Avirulence and altered physiological properties of cystic fibrosis strains of Pseudomonas aeruginosa.

Twenty Pseudomonas aeruginosa strains isolated from patients with cystic fibrosis in good and poor clinical condition were typed by the American Scientific (Difco Laboratories, Detroit, Mich.) Typing Scheme. Only five strains were agglutinated with a single typing serum. Ten strains were agglutinated with more than one serum, and five were not agglutinated with any serum, suggesting some type of lipopolysaccharide alteration in the majority of these strains. Of the strains from patients in good clinical condition, 72% demonstrated proteolytic activity, while 60% of the strains from patients in poor clinical condition demonstrated no proteolytic activity. Twenty-three cystic fibrosis strains of P. aeruginosa examined demonstrated reduced bacteremic virulence when compared with a virulent burn strain with a 50% lethal dose (LD50) of 1.5 X 10(1) CFU in an invasive burned mouse model. Ninety-two percent of the strains tested were avirulent at doses of 10(3) to 10(5) CFU. The LD50s were determined for 10 selected strains which exhibited specific important morphological and physiological deficiencies. Five of the strains tested gave LD50s greater than 10(6) CFU. Reduced virulence of these strains was associated with loss of two or more physiological characteristics associated with virulence. The cystic fibrosis strains of P. aeruginosa which morphologically and physiologically resembled the virulent burn strain were the most virulent (LD50s of 10(2) to 10(4). Results suggest that some degree of virulence is associated only with classic strains prevalent in early infections. The data suggest that a selection transition occurs in the lungs of patients with cystic fibrosis that favors P. aeruginosa avirulence. The avirulent state may be caused by alterations in the cell envelope, including associated factors such as motility and chemotaxis and protease production.