INVASION OF CULTURED HUMAN-CELLS BY STREPTOCOCCUS-PYOGENES

The invasive capacity of streptococcal strains belonging to groups A a nd B was evaluated by infecting human epithelial and endothelial cells and monitoring the number of viable intracellular bacteria at differe nt times postinfection. All strains tested entered eukaryotic cells (H eLa, HEp2 and HUVE), with Streptococcus pyogenes exhibiting a higher i nvasion efficiency than group B streptococci (GBS). No intracellular m ultiplication was observed, and GBS remained viable 24 h postinfection , whereas S. pyogenes were gradually killed. We found that cytochalasi n D almost completely inhibited internalization of all bacterial strai ns, whereas colchicine had no effect, indicating that host microfilame nts play a major role in bacterial internalization. Moreover, the use of the lysosomotropic agent ammonium chloride enabled us to demonstrat e that a pH increase in the intracellular vesicles did not affect stre ptococcal entry. These results were documented by electron microscopic observations which revealed the different steps in the invasion pathw ay, including a fusion event between phagosomes containing S. pyogenes and lysosomes.