Functional and metabolic interactions of bovine alveolar macrophages with Pasteurella haemolytica (biotype A, serotype 1) and possible bacterial extrachromosomal genetic factors responsible for virulence

Abstract

Pasteurella haemolytica is generally recognized as the most economically important infectious agent involved in the bovine respiratory disease complex (BRDC). Alveolar macrophages (AMO) are a primary mechanism of pulmonary host defense against bacterial sepsis at the level of the alveoli. If P haemolytica is to establish a nidus of infection in the bovine lung then the organism must overwhelm or circumvent the AMO host defense system. A luminol-dependent chemiluminescence (LDCL) assay was developed for the study of the interaction of pathogenic (P haemolytica biotype A, serotype 1) and nonpathogenic (Escherichia coli K-12) bacteria with bovine AMO (BAMO) in pulmonary lavage cells (PLC). Efficient LDCL was produced by PLC phagocytically stimulated with E coli. The LDCL profile was characterized by a rapid rise to a peak at 60 min followed by a gradual decline in the next 60 min and then relative stability above control values for the remaining 60 min of the incubation period. In contrast the LDCL profile induced by P haemolytica was characterized by an extremely rapid rise in LDCL in the early stages after phagocytic stimulation followed by a precipitous decline and subsequent total cessation of LDCL. Kinetics of the LDCL response to living and heat-killed P haemolytica in LDCL, ('51)chromium release, and macrophage bactericidal assays. Collectively, the results suggested that living P haemolytica cells possess a heat-labile cytotoxic factor that functionally incapacitates BAMO and thereby reduces the antimicrobial effectiveness of an important pulmonary host defense mechanism.Basic aspects of P haemolytica biology were studied in relation to plasmid and inducible bacteriophage content. A bacteriophage was induced from all isolates and kinetics of induction and bacteriophage morphology were determined. Isolates of P haemolytica contained either 0, 2, or 3 apparent plasmids. The presence of plasmids were related to antibiotic resistance to ampicillin and penicillin, but not related to cytotoxic activity. P haemolytica cytotoxic factor appears to be coded for on the bacterial chromosome.