KILLING OF ALPHA-HEMOLYTIC AND NONHEMOLYTIC ESCHERICHIA-COLI STRAINS IN HUMAN SERUM AND POLYMORPHONUCLEAR LEUKOCYTES

The role of alpha-haemolysin (AH) in the resistance of Escherichia coli strains, isolated from patients with extra-intestinal diseases or diarrhoea, to the bactericidal activity of human serum and intracellular killing in polymorphonuclear leucocytes (PMNL) was investigated in vitro. Sets of alpha-haemolytic and non-haemolytic E. coli strains and sets of isogenic E. coli strains, which included wild-type alpha-haemolytic strains and derived strains with a reduced production of AH, were used. Compared with non-haemolytic strains, alpha-haemolytic strains were significantly more resistant to the bactericidal activity of 10 % and 100 % human serum and to intracellular killing in PMNL. Higher resistance to serum killing and to intracellular killing in PMNL was also found in wild-type alpha-haemolytic E. coli than in isogenic bacteria with reduced production of AH. These results provide evidence that production of AH in E. coli strains counteracts both the bactericidal activity of serum and intracellular killing in PMNL.