In vitro cytotoxic effect of α-hemolytic Escherichia coli on human blood granulocytes

Abstract

The cytotoxic effect of Escherichia coli bacteria on human blood granulocytes was measured by recording numbers of nonlysed cells and percentages of viable cells after in vitro incubation with bacteria in the presence of plasma. A total of 179 strains from various sources of infection were tested. Of 117 α-hemolytic strains, 59 were cytotoxic. Five nonhemolytic mutant strains, derived from α-hemolytic cytotoxic strains, were nontoxic. None of the 62 nonhemolytic strains were toxic. Four spontaneously occurring α-hemolytic, nontoxic mutant strains were isolated from cytotoxic ones. Cytotoxicity of bacteria reached a maximum after log-phase growth at 30 to 37°C for 2.5 h, and the toxic capacity was equal after growth in various media, including human urine and plasma. The cytotoxic effect increased with the length of exposure of granulocytes to bacteria and with increasing numbers of bacteria per granulocyte. Cytotoxic strains showed different degrees of toxicity, highly cytotoxic strains lysing about 90% of the granulocytes and killing about one-half of nonlysed cells in 1 h. Bacteria killed by heat, formaldehyde, or UV light were nontoxic. Alpha-hemolytic strains of O groups 2, 4, 6, 25, and 75 originating from various infections in humans were more frequently cytotoxic than α-hemolytic strains of other O groups derived from human infections. Culture supernatants containing free α-hemolysin were highly cytotoxic to human blood granulocytes, monocytes, and lymphocytes in vitro, whether supernatants originated from cytotoxic or noncytotoxic bacteria. Cytotoxicity to phagocytes, which is mediated by or closely linked genetically to α-hemolysin, may be a mechanism by which α-hemolytic strains of E. coli strengthen their ability to establish and maintain infections.