Two distinct pathways for the invasion of Streptococcus pyogenes in non-phagocytic cells

Adherence to and invasion of epithelial cells represent important pathogeni c mechanisms of Streptococcus pyogenes. A fibronectin-binding surface prote in of S. pyogenes, SfbI protein, has been implicated in both adherence and invasion processes. Invasion of SfbI-containing strains has been suspected to be responsible for the failure of antibiotics treatment to eradicate S. pyogenes. In this study, we tested the adherence and invasion properties of two well-characterized clinical isolates: A40, which expresses SfbI; and A 8, which is SfbI negative and is unable to bind fibronectin. In strain A40, SfbI was the main factor required for attachment and invasion by using fib ronectin as a bridging molecule and the alpha(5)beta(1) integrin as cellula r receptor. The uptake process was characterized by the generation of large membrane invaginations at the bacteria-cell interface without evidence of actin recruitment or cellular injury. A40 cells were located in phagosomes and, only 24 h after infection, a consistent part of the bacterial populati on reached the cytoplasm. In contrast, uptake of strain A8 required major r earrangements of cytoskeletal proteins underneath attached bacteria. In A8, a proteinaceous moiety was involved, which does not interact with alpha(5) beta(1) or need any known bridging molecule. Bacterial attachment stimulate d elongation and massive recruitment of neighbouring microvilli, which fuse d to surround streptococcal chains. They led to the generation of large pse udopod-like structures, which engulfed bacteria that were rapidly released and replicated in the cytoplasm. The identification of two completely diffe rent uptake pathways reported here provided further evidence regarding the diversity of S. pyogenes isolates and might contribute towards understandin g the pathogenesis and persistence of S. pyogenes.