Evidence of phenotypic dichotomy within an individual Pasteurella multocida type strain and among some haemorrhagic septicaemia-causing field isolates

Haemorrhagic septicaemia-causing strains of Pasteurella multocida were identified by a disease-specific ELISA. Some strains, however, were of the same serotype as those which cause haemorrhagic septicaemia (HS) but were negative when tested in the disease specific ELISA. The suspect false negative isolates were passaged in mice and retested in the HS ELISA with the same result. Immunoelectron microscopy was used to examine further these suspect HS-causing strains. Monoclonal antibodies and protein A-gold showed that the suspect negative organisms were a mixture of phenotypes with less than 10 per cent, and usually less than 2 per cent, of the population expressing HS-associated epitopes. The degree of staining on the organisms expressing the HS-epitopes was of the same intensity as the positive control organism. Expression of the HS-associated epitopes is presumably too low to allow detection in the current HS ELISA.