CONSTRUCTION OF AN ISOGENIC LEUKOTOXIN DELETION MUTANT OF PASTEURELLA-HAEMOLYTICA SEROTYPE-1 - CHARACTERIZATION AND VIRULENCE

Allelic replacement was used to generate two isogenic IMA deletion mut ants of Pasteurella haemolytica serotype 1 that were incapable of synt hesizing leukotoxin (Lkt). Southern blot data confirmed that lktA sequ ences were absent in the two P. haemolytica deletion mutants. Culture supernatants and whole cell lysates from the wild type P. haemolytica, D153 parent strain, but not the IMA deletion mutants, contained immun oreactive and bioactive leukotoxic protein. In addition, only the pare nt strain was haemolytic when grown on bovine and sheep blood agar pla tes. Virulence of the IMA deletion mutant, IMA 77, was compared with t he parent in an experimentally infected calf model of pneumonic pasteu rellosis. Results revealed significant reduction in virulence in the I MA mutant as measured by clinical and lung lesion scores. Notable diff erences in histological changes such as markedly reduced necrosis and lack of leukocyte degeneration occurred in calves infected with the IM A mutant in comparison with those infected with the parent wild-type s train. Thus, it appears that leukotoxin plays a important role in the pathogenesis of lung injury in bovine pneumonic pasteurellosis. (C) 19 98 Academic Press Limited.