MATERIALS AND METHODS
S. suis strains and growth conditions. Encapsulated S. suis type 2 strains
P1/7 (45) and 89-1591 (46), isolated from pigs with meningitis in Europe
(United Kingdom) and North America (Canada), respectively, were used
for experimental infections. A nonencapsulated cpsF mutant derived
from P1/7 (5) and M2, derived from 89-1591 (46), were used as the coating
for enzyme-linked immunosorbent assay (ELISA) evaluation of the
noncapsular, mainly protein-specific Ab response (see below). Bacteria
were grown as previously described (47). Briefly, bacteria were grown
overnight onto sheep blood agar plates at 37°C, and isolated colonies were
cultured in 5ml of Todd-Hewitt broth (THB [Becton Dickinson]) for 8 h
at 37°C. Then, 10 l of 1/1,000 dilutions of 8-h cultures were transferred
into 30 ml of THB and incubated for 16 h at 37°C. Stationary-phase
bacteria were washed in phosphate-buffered saline (PBS [pH 7.3]). The
bacterial pellet was then resuspended in vehicle solution and adjusted to
the desired concentrations. Aliquots of the final bacterial suspension were
plated using an Autoplate 4000 automated spiral plater (Spiral Biotech)
onto sheep blood agar plates, and colonies were accurately counted after
overnight incubation at 37°C.