In vitro interactions of S pyogenes with murine BMMCs. (A) FESEM images of uninfected murine BMMCs after 21 days in culture medium supplemented with recombinant mouse IL-3 (bar, 2 μm). (B) S pyogenes (arrow) attached to the surface of BMMCs (bar, 1 μm). (C) A clump of S pyogenes (arrow) trapped by extracellular fibers produced by BMMCs (bar, 10 μm). Insert in top-right corner shows a higher magnification of an entrapped bacterial clump (bar, 2 μm). (D) MCs in the process of producing MCETs. Some streptococci trapped by incipient MCETs are indicated by white arrows (bar, 5 μm). (E) S pyogenes captured in MCETs (white arrows; bar, 5 μm). (F) Immunofluorescence photograph showing blue labeled DNA (BMMC nuclei and extracellular MCET fibers) and associated red-labeled S pyogenes (bar, 10 μm). (G) Growth of S pyogenes in medium alone, in coculture with MCs, in coculture with cytochalasin D–treated MCs, or in coculture with MCs separated by a transwell system. Data are expressed as x-fold increase in bacterial growth with respect to the original inoculum. Each point represents the mean plus or minus SD of 3 independent experiments. *P < .05 by F-test for S pyogenes growth in medium control versus S pyogenes growth in the presence of either untreated or cytochalasin D–treated BMMCs.