SUMMARY: Twenty-six strains belonging to the five main species of the genus were examined for production of thiol-dependent exotoxins. All strains of cultured in charcoal-treated broth secreted a haemolytic factor at a level ranging from 200 to 800 haemolytic units (HU) ml-1, except for the strain EGD (1500 HU ml) and the type strain CIP 82110T (10 HU ml). The haemolytic activity reached a maximum level by 8-10 h and then rapidly declined as soon as bacterial exponential growth ceased. The titres of haemolytic activity were markedly reduced when bacteria were grown in charcoal-untreated broth. The haemolytic factor produced by strains was characterized as listeriolysin O ( about 60000), a member of the group of thiol-dependent exotoxins. Strains of also produced high levels of thiol-dependent exotoxin (about 2500 HU ml), in both charcoal-treated and untreated broth. Small amounts of haemolytic factor (about 9-30 HU ml) were also produced by in charcoal-treated broth. The haemolysin produced by was identified for the first time as a thiol-dependent exotoxin of about 60000, antigenically related to listeriolysin O. As expected, we failed to detect thiol-dependent exotoxin in the two nonhaemolytic species, and .


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