Cytotoxic effect of an alpha-hemolytic Escherichia coli strain on human blood monocytes and granulocytes in vitro.

The alpha-hemolytic Escherichia coli strain C134-73 Hly+ was primarily cytocidal to human blood monocytes and granulocytes in vitro in the presence of fresh autologous plasma. Monocytes and granulocytes underwent marked morphological changes during incubation with the bacteria, and the percentages of intact phagocytes decreased progressively with the time of incubation. The cytotoxic effect increased with the number of bacteria per phagocyte and was produced by log-phase microorganisms only. Neither free hemolysin nor free cytotoxic activity to leukocytes could be detected in the incubation medium if the bacteria were removed from the test system. Bacteria-free culture supernatants containing alpha-hemolysin were cytotoxic to monocytes, granulocytes, and lymphocytes, monocytes and granulocytes being the most sensitive. However, this effect was abolished by fresh autologous plasma. Two nonhemolytic strains, a mutant derivative of C134-73 Hly+ and strain X43, were not cytotoxic. These observations suggest that alpha-hemolysin associated with the bacterial cells may enhance the virulence of E. coli by injuring phagocytes, whereas free alpha-hemolysin may be of minor importance because its cytotoxic effect is neutralized by host plasma.