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

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Abstract
The .alpha.-hemolytic E. 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; 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 was 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. This effect was abolished by fresh autologous plasma. Two nonhemolytic strains, a mutant derivative of C134-73 Hly+ and strain X43 were not cytotoxic. Evidently, .alpha.-hemolysin associated with the bacterial cells may enhance the virulence of E. coli by injuring phagocytes; free .alpha.-hemolysin may be of minor importance because its cytotoxic effect is neutralized by host plasma.