β-Hemolysis May Not Be a Reliable Indicator of Leukotoxicity of Mannheimia haemolytica Isolates.

Mannheimia ( Pasteurella ) haemolytica causes bronchopneumonia in domestic and wild ruminants. Leukotoxin is the critical virulence factor of M. haemolytica . Since β-hemolysis is caused by a large number of leukotoxin-positive M. haemolytica isolates, all β-hemolytic M. haemolytica isolates are considered to be leukotoxic as well. However, conflicting reports exist in literature as to the leukotoxic and hemolytic properties of M. haemolytica . One group of researchers reported their leukotoxin-deletion mutants to be hemolytic while another reported their mutants to be non-hemolytic. The objective of this study was to determine whether β-hemolysis is a reliable indicator of leukotoxicity of M. haemolytica isolates. Ninety-five isolates of M. haemolytica were first confirmed for presence of leukotoxin gene ( lkt A) by a leukotoxin-specific PCR assay. Culture supernatant fluids from these isolates were then tested for presence of leukotoxin protein by an ELISA, and for leukotoxic activity by a cytotoxicity assay. All isolates were tested for β-hemolysis by culture on blood agar plates. Sixty-two isolates (65%) produced leukotoxin protein while 33 isolates (35%) did not. Surprisingly, 18 of the 33 isolates (55%), that did not produce leukotoxin protein, were hemolytic. Of the 62 isolates that produced leukotoxin, 55 (89%) were leukotoxic while 7 (11%) were not. All except one of the 55 leukotoxic isolates (98%) were also hemolytic. All seven isolates that were not leukotoxic were hemolytic. Taken together, these results suggest that β-hemolysis may not be a reliable indicator of leukotoxicity of M. haemolytica isolates. Furthermore, all M. haemolytica isolates that possess lkt A gene may not secrete active leukotoxin.