Rationale: Respiratory syncytial virus (RSV) and Streptococcus pneumoniae are major respiratory pathogens. Coinfection with RSV and S. pneumoniae is associated with severe and often fatal pneumonia but the molecular basis for this remains unclear.Objectives: To determine if interaction between RSV and pneumococci enhances pneumococcal virulence.Methods: We used confocal microscopy and Western blot to identify the receptors involved in direct binding of RSV and pneumococci, the effects of which were studied in both in vivo and in vitro models of infection. Human ciliated respiratory epithelial cell cultures were infected with RSV for 72 hours and then challenged with pneumococci. Pneumococci were collected after 2 hours exposure and changes in gene expression determined using quantitative real-time polymerase chain reaction.Measurements and Main Results: Following incubation with RSV or purified G protein, pneumococci demonstrated a significant increase in the inflammatory response and bacterial adherence to human ciliated epithelial cultures and markedly increased virulence in a pneumonia model in mice. This was associated with extensive changes in the pneumococcal transcriptome and significant up-regulation in the expression of key pneumococcal virulence genes, including the gene for the pneumococcal toxin, pneumolysin. We show that mechanistically this is caused by RSV G glycoprotein binding penicillin binding protein 1a.Conclusions: The direct interaction between a respiratory virus protein and the pneumococcus resulting in increased bacterial virulence and worsening disease outcome is a new paradigm in respiratory infection.Keywords: respiratory syncytial virus; pneumococcus; cilia; virulence; G protein Respiratory syncytial virus (RSV) and Streptococcus pneumoniae, also known as the pneumococcus, are major respiratory pathogens causing a huge global healthcare burden, predominantly in young children and the elderly (1). Global RSV disease burden is estimated at 64 million cases and 160,000 deaths every year (2). Pneumococcal pneumonia results in more than 1 million deaths each year worldwide with approximately half a million in children younger than 5 years (3). There is increasing evidence that RSV and the pneumococcus interact to increase the severity of respiratory disease (4-6). Seasonal increases in infections This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org This article has embedded videos, which play in place from both the online PDF or HTML versions. If you cannot view Flash videos on your device, please try playing the videos in the online PDF, or access the original uncompressed videos at http://www.atsjournals.org/doi/suppl/10.1164/rccm.201311-2110OC. To play, mouse over the image and click on the arrow that will appear in the center or on the lower left.