(A) Mice treated with Ga are protected from lethal P. aeruginosa pneumonia. P. aeruginosa strain PA103 (5 × 10⁵ bacteria) was administered to mice by the intratracheal route. Mice were treated with a single dose of inhaled Ga (or vehicle alone) 3 hours after infection. Inhalation treatment was achieved by placing a 50-μl drop of concentrated Ga(NO₃)₃ (250 mm) or Ga-free vehicle on the nares of the mice. Because mice are obligate nose breathers, some of the drop was inhaled. Data are representative of 4 separate experiments (n = 6 mice in each group); *P < 0.005 versus vehicle control. Ga was also effective against acute pneumonia produced by strain PA01 (Supplemental Figure 10). (B) Preloading the lung with Fe blocks Ga’s antimicrobial effects. Ten microliters of 2 mM FeCl₃ (or Fe-free vehicle) was administered to mice by the intratracheal route immediately prior to infection. Mice (n = 8 per group) were treated with Ga as described in A. Data are representative of 3 separate experiments; *P < 0.005 versus Fe-free control. (C) Inhaled Ga is effective in a mouse airway biofilm infection model. Infections were established as described in Methods. Mice were treated with vehicle alone or Ga (as described in A) 3 times a day for 3 days. Lungs were homogenized, and bacterial counts determined by plating (n = 4 mice per group). Data are representative of 3 separate experiments; *P < 0.005 versus vehicle control.