Pretreatment of epithelial cells with live streptococcus pneumoniae had no detectable effect on influenza A virus replication in vitro (MPF1P.774)

Influenza A virus (IAV) and Streptococcus pneumoniae (pneumococcus) are two major upper respiratory tract pathogens responsible for exacerbated disease in coinfected individuals. Information on the direct effect of S. pneumoniae on IAV replication in humans is unknown. Such an investigation is important because S. pneumoniae is a commensal of the human upper respiratory tract. The aim of this study was to determine whether treatment of epithelial cells with S. pneumoniae affects IAV replication, determined with the help of a standard immunofluorescence assay (IFA). For this study, four IAV permissive epithelial cell lines including two human-derived cell lines, 12 pneumococcal strains including recent human clinical isolates which represent different genetic backgrounds and serotypes, and six IAV strains of varying genetic nature and pathogenic potential including the pandemic 2009 H1N1 virus were used. Our results suggested that pretreatment of MDCK cells with 7.5×10^6 colony-forming unit (CFU) of live S. pneumoniae resulted in gradual cell-death in a time-dependent manner (1/2 hr to 4 hr). However, pretreatment of cells with 7.5×10^5 and lower CFU of S. pneumoniae had no detectable effect on either the morphology of epithelial cells or on the IAV replication. However, the effect of pneumococci on IAV replication may be different during a coinfection in vivo in the human upper respiratory tract.