Plasmid-mediated contact haemolytic activity in Shigella species: correlation with penetration into HeLa cells.

The main feature of virulent strains of Shigella is their ability to invade eucaryotic cells. This phenotype is both plasmid-mediated and temperature-regulated. In the present report, we demonstrate a plasmid-mediated and temperature-regulated haemolytic activity in S. flexneri, S. dysenteriae and S. sonnei. Detection of this haemolytic activity requires centrifugation of suspensions containing bacteria and erythrocytes, followed by incubation of the pellets at 37 degrees C. No soluble intra- or extracellular haemolytic activity could be detected. Dose-effect and electron microscopic studies demonstrated that direct contact of several virulent bacteria per erythrocyte was critical for haemolysis to occur. However, no local morphological alteration of the erythrocyte membrane at the site of contact with bacteria could be detected. Intensity of this haemolytic activity was fully correlated with the efficiency of penetration within HeLa cells, suggesting a common mechanism for induction of phagocytosis and lysis of erythrocytes.