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

Abstract

Adherence to and invasion of epithelial cells represent important pathogenic mechanisms of Streptococcus pyogenes. A fibronectin‐binding surface protein 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 A8, 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 fibronectin as a bridging molecule and the α5β1 integrin as cellular 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 population reached the cytoplasm. In contrast, uptake of strain A8 required major rearrangements of cytoskeletal proteins underneath attached bacteria. In A8, a proteinaceous moiety was involved, which does not interact with α5β1 or need any known bridging molecule. Bacterial attachment stimulated elongation and massive recruitment of neighbouring microvilli, which fused to surround streptococcal chains. They led to the generation of large pseudopod‐like structures, which engulfed bacteria that were rapidly released and replicated in the cytoplasm. The identification of two completely different uptake pathways reported here provided further evidence regarding the diversity of S. pyogenes isolates and might contribute towards understanding the pathogenesis and persistence of S. pyogenes.

Fate of Streptococcus pyogenes and epithelial cells following internalization

“…Our results agree well with recent findings that, following internalization, GAS escape from endocytic vacuoles into the cytosol, where they can multiply (Molinari et al, 2000).…”

“…Recent studies which demonstrated that some GAS strains are capable of intracellular multiplication (Molinari et al, 2000(Molinari et al, , 2001) support the first and last hypotheses.…”

“…Although GAS was considered an extracellular pathogen, it has been established that the bacterium can efficiently enter a variety of mammalian cells (LaPenta et al, 1994;Greco et al, 1995;Jadoun et al, 1998;Ö sterlund & Engstrand, 1995;Molinari & Chhatwal, 1999). Internalization of GAS strains by epithelial cells occurs via Fn-dependent or Fn-independent mechanisms that involve different bacterial ligands and cellular membrane receptors (Molinari et al, 2000).…”

Streptococcal modulation of cellular invasion via TGF-β1 signaling

“…The capacity to bind Fn renders these bacteria capable of employing integrins as receptors for efficient internalization. Extensive studies established that ␣5␤1 integrin is the primary receptor for M1-mediated (8,34) and PrtF1-mediated GAS invasion of epithelial cells (9,11). Integrin signaling required for Streptococcus internalization has only recently been investigated (10,12,35).…”

“…The bacteria can express several invasins, but the Fn-binding proteins (FnBPs), M1 and PrtF1͞Sfb1, have been studied in most detail (9)(10)(11). Engagement of ␣5␤1 integrin with Fn bound to these proteins initiates phosphoinositide 3-kinase (10,12) and integrin-linked kinase-dependent intracellular signals that induce cytoskeletal rearrangement and ingestion of streptococci (10).…”