Extracellular DNA accumulates in the matrix of S. Typhimurium biofilms. Biofilms of strain 14028 were cultivated in flow chambers at 37°C for 2 days in LB medium and stained for extracellular DNA. Cells in the biofilm were stained with the membrane staining dye FM 4–64 (A). The middle panel depicts the accumulation of extracellular DNA with TOTO-1 staining (B). The images are merged on the right (C). The large image shows the xy plane and the bottom panel shows the xz plane. The scale bar equals 15 μM. The wild-type 14028 strain carrying the pmrH-gfp construct forms aggregates on the surface of glass (D). The merged image of green fluorescence from pmr expression and red from propidium iodide staining, which stains both dead cells and extracellular DNA (E).