The immunopathology of respiratory syncytial virus (RSV) infection, the most common cause of lower respiratory tract infections (LRTI) in the pediatric population, with severe disease being the exception. The variability of the clinical presentation is incompletely explained by host, viral and environmental factors but, in infants and young children, disease severity is certainly linked to the physiological immune immaturity. There is evidence that the maturation of the host immune response is, at least in part, promoted by the composition of the nasopharyngeal microbiome that, modulating excessive inflammation, can counteract the predisposition to develop viral respiratory infections and lower the risk of disease severity. However, interaction between the nasopharyngeal microbiota and respiratory viruses can be bidirectional. Microbial dysbiosis can drive disease pathogenesis but may also represents a reflection of the disease-induced alterations of the local milieu. Moreover, viruses like RSV, can also increase the virulence of potential pathogens in nasopharynx, which is a main reservoir of bacteria, and therefore promote their spread to the lower airways causing superinfection. Negative changes in microbial community composition in early life may constitute a heightened risk towards severe RSV respiratory infection and bacterial superinfection, whilst specific groups of microorganisms can be associated with protection. A better understanding into the potential negative and positive role of the different nasopharyngeal bacterial species in disease prevention as well as into the possible benefits of microbiome therapeutic manipulation, may improve patient outcomes.

Background. In secondary tracheomalacia due to mediastinal vascular anomalies one of the most prevalent symptom is recurrent lower respiratory tract (LRT) infections, related to defective airway clearance. Whether this condition could result in persistent LRT inflammation and subclinical infection is not known. Patients and methods. Children with tracheomalacia due to mediastinal vascular anomalies, recurrent (>3/y) LRT infections were evaluated while in stable condition. Computed tomography (CT) scan and bronchoscopy with bronchoalveolar lavage (BAL) were performed. Results. 31 children were included in the study: 21 with aberrant innominate artery (AIA), 4 with right aortic arch (RAA) and 1 with double aortic arch (DAA) and 5 with AIA associated with RAA. Cytological evaluation of BAL fluid showed increased neutrophil percentages and normal lymphocyte and eosinophil proportions. Microorganism growth was detected in 35.5% of BAL samples, with a bacterial load >105 colony-forming-units (CFU)/mL only in 10,2% of them. Most isolates were positive for Haemophilus influenzae, followed by Streptococcus pneumoniae, Group A β-hemolytic streptococci and Moraxella catarrhalis. Chest CT scan demonstrated the presence of bronchiectasis in 13% of the children, of which only one had a positive BAL culture for Haemophilus influenzae. Conclusions. Only in a small subgroup of children, persistent neutrophilic alveolitis was associated with a significant bacterial load and the presence of bronchiectasis. Because most pathogens detected in BAL samples cultures can produce biofilms, caution should be used in inappropriate antibiotic prescription in these patients that, chiefly in those with bronchiectasis, in which chest physiotherapy can be of great benefit.