Haemophilus somnus is a catalase-negative, gram-negative pathogen of cattle which is refractory to killing by bovine neutrophils. In this report, we showed that H. somnus rapidly inhibited Luminol-dependent chemiluminescence of bovine neutrophils costimulated with opsonized zymosan or phorbol myristate acetate. We have postulated that this inhibition resulted in part from H. somnus preventing the accumulation of hydrogen peroxide (H202) during the oxidative burst. In support of this hypothesis,

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... we have demonstrated that when stimulated with viable H. somnus, bovine neutrophils accumulate lower levels of H202 than did neutrophils stimulated with heat-killed H. somnus or opsonized zymosan. We have presented evidence that four separate strains of H. somnus, despite being catalase negative by conventional criteria, removed H202 from solution. Viable cells ofH. somnus were required for the removal of H202 from solution; little or no activity was observed when suspensions of heat-killed, formalin-killed, or sonicated cells of H. somnus were incubated with H202. In addition, the elimination of H202 occurred only in the presence of carbon sources that could be utilized by H. somnus, indicating that elimination of H202 was an energy-dependent process. The amount of H202 that could be eliminated by 107 cells of H. somnus was greater than 10 nmol, an amount comparable to that produced by a similar number of stimulated bovine neutrophils. Thus, we suggest that the ability of H. somnus to remove H202 from solution may be an important virulence mechanism that contributes to the survival of the organism following ingestion by bovine neutrophils.