CHARACTERIZATION OF A PASTEURELLA-HAEMOLYTICA A1 MUTANT DEFICIENT IN PRODUCTION OF 3 MEMBRANE LIPOPROTEINS

Objective-To determine whether a Pasteurella haemolytica A1 mutant tha t is unable to produce membrane lipoproteins has reduced susceptibilit y to complement-mediated killing, and to characterize the mutant strai n. Sample Population-12 sera from cattle resistant to P haemolytica ch allenge exposure after vaccination with P haemolytica or its antigens, or after natural exposure. Procedures-Complement-mediated killing ass ays were performed, using wild-type and mutant strains and, as antibod y source, various immune sera from cattle that were resistant to P hae molytica challenge exposure. Antibody response to whole-cell antigens produced by mutant and wild-type strains, production of outer membrane proteins and iron-regulated outer membrane proteins by the 2 strains, and growth of the 2 strains in various media were analyzed. Results-C ompared with wild-type P haemolytica, the lipoprotein mutant strain ha d increased susceptibility to bovine complement-mediated killing. Asid e from the lipoproteins that are not produced by the mutant, immunoblo t analysis did not reveal differences between immunoreactive antigens that are produced by the 2 strains. Some iron-regulated, outer membran e proteins, which usually are only produced by P haemolytica under iro n-deficient conditions, were produced constitutively by the mutant. Th e mutant grew to a lower final cell density and at a lower rate under conditions likely to reflect those encountered in vivo.Conclusions-Lac k of 3 membrane lipoproteins resulted in enhanced susceptibility to bo vine complement-mediated killing. Site-specific mutagenesis of genes e ncoding P haemolytica membrane lipoproteins alters production of iron- regulated outer membrane proteins by P haemolytica. Growth characteris tics of the mutant suggested that it may have reduced capacity for sur vival in vivo.