Colocalization of M1 protein and the fXIII cross-linking site in human biopsies. (A) Tissue biopsies from patients with streptococcal necrotizing fasciitis were sectioned and immunofluorescently stained for M1 protein (green) in combination with anti–N-ϵ-γ-glutamyl-lysine (red). Confocal microscopy revealed colocalization of both antibodies, seen at higher magnification in the inset figure. Cell nuclei are stained in blue with DAPI. (B) Scanning electron microscopy showing bacteria entrapped in the fibrin network (arrows) in a biopsy from a patient with streptococcal necrotizing fasciitis. Scale bar indicates 5 μm. (C) Transmission electron micrograph displaying fXIII-mediated cross-linking of bacterial surface proteins to fibrin by detection of the gold-labeled antibody against N-ϵ-γ-glutamyl-lysine. The scale bar indicates 100 nm. (D) Transmission electron microscopy shows dead bacteria inside the fibrin network in a biopsy from a patient with streptococcal necrotizing fasciitis. The scale bar indicates 0.5 μm.