fXIII cross-links the streptococcal M1 protein with fibrinogen, leading to immobilization of bacteria within the clot. (A) The electron micrographs show negatively stained human fibrinogen (characterized by 3 domains) in complex with M1-protein (elongated) before (top) and after fXIII cross-linking (middle). Cross-linking was detected by immunostaining the fibrinogen M1 protein complex with the gold-labeled antibody against N-ϵ-γ-glutamyl-lysine (bottom). A schematic drawing of the fibrinogen (gray) and M1 protein (black) is included to highlight the interaction between fibrinogen and M1 protein. The scale bars indicate 25 nm. (B) Bacteria were incubated with normal or fXIII-deficient plasma and clotting was initiated by the addition of thrombin. Clots were washed briefly, covered with THB medium, and further incubated at 37°C. At the indicated time points, bacterial numbers were determined by plating of serial dilutions of the supernatant onto blood agar. The data represent the means ± SD of 3 independent experiments. *P < .05; ***P < .001.