Necrobacillosis and immunity in mice.

The study arose from the recent finding that sub-lethal numbers of certain bacterial species greatly enhanced the infectivity of Fusobacterium necrophorum. A severe F. necrophorum infection in mice, cured with metronidazole, produced significant though slight resistance, which was demonstrable by challenge with a minute dose of F. necrophorum (less than 20 organisms) suspended in a sub-lethal dose of Escherichia coli (300 x 10(6) organisms) to enhance fusobacterial infectivity. In an earlier comparable experiment, challenge with F. necrophorum alone, in necessarily large doses (greater than or equal to 3 x 10(6) organisms), failed to demonstrate that a single cured fusobacterial infection gave rise to resistance; such an infection neither protected against the fatal necrobacillosis produced by challenge nor prolonged survival. A sub-lethal E. coli infection was also shown by challenge with a minute dose of F. necrophorum (less than 10 organisms), suspended in a sub-lethal dose of E. coli (152 x 10(6) organisms), to produce significant though slight protection against necrobacillosis. The degrees of resistance demonstrated were too slight to give any encouragement to the prospect of an effective necrobacillosis vaccine.