Background: Pseudomonas aeruginosa is an opportunistic pathogen that causes chronic endobronchial infections in cystic fibrosis (CF) patients. The role of bacterial internalization in the clearance of P. aeruginosa from the airways is controversial.
Material/Methods: A xenograft model was used to study P. aeruginosa strain PAO-1 clearance and internalization by the human airways in vivo. Human lung and tracheal rudiments, obtained from therapeutic abortions (20±2 weeks of gestation), were subcutaneously implanted in the flanks of SCID mice and infected after 14–22 weeks of engraftment. Lungs were surgically exposed and P. aeruginosa was injected in the pulmonary parenchima. Opercula closing the tracheal openings were excised, mucus removed, and bacterial inoculum was injected into the lumen. Internalization was studied at 4 hours post-infection on single-cell suspensions, while clearance was evaluated after 24–72 hours from the infection on homogenized tissues.
Results: Tracheae and lungs were morphologically identical to the adult human tissues, as evaluated by standard histology. Both types of xenografts showed a very low level of bacterial internalization (0.004–0.25% of total recovered bacteria), although tracheal xenografts presented more than 100 times greater internalization than did lung xenografts. Both lung and tracheal xenografts did not clear the injected bacteria for each inocolum, even at very low doses (100 colony forming units).
Conclusions: P. aeruginosa internalization by epithelial cells occurs, albeit at very low levels, and is not sufficient to clear bacteria in the airway xenograft model. This model could be used for studying chronic respiratory infections in CF patients.

Keywords: Pseudomonas aeruginosa - physiology, Models, Animal, Mice, SCID, Lung Transplantation, Fetal Tissue Transplantation, Epithelial Cells - microbiology, Respiratory Tract Infections - microbiology, Trachea - transplantation, Transplantation, Heterologous