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Original human isolates of influenza virus are highly filamentous in morphology; even the avian H5N1 strain of influenza virus exhibits filamentous morphology. In contrast, most laboratory-adapted strains, including the commonly used influenza A/PR/8/34 strain of influenza virus, produce strictly spherical virions
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Here we report that filamentous influenza virus infection predisposes mice to fatal septicemia following superinfection with Streptococcus pneumoniae serotype 3.
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Early studies demonstrated that Staphylococcus aureus adheres more efficiently to influenza virus-infected MDCK cells (11, 12). More recently, Streptococcus pneumoniae has also been shown to bind more efficiently to influenza virus-infected epithelial cells in vitro (25) and to mouse respiratory epithelium in vivo (31).
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This increased binding is believed to be a result of destruction of ciliated cells and complete desquamation of the respiratory epithelium. In our studies, we also found that S. pneumoniae colonizes the respiratory tracts of mice more efficiently in animals with an ongoing influenza virus infection. However, the observation that animals superinfected after virus clearance remained highly susceptible to synergistic fatal septicemia suggests that direct viral and bacterial interactions are not required for enhanced colonization or exacerbative disease.