Two mutants of Pasteurella haemolytica A1 that do not produce leukotox
in were isolated. Following mutagenesis, colonies were screened with a
ntiserum by a filter assay for absence of the secreted leukotoxin. The
two mutants both appeared to produce normal amounts of other antigens
, as judged by reactivity with polyclonal serum from an animal with pa
steurellosis, and were not altered in beta-hemolytic activity as seen
on blood agar plates. There was no evidence of either cell-associated
or secreted leukotoxin protein when Western blots (immunoblots) were c
arried out with the polyclonal serum or with a monoclonal antibody dir
ected against the leukotoxin. Southern blots revealed that both mutant
s show the wild-type restriction pattern at the leukotoxin locus, alth
ough the strain with the lkt42 mutation showed differences in other re
gions of the chromosome on analysis by pulsed-field gel electrophoresi
s. The strain with the lktA2 mutation grew more slowly than did the wi
ld-type strain, while the strain with the lktA1 mutation was indisting
uishable from the wild-type strain in its growth properties. The strai
n with the lktA1 mutation should be valuable in determining the role o
f the leukotoxin in virulence as well as in identifying other virulenc
e factors of P. haemolytica.