A collection of 198 clinical isolates of strains belonging to the tribe was examined for haemolytic activity on blood agar and in Brain Heart Infusion Broth. The strains were of diverse bacteriocin and O-antigenic types and from a wide variety of sources. They included representatives of all species of and . Approximately half of the strains were haemolytic on blood agar. This activity was not associated with any particular bacteriocin type. The haemolysin was also produced during exponential growth in broth and was thermolabile and calcium dependent. All strains and some strains were non-haemolytic on blood agar. However, most strains of the spp., irrespective of their bacteriocin and antigenic type, produced, over a short period during exponential growth in broth, a heat-stable, cell-associated calcium-independent haemolysin. A smaller proportion of and strains produced, in addition, a thermolabile, calcium-dependent haemolysin which was associated with the formation of large haemolytic zones on blood agar. The relationship of these haemolysins to haemolysin and their possible role in virulence is discussed. Haemolysin production was not found in any of the 74 strains of four species of .


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