Effect of intrapartum azithromycin on gut microbiota development in early childhood: A post hoc analysis of a double-blind randomized trial.

Intrapartum azithromycin prophylaxis has shown the potential to reduce maternal infections but showed no effect on neonatal sepsis and mortality. Antibiotic exposure early in life may affect gut microbiota development, leading to undesired consequences. Therefore, we here assessed the impact of 2 g oral intrapartum azithromycin on gut microbiota development from birth to the age of 3 years, by 16S-rRNA gene profiling of rectal samples from 127 healthy Gambian infants selected from a double-blind randomized placebo-controlled clinical trial (PregnAnZI-2). Microbiota trajectories showed, over the first month of life, a slower community transition and increase of Enterobacteriaceae (p = 0.001) and Enterococcaceae (p = 0.064) and a decrease of Bifidobacterium (p < 0.001) in the azithromycin compared to the placebo arm. Intrapartum azithromycin alters gut microbiota development and increases proinflammatory bacteria in the first month of life, which may have undesirable effects on the child.

Salivary polyreactive antibodies and Haemophilus influenzae are associated with respiratory infection severity in young children with recurrent respiratory infections.

BACKGROUND: Recurrent respiratory tract infections (rRTIs) are a common reason for immunodiagnostic testing in children, which relies on serum antibody level measurements. However, because RTIs predominantly affect the respiratory mucosa, serum antibodies may inaccurately reflect local immune defences. We investigated antibody responses in saliva and their interplay with the respiratory microbiota in relation to RTI severity and burden in young children with rRTIs. METHODS: We conducted a prospective cohort study including 100 children aged <10 years with rRTIs, their family members and healthy healthcare professionals. Total and polyreactive antibody concentrations were determined in serum and saliva (ELISA); respiratory microbiota composition (16S rRNA sequencing) and respiratory viruses (quantitative PCR) were characterised in nasopharyngeal swabs. Proteomic analysis (Olink) was performed on saliva and serum samples. RTI symptoms were monitored with a daily mobile phone application and assessed using latent class analysis and negative binomial mixed models. RESULTS: Serum antibody levels were not associated with RTI severity. Strikingly, 28% of salivary antibodies and only 2% of serum antibodies displayed polyreactivity (p<0.001). Salivary polyreactive IgA was negatively associated with recurrent lower RTIs (adjusted OR 0.80, 95% CI 0.67-0.94) and detection of multiple respiratory viruses (adjusted OR 0.76, 95% CI 0.61-0.96). Haemophilus influenzae abundance was positively associated with RTI symptom burden (regression coefficient 0.05, 95% CI 0.02-0.08). CONCLUSION: These results highlight the importance of mucosal immunity in RTI severity and burden, and suggest that the level of salivary polyreactive IgA and H. influenzae abundance may serve as indicators of infection severity and burden in young children with rRTIs.

Host and environmental factors shape upper airway microbiota and respiratory health across the human lifespan.

Our understanding of the normal variation in the upper respiratory tract (URT) microbiota across the human lifespan and how these relate to host, environment, and health is limited. We studied the microbiota of 3,104 saliva (<10 year-olds)/oropharynx (≥10 year-olds) and 2,485 nasopharynx samples of 3,160 Dutch individuals 0-87 years of age, participating in a cross-sectional population-wide study (PIENTER-3) using 16S-rRNA sequencing. The microbiota composition was strongly related to age, especially in the nasopharynx, with maturation occurring throughout childhood and adolescence. Clear niche- and age-specific associations were found between the microbiota composition and host/environmental factors and health outcomes. Among others, social interaction, sex, and season were associated with the nasopharyngeal microbial community. By contrast, the oral microbiota was more related to antibiotics, tobacco, and alcohol use. We present an atlas of the URT microbiota across the lifespan in association with environment and health, establishing a baseline for future research.

Impaired upper respiratory tract barrier function during postnatal development predisposes to invasive pneumococcal disease.

Infants are highly susceptible to invasive respiratory and gastrointestinal infections. To elucidate the age-dependent mechanism(s) that drive bacterial spread from the mucosa, we developed an infant mouse model using the prevalent pediatric respiratory pathogen, Streptococcus pneumoniae (Spn). Despite similar upper respiratory tract (URT) colonization levels, the survival rate of Spn-infected infant mice was significantly decreased compared to adults and corresponded with Spn dissemination to the bloodstream. An increased rate of pneumococcal bacteremia in early life beyond the newborn period was attributed to increased bacterial translocation across the URT barrier. Bacterial dissemination in infant mice was independent of URT monocyte or neutrophil infiltration, phagocyte-derived ROS or RNS, inflammation mediated by toll-like receptor 2 or interleukin 1 receptor signaling, or the pore-forming toxin pneumolysin. Using molecular barcoding of Spn, we found that only a minority of bacterial clones in the nasopharynx disseminated to the blood in infant mice, indicating the absence of robust URT barrier breakdown. Rather, transcriptional profiling of the URT epithelium revealed a failure of infant mice to upregulate genes involved in the tight junction pathway. Expression of many such genes was also decreased in early life in humans. Infant mice also showed increased URT barrier permeability and delayed mucociliary clearance during the first two weeks of life, which corresponded with tighter attachment of bacteria to the respiratory epithelium. Together, these results demonstrate a window of vulnerability during postnatal development when altered mucosal barrier function facilitates bacterial dissemination.

Nasopharyngeal microbiota in children is associated with severe asthma exacerbations.

BACKGROUND: The respiratory microbiome has been associated with the etiology and disease course of asthma. OBJECTIVE: We sought to assess the nasopharyngeal microbiota in children with a severe asthma exacerbation and their associations with medication, air quality, and viral infection. METHODS: A cross-sectional study was performed among children aged 2 to 18 years admitted to the medium care unit (MCU; n = 84) or intensive care unit (ICU; n = 78) with an asthma exacerbation. For case-control analyses, we matched all cases aged 2 to 6 years (n = 87) to controls in a 1:2 ratio. Controls were participants of either a prospective case-control study or a longitudinal birth cohort (n = 182). The nasopharyngeal microbiota was characterized by 16S-rRNA-gene sequencing. RESULTS: Cases showed higher Shannon diversity index (ICU and MCU combined; P = .002) and a distinct microbial community composition when compared with controls (permutational multivariate ANOVA R2 = 1.9%; P < .001). We observed significantly higher abundance of Staphylococcus and "oral" taxa, including Neisseria, Veillonella, and Streptococcus spp. and a lower abundance of Dolosigranulum pigrum, Corynebacterium, and Moraxella spp. (MaAsLin2; q < 0.25) in cases versus controls. Furthermore, Neisseria abundance was associated with more severe disease (ICU vs MCU MaAslin2, P = .03; q = 0.30). Neisseria spp. abundance was also related with fine particulate matter exposure, whereas Haemophilus and Streptococcus abundances were related with recent inhaled corticosteroid use. We observed no correlations with viral infection. CONCLUSIONS: Our results demonstrate that children admitted with asthma exacerbations harbor a microbiome characterized by overgrowth of Staphylococcus and "oral" microbes and an underrepresentation of beneficial niche-appropriate commensals. Several of these associations may be explained by (environmental or medical) exposures, although cause-consequence relationships remain unclear and require further investigations.

Mycoplasma pneumoniae carriage in children with recurrent respiratory tract infections is associated with a less diverse and altered microbiota.

BACKGROUND: Mycoplasma pneumoniae is a common cause of community-acquired pneumonia in school-aged children and can be preceded by asymptomatic carriage. However, its role in recurrent respiratory tract infections is unclear. We studied the prevalence of M.pneumoniae carriage in children with recurrent respiratory infections and identified associated factors. METHODS: We tested M.pneumoniae carriage by qPCR in children with recurrent infections and their healthy family members in a cross-sectional study. Serum and mucosal total and M.pneumoniae-specific antibody levels were measured by ELISA and nasopharyngeal microbiota composition was characterized by 16S-rRNA sequencing. FINDINGS: Prevalence of M.pneumoniae carriage was higher in children with recurrent infections (68%) than their family members without infections (47% in siblings and 27% in parents). M.pneumoniae carriage among family members appeared to be associated with transmission within the household, likely originating from the affected child. In logistic regression corrected for age and multiple comparisons, IgA (OR 0.16 [0.06-0.37]) and total IgG deficiency (OR 0.15 [0.02-0.74]) were less prevalent in M.pneumoniae carriers (n = 78) compared to non-carriers (n = 36). In multivariable analysis, the nasopharyngeal microbiota of M.pneumoniae carriers had lower alpha diversity (OR 0.27 [0.09-0.67]) and a higher abundance of Haemophilus influenzae (OR 45.01 [2.74-1608.11]) compared to non-carriers. INTERPRETATION: M.pneumoniae carriage is highly prevalent in children with recurrent infections and carriers have a less diverse microbiota with an overrepresentation of disease-associated microbiota members compared to non-carriers. Given the high prevalence of M.pneumoniae carriage and the strong association with H. influenzae, we recommend appropriate antibiotic coverage of M.pneumoniae and H. influenzae in case of suspected pneumonia in children with recurrent respiratory tract infections or their family members. FUNDING: Wilhelmina Children's Hospital Research Fund, 'Christine Bader Stichting Irene KinderZiekenhuis', Sophia Scientific Research Foundation, ESPID Fellowship funded by Seqirus, Hypatia Fellowship funded by Radboudumc and The Netherlands Organisation for Health Research and Development (ZonMW VENI grant to LM Verhagen).

Mother-to-infant microbiota transmission and infant microbiota development across multiple body sites.

Early-life microbiota seeding and subsequent development is crucial to future health. Cesarean-section (CS) birth, as opposed to vaginal delivery, affects early mother-to-infant transmission of microbes. Here, we assess mother-to-infant microbiota seeding and early-life microbiota development across six maternal and four infant niches over the first 30 days of life in 120 mother-infant pairs. Across all infants, we estimate that on average 58.5% of the infant microbiota composition can be attributed to any of the maternal source communities. All maternal source communities seed multiple infant niches. We identify shared and niche-specific host/environmental factors shaping the infant microbiota. In CS-born infants, we report reduced seeding of infant fecal microbiota by maternal fecal microbes, whereas colonization with breastmilk microbiota is increased when compared with vaginally born infants. Therefore, our data suggest auxiliary routes of mother-to-infant microbial seeding, which may compensate for one another, ensuring that essential microbes/microbial functions are transferred irrespective of disrupted transmission routes.

Asymptomatic Viral Presence in Early Life Precedes Recurrence of Respiratory Tract Infections.

BACKGROUND: Respiratory tract infections (RTIs) in infants are often caused by viruses. Although respiratory syncytial virus (RSV), influenza virus and human metapneumovirus (hMPV) can be considered the most pathogenic viruses in children, rhinovirus (RV) is often found in asymptomatic infants as well. Little is known about the health consequences of viral presence, especially early in life. We aimed to examine the dynamics of (a)symptomatic viral presence and relate early viral detection to susceptibility to RTIs in infants. METHODS: In a prospective birth cohort of 117 infants, we tested 1304 nasopharyngeal samples obtained from 11 consecutive regular sampling moments, and during acute RTIs across the first year of life for 17 respiratory viruses by quantitative PCR. Associations between viral presence, viral (sub)type, viral load, viral co-detection and symptoms were tested by generalized estimating equation (GEE) models. RESULTS: RV was the most detected virus. RV was negatively associated [GEE: adjusted odds ratio (aOR) 0.41 (95% CI 0.18-0.92)], and hMPV, RSV, parainfluenza 2 and 4 and human coronavirus HKU1 were positively associated with an acute RTI. Asymptomatic RV in early life was, however, associated with increased susceptibility to and recurrence of RTIs later in the first year of life (Kaplan-Meier survival analysis: P = 0.022). CONCLUSIONS: Respiratory viruses, including the seasonal human coronaviruses, are often detected in infants, and are often asymptomatic. Early life RV presence is, though negatively associated with an acute RTI, associated with future susceptibility to and recurrence of RTIs. Further studies on potential ecologic or immunologic mechanisms are needed to understand these observations.

Higher off-target amplicon detection rate in MiSeq v3 compared to v2 reagent kits in the context of 16S-rRNA-sequencing.

One of the most widely used techniques in microbiota research is 16S-rRNA-sequencing. Several laboratory processes have been shown to impact sequencing results, especially in low biomass samples. Low biomass samples are prone to off-target amplification, where instead of bacterial DNA, host DNA is erroneously amplified. Knowledge on the laboratory processes influencing off-target amplification and detection is however scarce. We here expand on previous findings by demonstrating that off-target amplification is not limited to invasive biopsy samples, but is also an issue in low bacterial biomass respiratory (mucosal) samples, especially when below 0.3 pg/µL. We show that off-target amplification can partly be mitigated by using gel-based library purification methods. Importantly, we report a higher off-target amplicon detection rate when using MiSeq reagent kit v3 compared to v2 (mean 13.3% vs 0.1% off-target reads/sample, respectively), possibly as a result of differences in reagents or sequencing recipes. However, since after bioinformatic removal of off-target reads, MiSeq reagent kit v3 still results in a twofold higher number of reads when compared to v2, v3 is still preferred over v2. Together, these results add to the growing knowledge base on off-target amplification and detection, allowing researchers to anticipate this problem in 16S-rRNA-based microbiome studies involving low biomass samples.

Air pollution from livestock farms and the oropharyngeal microbiome of COPD patients and controls.

Air pollution from livestock farms is known to affect respiratory health of patients with chronic obstructive pulmonary disease (COPD). The mechanisms behind this relationship, however, remain poorly understood. We hypothesise that air pollutants could influence respiratory health through modulation of the airway microbiome. Therefore, we studied associations between air pollution exposure and the oropharyngeal microbiota (OPM) composition of COPD patients and controls in a livestock-dense area. Oropharyngeal swabs were collected from 99 community-based (mostly mild) COPD cases and 184 controls (baseline), and after 6 and 12 weeks. Participants were non-smokers or former smokers. Annual average livestock-related outdoor air pollution at the home address was predicted using dispersion modelling. OPM composition was analysed using 16S rRNA-based sequencing in all baseline samples and 6-week and 12-week repeated samples of 20 randomly selected subjects (n = 323 samples). A random selection of negative control swabs, taken every sampling day, were also included in the downstream analysis. Both farm-emitted endotoxin and PM10 levels were associated with increased OPM richness in COPD patients (p < 0.05) but not in controls. COPD case-control status was not associated with community structure, while correcting for known confounders (multivariate PERMANOVA p > 0.05). However, members of the genus Streptococcus were more abundant in COPD patients (Benjamini-Hochberg adjusted p < 0.01). Moderate correlation was found between ordinations of 20 subjects analysed at 0, 6, and 12 weeks (Procrustes r = 0.52 to 0.66; p < 0.05; Principal coordinate analysis of Bray-Curtis dissimilarity), indicating that the OPM is relatively stable over a 12 week period and that a single sample sufficiently represents the OPM. Air pollution from livestock farms is associated with OPM richness of COPD patients, suggesting that the OPM of COPD patients is susceptible to alterations induced by exposure to air pollutants.

Effect of intra-partum azithromycin on the development of the infant nasopharyngeal microbiota: A post hoc analysis of a double-blind randomized trial.

BACKGROUND: Sepsis is a leading cause of neonatal death. Intrapartum azithromycin reduces neonatal nasopharyngeal carriage of potentially pathogenic bacteria, a prerequisite for sepsis. Early antibiotic exposure has been associated with microbiota perturbations with varying effects. This study aims to understand the effect of intrapartum azithromycin intervention on the developing nasopharyngeal microbiota of the child. METHODS: Using 16S rRNA gene sequencing, we analysed the microbiota of 343 nasopharyngeal samples collected from birth to 12 months from 109 healthy infants selected from a double-blind randomized placebo-controlled clinical trial conducted in the Gambia (PregnAnZI-1). In the trial, 829 women were given 2g oral azithromycin or placebo (1:1) during labour with the objective of reducing bacterial carriage in mother and child during the neonatal period. The post-hoc analysis presented here assessed the effect of the intervention on the child nasopharyngeal microbiota development. FINDINGS: 55 children were from mothers given azithromycin and 54 from mothers given placebo. Comparing arms, we found an increase in alpha-diversity at day-6 (p = 0·018), and a significant effect on overall microbiota composition at days 6 and 28 (R2 = 4.4%, q = 0·007 and R2 = 2.3%, q = 0·018 respectively). At genus level, we found lower representation of Staphylococcus at day-6 (q = 0·0303) and higher representation of Moraxella at 12 months (q = 0·0443). Unsupervised clustering of samples by microbial community similarity showed different community dynamics between the intervention and placebo arms during the neonatal period. INTERPRETATION: These results indicate that intrapartum azithromycin caused short-term alterations in the nasopharyngeal microbiota with modest overall effect at 12 months of age. Further exploration of the effects of these variations on microbiome function will give more insight on the potential risks and benefits, for the child, associated with this intervention. FUNDING: This work was jointly funded by the Medical Research Council (UK) (MC_EX_MR/J010391/1/MRC), Bill & Melinda Gates Foundation (OPP1196513), and MRCG@LSHTM Doctoral Training Program.

Bacterial and fungal communities in tracheal aspirates of intubated COVID-19 patients: a pilot study.

Co-infections with bacterial or fungal pathogens could be associated with severity and outcome of disease in COVID-19 patients. We, therefore, used a 16S and ITS-based sequencing approach to assess the biomass and composition of the bacterial and fungal communities in endotracheal aspirates of intubated COVID-19 patients. Our method combines information on bacterial and fungal biomass with community profiling, anticipating the likelihood of a co-infection is higher with (1) a high bacterial and/or fungal biomass combined with (2) predominance of potentially pathogenic microorganisms. We tested our methods on 42 samples from 30 patients. We observed a clear association between microbial outgrowth (high biomass) and predominance of individual microbial species. Outgrowth of pathogens was in line with the selective pressure of antibiotics received by the patient. We conclude that our approach may help to monitor the presence and predominance of pathogens and therefore the likelihood of co-infections in ventilated patients, which ultimately, may help to guide treatment.

The microbiota in respiratory tract infections: from association to intervention.

PURPOSE OF REVIEW: The respiratory microbiota has a role in respiratory tract infection (RTI) pathogenesis. On the mucosa, the respiratory microbiota interacts with potential pathogenic viruses, bacteria and the host immune system, including secretory IgA (sIgA). This review discusses the role of the respiratory microbiota and its interaction with the (mucosal) immune system in RTI susceptibility, as well as the potential to exploit the microbiota to promote health and prevent RTIs. RECENT FINDINGS: Recent studies confirm that specific microbiota profiles are associated with RTI susceptibility and during susceptibility and found accompanying RTIs, although clear associations have not yet been found for SARS-CoV-2 infection. sIgA plays a central role in RTI pathogenesis: it stands under control of the local microbiota, while at the same time influencing bacterial gene expression, metabolism and defense mechanisms. Respiratory microbiota interventions are still newly emerging but promising candidates for probiotics to prevent RTIs, such as Corynebacterium and Dolosigranulum species, have been identified. SUMMARY: Improved understanding of the respiratory microbiota in RTIs and its interplay with the immune system is of importance for early identification and follow-up of individuals at risk of infection. It also opens doors for future microbiota interventions by altering the microbiota towards a healthier state to prevent and/or adjunctively treat RTIs.

Early-life viral infections are associated with disadvantageous immune and microbiota profiles and recurrent respiratory infections.

The respiratory tract is populated by a specialized microbial ecosystem, which is seeded during and directly following birth. Perturbed development of the respiratory microbial community in early-life has been associated with higher susceptibility to respiratory tract infections (RTIs). Given a consistent gap in time between first signs of aberrant microbial maturation and the observation of the first RTIs, we hypothesized that early-life host-microbe cross-talk plays a role in this process. We therefore investigated viral presence, gene expression profiles and nasopharyngeal microbiota from birth until 12 months of age in 114 healthy infants. We show that the strongest dynamics in gene expression profiles occurred within the first days of life, mostly involving Toll-like receptor (TLR) and inflammasome signalling. These gene expression dynamics coincided with rapid microbial niche differentiation. Early asymptomatic viral infection co-occurred with stronger interferon activity, which was related to specific microbiota dynamics following, including early enrichment of Moraxella and Haemophilus spp. These microbial trajectories were in turn related to a higher number of subsequent (viral) RTIs over the first year of life. Using a multi-omic approach, we found evidence for species-specific host-microbe interactions related to consecutive susceptibility to RTIs. Although further work will be needed to confirm causality of our findings, together these data indicate that early-life viral encounters could impact subsequent host-microbe cross-talk, which is linked to later-life infections.

Decreased production of epithelial-derived antimicrobial molecules at mucosal barriers during early life.

Young age is a risk factor for respiratory and gastrointestinal infections. Here, we compared infant and adult mice to identify age-dependent mechanisms that drive susceptibility to mucosal infections during early life. Transcriptional profiling of the upper respiratory tract (URT) epithelium revealed significant dampening of early life innate mucosal defenses. Epithelial-mediated production of the most abundant antimicrobial molecules, lysozyme, and lactoferrin, and the polymeric immunoglobulin receptor (pIgR), responsible for IgA transcytosis, was expressed in an age-dependent manner. This was attributed to delayed functional development of serous cells. Absence of epithelial-derived lysozyme and the pIgR was also observed in the small intestine during early life. Infection of infant mice with lysozyme-susceptible strains of Streptococcus pneumoniae or Staphylococcus aureus in the URT or gastrointestinal tract, respectively, demonstrated an age-dependent regulation of lysozyme enzymatic activity. Lysozyme derived from maternal milk partially compensated for the reduction in URT lysozyme activity of infant mice. Similar to our observations in mice, expression of lysozyme and the pIgR in nasopharyngeal samples collected from healthy human infants during the first year of life followed an age-dependent regulation. Thus, a global pattern of reduced antimicrobial and IgA-mediated defenses may contribute to increased susceptibility of young children to mucosal infections.

Benchmarking laboratory processes to characterise low-biomass respiratory microbiota.

The low biomass of respiratory samples makes it difficult to accurately characterise the microbial community composition. PCR conditions and contaminating microbial DNA can alter the biological profile. The objective of this study was to benchmark the currently available laboratory protocols to accurately analyse the microbial community of low biomass samples. To study the effect of PCR conditions on the microbial community profile, we amplified the 16S rRNA gene of respiratory samples using various bacterial loads and different number of PCR cycles. Libraries were purified by gel electrophoresis or AMPure XP and sequenced by V2 or V3 MiSeq reagent kits by Illumina sequencing. The positive control was diluted in different solvents. PCR conditions had no significant influence on the microbial community profile of low biomass samples. Purification methods and MiSeq reagent kits provided nearly similar microbiota profiles (paired Bray-Curtis dissimilarity median: 0.03 and 0.05, respectively). While profiles of positive controls were significantly influenced by the type of dilution solvent, the theoretical profile of the Zymo mock was most accurately analysed when the Zymo mock was diluted in elution buffer (difference compared to the theoretical Zymo mock: 21.6% for elution buffer, 29.2% for Milli-Q, and 79.6% for DNA/RNA shield). Microbiota profiles of DNA blanks formed a distinct cluster compared to low biomass samples, demonstrating that low biomass samples can accurately be distinguished from DNA blanks. In summary, to accurately characterise the microbial community composition we recommend 1. amplification of the obtained microbial DNA with 30 PCR cycles, 2. purifying amplicon pools by two consecutive AMPure XP steps and 3. sequence the pooled amplicons by V3 MiSeq reagent kit. The benchmarked standardized laboratory workflow presented here ensures comparability of results within and between low biomass microbiome studies.

Early origins of lung disease: towards an interdisciplinary approach.

The prenatal and perinatal environments can have profound effects on the development of chronic inflammatory diseases. However, mechanistic insight into how the early-life microenvironment can impact upon development of the lung and immune system and consequent initiation and progression of respiratory diseases is still emerging. Recent studies investigating the developmental origins of lung diseases have started to delineate the effects of early-life changes in the lung, environmental exposures and immune maturation on the development of childhood and adult lung diseases. While the influencing factors have been described and studied in mostly animal models, it remains challenging to pinpoint exactly which factors and at which time point are detrimental in lung development leading to respiratory disease later in life. To advance our understanding of early origins of chronic lung disease and to allow for proper dissemination and application of this knowledge, we propose four major focus areas: 1) policy and education; 2) clinical assessment; 3) basic and translational research; and 4) infrastructure and tools, and discuss future directions for advancement. This review is a follow-up of the discussions at the European Respiratory Society Research Seminar "Early origins of lung disease: towards an interdisciplinary approach" (Lisbon, Portugal, November 2019).

Early Life Microbiota and Respiratory Tract Infections.

Over the last decade, it has become clear that respiratory and intestinal tract microbiota are related to pathogenesis of respiratory tract infections (RTIs). Host and environmental factors can drive respiratory microbiota maturation in early life, which in turn is related to consecutive susceptibility to RTIs. Moreover, during RTIs, including viral bronchiolitis, the local microbiome appears to play an immunomodulatory role through complex interactions, though causality has not yet been fully demonstrated. The microbiota is subsequently associated with recovery after RTIs and can be related to persistent or long-term sequelae. In this Review, we explore the epidemiological evidence supporting these associations and link to mechanistic insights. The long-term consequences of childhood RTIs and the comprehensive role of the microbiota at various stages in RTI pathogenesis call for early life preventative and therapeutic interventions to promote respiratory health.

Interaction between the nasal microbiota and S. pneumoniae in the context of live-attenuated influenza vaccine.

Streptococcus pneumoniae is the main bacterial pathogen involved in pneumonia. Pneumococcal acquisition and colonization density is probably affected by viral co-infections, the local microbiome composition and mucosal immunity. Here, we report the interactions between live-attenuated influenza vaccine (LAIV), successive pneumococcal challenge, and the healthy adult nasal microbiota and mucosal immunity using an experimental human challenge model. Nasal microbiota profiles at baseline are associated with consecutive pneumococcal carriage outcome (non-carrier, low-dense and high-dense pneumococcal carriage), independent of LAIV co-administration. Corynebacterium/Dolosigranulum-dominated profiles are associated with low-density colonization. Lowest rates of natural viral co-infection at baseline and post-LAIV influenza replication are detected in the low-density carriers. Also, we detected the fewest microbiota perturbations and mucosal cytokine responses in the low-density carriers compared to non-carriers or high-density carriers. These results indicate that the complete respiratory ecosystem affects pneumococcal behaviour following challenge, with low-density carriage representing the most stable ecological state.