FbsA mediates absorption of Fbg from human plasma and attachment of S agalactiae to Fbg. In panels A and B the FbsA-dependent absorption of plasma Fbg by S agalactiae strains is reported. The FbsA⁺S agalactiae strain 6313 and its fbsA^– mutant were mixed with human plasma. Subsequently, bacteria-bound proteins were eluted and subjected to electrophoresis on an 7.5% polyacrylamide gel under reducing (+) and nonreducing conditions (–) and then Coomassie stained (A) or probed by Western blot (B). The Coomassie-stained gel and Western blot show a reference sample of pure human Fbg and proteins absorbed and eluted from bacteria, as indicated. Immunostaining was performed by incubating the membrane with mouse IgG raised against human Fbg. No Fbg bands were detected on a blot containing plasma proteins bound to the fbsA^– mutant. Molecular mass markers (left) are in kilodaltons. In panel C, the adherence of S agalactiae strains to Fbg is reported. Microtiter plates were coated with fibrinogen (10 μg/mL) and incubated for 2 hours with 5 × 10⁷ cells belonging to different serotypes. After extensive washes, 1 μg rabbit anti–GBS IgG was added to each well, followed by an incubation for 90 minutes. Attachment of the bacteria was quantified by addition of peroxidase-conjugated goat anti–rabbit IgG and the plates were developed with o-phenylenediamine. The bars show SDs of triplicate samples.