition, the number of GAS organisms adhering to IAV-infected alveolar epithelial cells was markedly reduced with the capsule-depleted mutant, although this was not the case with the other mutants. Wild-type GAS was found to bind directly to IAV particles, whereas the nonencapsulated mutant showed much less ability to bind. These results suggest that the capsule plays a key role in the invasion of host tissues by GAS following superinfection with IAV and GAS.

Proapoptotic effect of proteolytic activation of matrix metalloproteinases by Streptococcus pyogenes thiol proteinase (Streptococcus pyrogenic exotoxin B)

Streptococcus pyogenes thiol proteinase, also known as streptococcal pyrogenic exotoxin B (SpeB), has been suggested to be a major virulence factor in S. pyogenes infection. SpeB was reported to induce apoptosis of host cells, but its mechanism of action is not yet fully understood. In this study, we examined the involvement of matrix metalloproteinases (MMPs) in SpeB-induced apoptosis. We first developed a large-scale preparation of recombinant SpeB and precursors of human MMP-9 and -2 (proMMPs) by using Escherichia coli Rosetta (DE3)pLysS and baculovirus-insect cell expression systems, respectively. Treatment with SpeB induced effective proteolytic activation of both proMMP-9 and -2. When RAW264 murine macrophages were incubated with SpeB-activated proMMP-9, the level of tumor necrosis factor alpha (TNF-alpha) in conditioned medium (CM), assessed by an enzyme immunoassay, was elevated. This increase was completely inhibited by addition of the MMP inhibitor SI-27 to the cell culture. The CM also produced marked induction of apoptosis of U937 human monocytic cells. Similarly, soluble Fas ligand (sFasL) was detected in CM of cultures of SW480 cells expressing FasL after treatment with SpeB-activated proMMPs; this CM also induced apoptosis in U937 cells. SpeB had a direct effect as well and caused the release of TNF-alpha and sFasL from the cells. SpeB-dependent production of MMP-9 and -2 and proapoptotic molecules (TNF-alpha and sFasL) was evident in a murine model of severe invasive S. pyogenes infection. These results suggest that SpeB or SpeB-activated MMPs contribute to tissue damage and streptococcal invasion in the host via extracellular release of TNF-alpha and sFasL.

How do viral infections predispose patients to bacterial infections?

PURPOSE OF REVIEW

Bacterial sepsis is a leading cause of death in the United States, accounting for over 200,000 fatalities annually. Approximately half of bacterial sepsis cases occur following acute respiratory infections, and the lungs are the most common organs to fail. Notably, outbreaks of respiratory viral infections are associated with an increased incidence or severity of bacterial co-infections, with normally innocuous infections often becoming fatal. Understanding the 'lethal synergism' associated with concomitant infections may point the way toward improved anti-sepsis treatments.

RECENT FINDINGS

Murine models of viral and bacterial co-infection mimic the lethal synergism observed in humans and reveal at least two mechanisms of interaction. First, bacterial infiltration is heightened during acute viral infection. Secondly, the nature of responding cell populations is dramatically altered during concomitant infections. Although natural killer cells and macrophages are predominant cell populations responding to bacterial infection in a naïve host, there is also a large T cell component that is activated upon viral infection. Inflammatory cytokines produced by these cells contribute to lethal immunopathology, and therapeutic strategies need to target the initial causative microbes as well as subsequent inflammatory responses. Current therapies directed only at the host immune response have not been overly successful, owing largely to difficulties in reversing the severe immunopathology associated with sepsis.

SUMMARY

Respiratory viral infections may facilitate secondary bacterial infections and increase host immunopathology through the overproduction of inflammatory cytokines. Preventive measures, including vaccination and aggressive antimicrobial therapy early in the course of infection, may significantly reduce the morbidity and mortality of sepsis.

Respiratory viruses predisposing to bacterial infections: role of neuraminidase

BACKGROUND AND METHODS

Viral-bacterial coinfections in humans are well-documented. Viral infections often lead to bacterial superinfections. In vitro and animal models for influenza, as well as molecular microbiology study of viruses and bacteria, provide an understanding of the mechanisms that explain how respiratory viruses and bacteria combine to cause disease. This article focuses on viral and bacterial combinations, particularly synergism between influenza and Streptococcus pneumoniae.

RESULTS

Potential mechanisms for synergism between viruses and bacteria include: virus destruction of respiratory epithelium may increase bacterial adhesion; virus-induced immunosuppression may cause bacterial superinfections; and inflammatory response to viral infection may up-regulate expression of molecules that bacteria utilize as receptors. Influenza and parainfluenza viruses possess neuraminidase (NA) activity, which appears to increase bacterial adherence after viral preincubation. Experimental studies demonstrate that viral NA exposes pneumococcal receptors on host cells by removing terminal sialic acids. Other studies show that inhibition of viral NA activity reduces adherence and invasion of S. pneumoniae, independently of effects on viral replication. Clinical studies reveal that influenza vaccination reduces the incidence of secondary bacterial respiratory tract infections.

CONCLUSIONS

Detection of viral factors (e.g. high NA activity) that increase the likely potential of epidemic/pandemic influenza strains for causing morbidity and mortality from secondary bacterial infections provides new possibilities for intervention. Additional study is needed to identify the mechanisms for the development of bacterial complications after infections with respiratory syncytial virus and other important respiratory viruses that lack NA activity. Prevention of bacterial superinfection is likely to depend on effective antiviral measures.

Pneumonies communautaires et infection grippale

Children without chronic or serious medical conditions are at increased risk for hospitalization during influenza seasons, mainly with respiratory tract infections. But influenza virus infections frequently remain undiagnosed, even in hospitalized patients. We prospectively studied the rate of concomitant and preceding influenza infections in children hospitalized with a community acquired pneumonia (CAP).

POPULATION AND METHODS

All 1-15-year-old children with CAP requiring hospitalization between 1st April 2000 and 2002 had nasopharyngeal aspirate for viruses, immunofluorescence and serologies for respiratory pathogens. The peak of influenza IgG measured by complement fixation (CF) is transient, and a titer of 1/64 or more indicates an acute influenza infection in the preceding weeks. Children with chronic disease were excluded and a control group of patients from outpatient clinic was measured.

RESULTS

Among 33 previously healthy children (age 4.9 years, range 1.2-14 years), 8 had a pneumococcal pneumonia, 10 a pneumonia caused by Mycoplasma pneumoniae (MP), 1 by Chlamydia pneumonia, and 8 of unknown origin. In six patients immunofluorescence was positive: Respiratory Syncitial Virus, 2, Adenovirus, 1 and Influenza A, 3 (including a patient with concomitant MP infection). Thirteen of the 33 children (39.4%) had evidence of a recent influenza A infection with CF ab > or = 1/64: with pneumococcal pneumonia, 5/10 with MP pneumonia, 3/8 with unknown origin pneumonia, 9/13 of these previous influenza infections being clinically inapparent. Only 1/30 children of control group (3.3%) had CF ab > or = 1/64.

CONCLUSION

In this study, influenza infection is the direct cause of CAP of children in 12% of cases. In other children with CAP, 39.4% of patients had an influenza infection in the preceding weeks which leads to secondary infection caused by Streptococcus pneumoniae or by MP or other pathogens.

Genome sequence of an M3 strain of Streptococcus pyogenes reveals a large-scale genomic rearrangement in invasive strains and new insights into phage evolution

Group Astreptococcus (GAS) is a gram-positive bacterial pathogen that causes various suppurative infections and nonsuppurative sequelae. Since the late 1980s, streptococcal toxic-shock like syndrome (STSS) and severe invasive GAS infections have been reported globally. Here we sequenced the genome of serotype M3 strain SSI-1, isolated from an STSS patient in Japan, and compared it with those of other GAS strains. The SSI-1 genome is composed of 1,884,275 bp, and 1.7 Mb of the sequence is highly conserved relative to strain SF370 (serotype M1) and MGAS8232 (serotype M18), and almost completely conserved relative to strain MGAS315 (serotype M3). However, a large genomic rearrangement has been shown to occur across the replication axis between the homologous rrn-comX1 regions and between two prophage-coding regions across the replication axis. Atotal of 1 Mb of chromosomal DNA is inverted across the replication axis. Interestingly, the recombinations between the prophage regions are within the phage genes, and the genes encoding superantigens and mitogenic factors are interchanged between two prophages. This genomic rearrangement occurs in 65% of clinical isolates (64/94) collected after 1990, whereas it is found in only 25% of clinical isolates (7/28) collected before 1985. These observations indicate that streptococcal phages represent important plasticity regions in the GAS chromosome where recombination between homologous phage genes can occur and result not only in new phage derivatives, but also in large chromosomal rearrangements.