Pneumolysin with low hemolytic activity confers an early growth advantage to Streptococcus pneumoniae in the blood.

Streptococcus pneumoniae is a leading cause of human diseases such as pneumonia, bacteremia, meningitis, and otitis media. Pneumolysin (Ply) is an important virulence factor of S. pneumoniae and a promising future vaccine target. However, the expansion of clones carrying ply alleles with reduced hemolytic activity has been observed in serotypes associated with outbreaks of invasive disease and includes an allele identified in a highly virulent serotype 1 isolate (ply4496). The virulence of Ply-deficient and ply allelic-replacement derivatives of S. pneumoniae D39 was compared with that of wild-type D39. In addition, the protective immunogenicity of Ply against pneumococci with low versus high hemolytic activity was also investigated. Replacement of D39 ply with ply4496 resulted in a small but statistically significant reduction of virulence. However, both native Ply- and Ply4496-expressing strains were significantly more virulent than a Ply-deficient mutant. While the numbers of both Ply- and Ply4496-expressing isolate cells were higher in the blood than the numbers of Ply-deficient mutant cells, the growth of the Ply4496-expressing strain was superior to that of the wild type in the first 15 h postchallenge. Ply immunization provided protection regardless of the hemolytic activity of the challenge strain. In summary, we show that low-hemolytic-activity Ply alleles contribute to systemic virulence and may provide a survival advantage in the blood. Moreover, pneumococci expressing such alleles remain vulnerable to Ply-based vaccines.