The role of α-haemolysin (AH) in the resistance of 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 Sets of α-haemolytic and non-haemolytic strains and sets of isogenic strains, which included wild-type α-haemolytic strains and derived strains with a reduced production of AH, were used. Compared with non-haemolytic strains, α-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 α-haemolytic than in isogenic bacteria with reduced production of AH. These results provide evidence that production of AH in strains counteracts both the bactericidal activity of serum and intracellular killing in PMNL.


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