Pasteurella Pseudotuberculosis 'Swarming Colonies'

at room temperature (70? to 80?F.) it grows in the form of spreading colonies similar to the ' swarming' Proteus colonies. When incubated at 37?C., the growth is limited to the area of the inoculum. At room temperature the growth continues to spread for three days. If a plate that has been incubated at room temperature and shows a spreading form of growth is subsequently incubated at 37?C., the further ' swarming' of the colony stops. A colony grown at 37 ?C., and restricted mainly to the area of inoculation, when incubated at room temperature, spreads over the surface of the medium. Smears made from the margin of a swarming colony show actively motile bacilli, but smears from the centre of the colony show mainly non-motile forms. Smears from the growth incubated at 37?C. show no motility. When stained for flagella by the Plimmer and Paine (1921) method, the growth from the edge of the colony incubated at room temperature shows the majority of the organisms with two polar flagella, whereas the growth from the centre of such a colony or from a growth at 37?C. shows