F9 and pneumonia: The well-established role of FVII, FIX, and FX is to initiate coagulation via their serine protease activity.8–10 Nevertheless, an involvement of these coagulation factors in anti-infection mechanisms of organisms has also been suggested because their deficiency is documented to have a significant correlation with bacterial infectious diseases such as sepsis and pneumonia.18–20 In the current study, we reveal their potent antibacterial activity against Gram-negative bacteria, proposing a concept that FVII, FIX and FX constitute a class of important antimicrobial proteins.