[Serum sensitivity of Pseudomonas aeruginosa isolated from sputum as a virulence factor in the lower respiratory tract]. 
To elucidate the clinical significance of serum-sensitivity of respiratory pathogenic Pseudomonas aeruginosa (P. aeruginosa) strains, we examined serum-sensitivity of P. aeruginosa isolated from 16 patients with lower respiratory tract infections and clinical backgrounds of these patients.  We also evaluated the virulence of four serum-resistant and four serum-sensitive P. aeruginosa strains in murine pneumonia model induced by intratracheal challenge, and the silver-stained profiles of purified lipopolysaccharide (LPS) from these strains.  Serum-sensitive strains were isolated only from patients with chronic bronchitis, bronchiectasis, and diffuse panbronchiolitis colonized with P. aeruginosa, and rarely caused pneumonias, while serum-resistant strains caused pneumonias in some cases.  Intratracheal challenge of mice with 5 x 10(7) cfu per mouse of a serum resistant strain caused fatal hemorrhagic pneumonia with bacteremia.  In contrast, the same dose of a serum-sensitive strain provided non-fatal pneumonia without bacteremia.  LD50 of serum-sensitive strains in a murine model of P. aeruginosa pneumonia were at least 2-10 times higher than those of serum-resistant strain.  The LPS profiles of two serum-resistant strains and one serum-sensitive strain showed ladder-like patterns.  The similar analysis demonstrated that one serum-sensitive strain was lack of ladder-like patterns.  These data support that serum-sensitive P. aeruginosa strains are less virulent than serum-resistant P. aeruginosa strains in the lower respiratory tract, and serum sensitivity of P. aeruginosa strains is determined by the structure of the O-side chain of LPS; either lack of the O-side chain or the presence of sparse O-side chain.