The course of infection after the intracerebral inoculation of high-virulence and low-virulence strains of into mice was followed by making serial viable counts of the number of bacteria in the brain.

With all strains, some 90 per cent. of the inoculum was lost within 1 hr, but by 24 hr the number of bacteria in the brain reached or exceeded the number inoculated.

When small doses (. 5 X 10) of high-virulence strains were given, the bacterial count increased continuously until death. With one strain (no. G353) there was a pause in multiplication when the viable count reached 10, but this was not seen with 2 other high-virulence strains (no. 18/323 and 2-Atox).

Challenge doses of low-virulence strains smaller than 10 organisms gave rise to a transient infection, the maximum count was reached on the 1st or 2nd day, and most brains examined after 4 days contained less than 50 bacteria.

Resistance of the mouse brain to low-virulence strains was not due to the action of mouse complement or to the occurrence of modulation. The intracerebral injection of a small dose of a low-virulence strain made the mouse brain resistant to reinfection 4 1/2 days later by a low-virulence but not by a high-virulence strain.

When the interval between injections was reduced to 2 days, however, mice infected with a low-virulence strain were somewhat resistant to reinfection with a high-virulence strain.


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