Pasteurella haemolytica
leukotoxin is cytotoxic to bovine leukocytes, causing increased cell membrane permeability, osmotic swelling, release of cytosolic
proteins and cell lysis. These studies were designed to test if
leukotoxin causes release of the cytoskeletal
protein, actin, from
bovine leukemia cells and if purified actin-influenced bacterial growth or
leukotoxin production. Culture supernatants caused a 7-fold decrease in viability of
bovine leukemia cells and increased cell permeability that was accompanied by release of
beta-actin into the cell culture supernatant. Exposing P. haemolytica to purified actin solutions induced the conversion of monomeric
G-actin to polymerized
F-actin. This conversion was partially inhibited by bovine P. haemolytica immune, but not pre-immune, serum. Loss of
streptomycin resistance following treatment of the organism with
acridine orange ablated the polymerizing activity. Incubation of P. haemolytica in the presence of purified
F-actin did not affect growth but resulted in culture supernatant that had 3.0-3.9-fold greater leukotoxicity compared to medium alone or medium containing
G-actin, heat-denatured actin or
albumin. The effect of actin on leukotoxicity was concentration-dependent and directly associated with increases in secreted
leukotoxin. The interaction between P. haemolytica and actin is potentially detrimental to the host by inducing polymerization of actin into insoluble filaments and by enhancing leukotoxicity.