Evidence for the Pasteurella haemolytica cytotoxin as a product of actively growing bacteria.

The release of soluble cytotoxin from Pasteurella haemolytica type 1 cultured at 37 C in RPMI 1640 medium containing 7% bovine fetal serum was evaluated, using bovine alveolar macrophages in a microplate 51Cr-release assay. Heat-labile leukotoxic activity was detected in culture supernatant during the lag and logarithmic-growth phases, but not in stationary-phase culture; toxic activity could not be demonstrated in sonicates of logarithmic (1 hour) or stationary (18 hour) phase organisms. This pattern of toxin release, typical of a bacterial exotoxin or metabolic enzyme, is unique among bacterial leukotoxins, which are typically cell-associated and released after autolysis or sonication from outgrown cultures.