Circadian rhythm of the resistance of mice to acute pneumococcal infection.

The resistance of both male and female mice to an acute infection by type I pneumococci varied rhythmically in an approximately 24-hr cycle. Mice inoculated intraperitoneally with 10(5.4+/-0.1) organisms during the dark period of the daily cycle survived significantly longer than mice similarly inoculated during the light period. The variation in resistance was correlated with the rate of increase of bacteremia, the rate being greatest after inoculation during the light period. Neither the capacity of the blood to support growth of pneumococci in vitro nor clearance of pneumococci from the blood in vivo varied between phases of the daily cycle. Adrenalectomized mice were equally and highly susceptible to infection during both light and dark periods. Stress of mice during the light period, normally a time of relative inactivity of mice, disturbed the rhythm; resistance to infection initiated during the light period increased and equalled that during the dark period.