Facilitated expansion of pneumococcal colonization from the nose to the lower respiratory tract in mice preinfected with influenza virus

Clinical course and background of nasopharyngeal antibiotic-resistant bacteria carriers among preschool children hospitalized for lower respiratory tract infection

AbstractWe investigated the nasopharyngeal microbiota in preschool patients hospitalized with lower respiratory tract infection to clarify the relationships between culturable nasopharyngeal bacteria and prognosis. From 2016 to 2018, nasopharyngeal culture was performed on inpatients under 6 years of age with a lower respiratory tract infection. Among the 1,056 study patients, 1,046 provided nasopharyngeal samples that yielded positive cultures, yielding 1,676 isolated strains. Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis, were isolated in 25%, 27%, and 31% of the samples, respectively, and were the major causes of respiratory tract infection in these children. The only factor associated with the isolation of antibiotic-resistant strains from the nasopharynx was daycare attendance, which did not affect clinical severity, such as duration of fever and hospitalization. This study demonstrated that resistant bacteria in the nasopharynx did not affect the severity of lower respiratory tract infection and supports the use of narrow-spectrum antimicrobial agents in accordance with published guidelines when initiating therapy for pediatric patients with community-acquired pneumonia.

Respiratory Viral Infection-Induced Microbiome Alterations and Secondary Bacterial Pneumonia

Influenza and other respiratory viral infections are the most common type of acute respiratory infection. Viral infections predispose patients to secondary bacterial infections, which often have a more severe clinical course. The mechanisms underlying post-viral bacterial infections are complex, and include multifactorial processes mediated by interactions between viruses, bacteria, and the host immune system. Studies over the past 15 years have demonstrated that unique microbial communities reside on the mucosal surfaces of the gastrointestinal tract and the respiratory tract, which have both direct and indirect effects on host defense against viral infections. In addition, antiviral immune responses induced by acute respiratory infections such as influenza are associated with changes in microbial composition and function (“dysbiosis”) in the respiratory and gastrointestinal tract, which in turn may alter subsequent immune function against secondary bacterial infection or alter the dynamics of inter-microbial interactions, thereby enhancing the proliferation of potentially pathogenic bacterial species. In this review, we summarize the literature on the interactions between host microbial communities and host defense, and how influenza, and other acute respiratory viral infections disrupt these interactions, thereby contributing to the pathogenesis of secondary bacterial infections.

Secondary Bacterial Infections Associated with Influenza Pandemics

Lower and upper respiratory infections are the fourth highest cause of global mortality (Lozano et al., 2012). Epidemic and pandemic outbreaks of respiratory infection are a major medical concern, often causing considerable disease and a high death toll, typically over a relatively short period of time. Influenza is a major cause of epidemic and pandemic infection. Bacterial co/secondary infection further increases morbidity and mortality of influenza infection, with Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus reported as the most common causes. With increased antibiotic resistance and vaccine evasion it is important to monitor the epidemiology of pathogens in circulation to inform clinical treatment and development, particularly in the setting of an influenza epidemic/pandemic.

Lethal synergism of 2009 pandemic H1N1 influenza virus and Streptococcus pneumoniae coinfection is associated with loss of murine lung repair responses

Secondary bacterial infections increase disease severity of influenza virus infections and contribute greatly to increased morbidity and mortality during pandemics. To study secondary bacterial infection following influenza virus infection, mice were inoculated with sublethal doses of 2009 seasonal H1N1 virus (NIH50) or pandemic H1N1 virus (Mex09) followed by inoculation with Streptococcus pneumoniae 48 h later. Disease was characterized by assessment of weight loss and survival, titration of virus and bacteria by quantitative reverse transcription-PCR (qRT-PCR), histopathology, expression microarray, and immunohistochemistry. Mice inoculated with virus alone showed 100% survival for all groups. Mice inoculated with Mex09 plus S. pneumoniae showed severe weight loss and 100% mortality with severe alveolitis, denuded bronchiolar epithelium, and widespread expression of apoptosis marker cleaved caspase 3. In contrast, mice inoculated with NIH50 plus S. pneumoniae showed increased weight loss, 100% survival, and slightly enhanced lung pathology. Mex09-S. pneumoniae coinfection also resulted in increased S. pneumoniae replication in lung and bacteremia late in infection. Global gene expression profiling revealed that Mex09-S. pneumoniae coinfection did not induce significantly more severe inflammatory responses but featured significant loss of epithelial cell reproliferation and repair responses. Histopathological examination for cell proliferation marker MCM7 showed significant staining of airway epithelial cells in all groups except Mex09-S. pneumoniae-infected mice. This study demonstrates that secondary bacterial infection during 2009 H1N1 pandemic virus infection resulted in more severe disease and loss of lung repair responses than did seasonal influenza viral and bacterial coinfection. Moreover, this study provides novel insights into influenza virus and bacterial coinfection by showing correlation of lethal outcome with loss of airway basal epithelial cells and associated lung repair responses.

Secondary bacterial pneumonias lead to increased disease severity and have resulted in a significant percentage of deaths during influenza pandemics. To understand the biological basis for the interaction of bacterial and viral infections, mice were infected with sublethal doses of 2009 seasonal H1N1 and pandemic H1N1 viruses followed by infection with Streptococcus pneumoniae 48 h later. Only infection with 2009 pandemic H1N1 virus and S. pneumoniae resulted in severe disease with a 100% fatality rate. Analysis of the host response to infection during lethal coinfection showed a significant loss of responses associated with lung repair that was not observed in any of the other experimental groups. This group of mice also showed enhanced bacterial replication in the lung. This study reveals that the extent of lung damage during viral infection influences the severity of secondary bacterial infections and may help explain some differences in mortality during influenza pandemics.

Bacterial tracheitis: a multi-centre perspective

The published literature on bacterial tracheitis is limited. We report the first multi-centre study of bacterial tracheitis together with a concise review of the literature. We conducted a retrospective study of cases admitted during the period 1993-2007 to 3 tertiary paediatric centres in the United Kingdom and 1 in Australia. A total of 34 cases were identified. 31 patients (91%) required intubation. Complications included cardiorespiratory arrest in 1, ARDS in 1, hypotension in 10, toxic shock syndrome in 1 and renal failure in 1 patient(s). Staphylococcus aureus was the most commonly implicated bacterial organism, isolated from the respiratory tract in 55.8% of the cases overall. Other pathogens commonly isolated from the respiratory tract included Streptococcus pyogenes (5.9%), Streptococcus pneumoniae (11.8%) and Haemophilus influenzae (11.8%). Viral coinfection was identified in 9 (31%) of the 29 cases in whom immunofluorescence testing was performed (influenza A in 4 cases; parainfluenza 1 in 2 cases; parainfluenza 3 in 2 cases; adenovirus in 1 case). The combined experience from 4 major paediatric intensive care units suggests that bacterial tracheitis remains a rare condition with an estimated incidence of approximately 0.1/100,000 children per year. Short-term complications were common but long-term sequelae were rare. There were no fatal outcomes, which contrasts with the high historical mortality rates and likely reflects improvements in intensive care management.

Influenza virus infection decreases tracheal mucociliary velocity and clearance of Streptococcus pneumoniae

Influenza virus infections increase susceptibility to secondary bacterial infections, such as pneumococcal pneumonia, resulting in increased morbidity and mortality. Influenza-induced tissue damage is hypothesized to increase susceptibility to Streptococcus pneumoniae infection by increasing adherence to the respiratory epithelium. Using a mouse model of influenza infection followed by S. pneumoniae infection, we found that an influenza infection does not increase the number of pneumococci initially present within the trachea, but does inhibit pneumococcal clearance by 2 hours after infection. To determine whether influenza damage increases pneumococcal adherence, we developed a novel murine tracheal explant system to determine influenza-induced tissue damage and subsequent pneumococcal adherence. Murine tracheas were kept viable ex vivo as shown by microscopic examination of ciliary beating and cellular morphology using continuous media flow for up to 8 days. Tracheas were infected with influenza virus for 0.5-5 days ex vivo, and influenza-induced tissue damage and the early stages of repair to the epithelium were assessed histologically. A prior influenza infection did not increase pneumococcal adherence, even when the basement membrane was maximally denuded or during the repopulation of the basement membrane with undifferentiated epithelial cells. We measured mucociliary clearance in vivo and found it was decreased in influenza-infected mice. Together, our results indicate that exposure of the tracheal basement membrane contributes minimally to pneumococcal adherence. Instead, an influenza infection results in decreased tracheal mucociliary velocity and initial clearance of pneumococci, leading to an increased pneumococcal burden as early as 2 hours after pneumococcal infection.

Both influenza-induced neutrophil dysfunction and neutrophil-independent mechanisms contribute to increased susceptibility to a secondary Streptococcus pneumoniae infection

Since secondary Streptococcus pneumoniae infections greatly increase the mortality of influenza infections, we determined the relative roles of neutrophil-dependent and -independent mechanisms in increased susceptibility to S. pneumoniae during influenza infection. Mice infected with influenza for 6 days, but not 3 days, showed a significant increase in susceptibility to S. pneumoniae infection compared to mice not infected with influenza. There was significant neutrophil accumulation in the lungs of S. pneumoniae-infected mice regardless of whether or not they were infected with influenza for 3 or 6 days. Depletion of neutrophils in these mice resulted in increased susceptibility to S. pneumoniae in both the non-influenza-infected mice and mice infected with influenza for 3 days but not in the mice infected with influenza for 6 days, indicating that a prior influenza infection of 6 days may compromise neutrophil function, resulting in increased susceptibility to a S. pneumoniae infection. Neutrophils from the lungs of mice infected with influenza for 3 or 6 days exhibited functional impairment in the form of decreased phagocytosis and intracellular reactive oxygen species generation in response to S. pneumoniae. In addition, neutrophil-depleted mice infected with influenza for 6 days were more susceptible to S. pneumoniae than neutrophil-depleted mice not infected with influenza, indicating that neutrophil-independent mechanisms also contribute to influenza-induced increased susceptibility to S. pneumoniae. Pulmonary interleukin-10 levels were increased in coinfected mice infected with influenza for 6 days but not 3 days. Thus, an influenza infection of 6 days increases susceptibility to S. pneumoniae by both suppression of neutrophil function and by neutrophil-independent mechanisms such as enhanced cytokine production.

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.