Role of Pneumolysin's complement-activating activity during pneumococcal bacteremia in cirrhotic rats

Abstract

We investigated the role of pneumolysin's complement-activating activity during Streptococcus pneumoniae bacteremia in a hypocomplementemic, cirrhotic host. Isogenic mutant pneumococcal strains, in which pneumolysin was expressed from a plasmid, were used. These strains included H+C+, expressing wild-type pneumolysin with both cytolytic and complement-activating activity; PLY-, carrying the plasmid without the pneumolysin gene; and, H+C-, expressing pneumolysin with cytolytic activity only. In control rats, intravenous infection with 2.0 x 10(7) CFU of H+C+ per ml of blood resulted in a decrease in bacteremia of 3.5 log units by 18 h postinfection and 55% mortality. By contrast, cirrhotic rats infected similarly with the H+C+ strain demonstrated a 0.2-log-unit increase in bacteremia by 18 h postinfection and 100% mortality. Both control and cirrhotic rats cleared the PLY- strain more effectively from their bloodstreams by 18 h postinfection (6.2 and 5. 6 log unit decreases, respectively). Infection with the PLY- strain also resulted in low mortality (0 and 14%, respectively) for control and cirrhotic rats. When infected with the H+C- strain (without complement-activating activity), both groups cleared the organism from their bloodstreams nearly as well as they did the PLY- strain. Furthermore, the mortality rate for control and cirrhotic rats was identical after infection with the H+C- strain. These studies suggest that pneumolysin production contributes to decreased pneumococcal clearance from the bloodstream and higher mortality in both control and cirrhotic rats. However, pneumolysin's complement-activating activity may uniquely enhance pneumococcal virulence in the hypocomplementemic, cirrhotic host.

FIG. 1

Growth curves for WU2 pneumococcal mutant strain in vitro. Each strain was grown at 37°C in Todd-Hewitt broth supplemented with 5% rabbit serum and 10 μg of erythromycin per ml. Viable counts over time were determined by a plate count technique.

FIG. 2

Clearance of mutant organisms from the bloodstreams of control (top) and cirrhotic (bottom) rats. All rats (n = 5 to 9) were infected intravenously with 2.0 × 10⁷ CFU of the isogenic mutant strains per ml of blood. The number of CFU per milliliter of blood was determined at each time point by a plate count technique. ∗, significantly higher for cirrhotic than for control animals (P = 0.0002). SD, standard deviation.

FIG. 3

Survival of cirrhotic and control rats after infection with the isogenic mutant strains. Rats (n = 5 to 9) were infected with 2.0 × 10⁷ CFU of the isogenic mutant strains per ml of blood as indicated, and survival was determined for 10 days postinfection. Solid symbols represent cirrhotic rats (cir), and open symbols represent controls (con). ∗, survival was significantly higher for rats infected with the PLY− strain than for those infected with the H+C+ strain (P < 0.05).