Abstract Background The lung epithelium constitutes the first barrier against invading pathogens and also a major surface potentially exposed to nanoparticles. In order to ensure and preserve lung epithelial barrier function, the alveolar compartment possesses local defence mechanisms that are able to control bacterial infection. For instance, alveolar macrophages are professional phagocytic cells that engulf bacteria and environmental contaminants (including nanoparticles) and secrete pro-inflammatory cytokines to effectively eliminate the invading bacteria/contaminants. The consequences of nanoparticle exposure in the context of lung infection have not been studied in detail. Previous reports have shown that sequential lung exposure to nanoparticles and bacteria may impair bacterial clearance resulting in increased lung bacterial loads, associated with a reduction in the phagocytic capacity of alveolar macrophages. Results Here we have studied the consequences of SiO 2 nanoparticle exposure on Pseudomonas aeruginosa clearance, Pseudomonas aeruginosa -induced inflammation and lung injury in a mouse model of acute pneumonia. We observed that pre-exposure to SiO 2 nanoparticles increased mice susceptibility to lethal pneumonia but did not modify lung clearance of a bioluminescent Pseudomonas aeruginosa strain. Furthermore, internalisation of SiO 2 nanoparticles by primary alveolar macrophages did not reduce the capacity of the cells to clear Pseudomonas aeruginosa . In our murine model, SiO 2 nanoparticle pre-exposure preferentially enhanced Pseudomonas aeruginosa -induced lung permeability (the latter assessed by the measurement of alveolar albumin and IgM concentrations) rather than contributing to Pseudomonas aeruginosa -induced lung inflammation (as measured by leukocyte recruitment and cytokine concentration in the alveolar compartment). Conclusions We show that pre-exposure to SiO 2 nanoparticles increases mice susceptibility to lethal pneumonia but independently of macrophage phagocytic function. The deleterious effects of SiO 2 nanoparticle exposure during Pseudomonas aeruginosa -induced pneumonia are related to alterations of the alveolar-capillary barrier rather than to modulation of the inflammatory responses.