Upper and lower respiratory tract infection by Streptococcus pneumoniae is affected by pneumolysin deficiency and differences in capsule type

Pneumococci frequently colonize the upper respiratory tract, and these pneumococci are believed to act as a reservoir for infection of the lower respiratory tract and bacteremia. We investigated how the pneumococcal toxin pneumolysin affects the capacity of pneumococci to infect the upper and lower respiratory tract of the mouse. Wild-type Streptococcus pneumoniae serotype 2 and 3 strains, a serotype 2 pneumolysin-deficient mutant, and a serotype 2 mutant with the pneumolysin gene reinserted were used to study differences in colonization and disease. In addition, we also examined a pneumococcal chimeric mutant (capsule type switched from serotype 2 to serotype 3) to gain further insight into the role that capsule plays in nasopharyngeal infection. Absence of pneumolysin was found to be associated with significantly lower numbers of pneumococci in the nasopharynx, trachea, and lungs. Differences in pneumococcal capsule type were found to have significant effects on pneumococcal infection of the nasopharynx, trachea, and lungs. However, it was the combination of capsule type and genetic background that was important, and the influence of this combination varied with the site of infection. For example, in the nasopharynx the wild-type serotype 3 strain and the capsule-switched mutant behaved similarly, whereas in the lungs the mutant that was switched to serotype 3 survived less well than the wild-type serotype 3 strain. The combination of capsule type and genetic background also determined virulence. Thus, the wild-type serotype 3 strain was virulent, whereas the capsule-switched mutant was avirulent.