Infection with Conditionally Virulent Streptococcus pneumoniae pab Strains Induces Antibody to Conserved Protein Antigens but Does Not Protect against Systemic Infection with

Avirulent strains of a bacterial pathogen could be useful tools for investigating immunological responses to infection and potentially effective vaccines. We have therefore constructed an auxotrophic TIGR4 pab strain of Streptococcus pneumoniae by deleting the pabB gene Sp_0665. The TIGR4 pab strain grew well in complete medium but was unable to grow in serum unless it was supplemented with para-aminobenzoic acid (PABA). The TIGR4 pab strain was markedly attenuated in virulence in mouse models of S. pneumoniae nasopharyn-geal colonization, pneumonia, and sepsis. Supplementing mouse drinking water with PABA largely restored the virulence of TIGR4 pab. An additional pab strain constructed in the D39 capsular serotype 2 background was also avirulent in a sepsis model. Systemic inoculation of mice with TIGR4 pab induced antibody responses to S. pneumoniae protein antigens, including PpmA, PsaA, pneumolysin, and CbpD, but not capsular polysaccharide. Flow cytometry demonstrated that IgG in sera from TIGR4 pab-vaccinated mice bound to the surface of TIGR4 and D39 bacteria but not to a capsular serotype 3 strain, strain 0100993. Mice vaccinated with the TIGR4 pab or D39 pab strain by intraperitoneal inoculation were protected from developing septicemia when challenged with the homologous S. pneumoniae strain. Vaccination with the TIGR4 pa