Viral/bacterial respiratory tract co-infections in children

Abstract

Respiratory syncytial virus (RSV) is the most significant cause of paediatric acute respiratory infection (ARI). RSV and other respiratory viruses are commonly co-detected with potentially pathogenic bacteria, such as Streptococcus pneumoniae, in the upper airways of young children. While these bacteria are known to be carried asymptomatically, they are also capable of opportunistic infections. Interactions between RSV and S. pneumoniae have been demonstrated in both clinical and molecular studies. However, the clinical significance of these interactions remains debated, and the effect of clinically relevant strain variation in both virus and bacteria on these interactions is also unknown. This work aimed to better understand the role of S. pneumoniae colonisation during RSV infections. To determine the effect of RSV and S. pneumoniae co-detection on disease severity during ARI, molecular pathogen screening of nasopharyngeal samples was conducted in a cohort of young children under the age of two years presenting with ARI at the emergency department of the Royal Brisbane Children’s Hospital, Australia. S. pneumoniae was co-detected in approximately 52% of RSV infections, and co-detection was associated with increased disease severity, suggesting that S. pneumoniae plays a pathogenic role in severe paediatric RSV infections. Having established a clinical role for S. pneumoniae during RSV infection, the dynamics of pneumococcal colonisation of the nasal cavity was investigated in the four weeks prior and subsequent to an RSV infection in a longitudinal birth cohort of Brisbane-based children. Approximately 33% of infants and young children with RSV infections were colonised by S. pneumoniae prior to the RSV detection, and strain characterisation of S. pneumoniae in the weeks immediately surrounding the viral infection suggested that any observed growth of S. pneumoniae during RSV infection arose from the pre-existing strain colonising the URT. In another 14% of RSV infections, S. pneumoniae was first detected during the virus infection, suggesting simultaneous acquisition of RSV and S. pneumoniae and possible co-transmission of the two pathogens. The results from the two paediatric cohorts suggested an interaction between RSV and S. pneumoniae that was advantageous to the bacteria. It was therefore hypothesised that the molecular mechanisms governing RSV and S. pneumoniae interactions might be pneumococcal strain-specific and stronger in strains frequently co-detected with RSV. Studies have shown that RSV can increase S. pneumoniae colonisation by enhancing pneumococcal adherence to airway epithelia, and virions can bind directly to the bacterium to form co-pathogen complexes. These interactions are in part mediated through the binding interactions between S. pneumoniae surface receptors and the RSV G surface glycoprotein. RSV G is highly variable, both antigenically and genetically, and is the main protein used to type RSV into subtypes A and B. The genetic variation of RSV G was characterised from nasopharyngeal samples of young children under the age of five years with respiratory symptoms collected from Pathology Queensland Central at the Royal Brisbane Women’s Hospital. The core genetic variation, through multilocus sequence typing, and molecular serotype of S. pneumoniae isolates from the same cohort was also determined. S. pneumoniae co-detection was more common with RSV-A compared to RSV-B; however, there were no clear associations between virus co-detection and bacteria strains. An in vitro binding assay was developed to specifically investigate pneumococcal strain variation in adherence to immortalised airway epithelial cells during RSV infection or after pre-binding with RSV virions. S. pneumoniae strains were isolated from the paediatric pathology cohort to investigate whether strains isolated from patients with RSV infections exhibited greater adherence to RSV infected epithelial cells. The results suggested that the effect of RSV on pneumococcal adherence to the epithelial cells differed between S. pneumoniae strains and isolates, although no significant association was identified between strains isolated from children with RSV infections and increased RSV-mediated adherence. This project has demonstrated a pathogenic role for S. pneumoniae in RSV infection of young children. The results suggest that one mechanism of this interaction may be mediated through binding interactions between the bacteria and RSV surface glycoproteins, potentially resulting in outgrowth of S. pneumoniae colonising the URT during virus infection or possible co-transmission of virus and bacteria. The results from the paediatric cohorts have suggested that the presence of both S. pneumoniae and RSV should be considered during the diagnosis and treatment of paediatric ARI, while the molecular investigations have given insight into the strain-specific variation of interactions between the two pathogens, and provided a panel of characterised, clinically relevant S. pneumoniae isolates for use in further studies.