Fusobacterium necrophorum, a Gram-negative anaerobe, is the primary etiologic agent of
liver abscesses of beef cattle. The bacterium, a member of the microbial community of the rumen, travels to the liver via portal circulation to cause
abscesses. The severity of
liver abscesses vary from mild with one or two small
abscesses to severe with medium to large multiple
abscesses.
Leukotoxin, a secreted
protein, is the critical
virulence factor involved in the
infection. Our objective was to compare
leukotoxin production between strains of F. necrophorum isolated from mild and severe
liver abscesses collected from slaughtered cattle. The quantification of
leukotoxin was based on assays to measure cytotoxicity and
protein antigen concentration. One-hundred strains, 50 from mild and 50 from severe
abscesses, were utilized in the study. Cell-free supernatants were prepared from cultures grown in anaerobic broth at 9 and 24 h incubations. The leukotoxic activity was quantified by measuring cytotoxicity based on the release of lactic
dehydrogenase from bovine lymphocyte cells, BL3, treated with the culture supernatant.
Leukotoxin protein concentration was quantified by a sandwich ELISA assay with a
leukotoxin-specific
monoclonal antibody as the capture antibody. The
leukotoxin activity and concentration were highly variable among the strains within each severity of
liver abscesses. Although the leukotoxic activity was unaffected by incubation time,
leukotoxin protein concentration was consistently higher at 24 h compared to 9 h incubation. Strains from severe
liver abscesses had significantly higher leukotoxic activity and higher
protein concentration compared to strains from mild
liver abscesses (P < 0.0001) at both 9 and 24 h culture supernatants. Across all strains, the correlation coefficients between leukotoxic activity and
leukotoxin concentration at 9 and 24 h were 0.14 (P = 0.17) and 0.47 (P < 0.0001), respectively. In conclusion, strains isolated from severe
liver abscesses had significantly higher leukotoxic activities and
leukotoxin protein concentrations compared to strains isolated from mild
liver abscesses.