The data support an important role for S100A9 in lung innate immunity against S. pneumoniae: 1) Increased levels of S100A9 protein were found in BALF of patients with bacterial, but not viral ARDS, which were correlated with CRP, PCT and SOFA scores, suggesting S100A9 as novel biomarker for bacterial pneumonia. This evidence concerns the gene S100A9 and bacterial pneumonia.