Oscillating signals in the sensory pathway of halobacteria induced by periodic light stimuli

The swimming behaviour of Halobacterium salinarium can be modulated by light. Changes of the light intensity that induce reversals of the swimming direction are called repellent stimuli, those that suppress reversals, which otherwise would occur spontaneously from time to time, are called attractant stimuli. Bacteria were stimulated by periodic pulse-like stimuli, and the frequency of induced reversals was recorded. Stimulation with a period length between 16 and 6.5 s let the cells reverse periodically with the frequency of the external force. After the stimulation had been stopped, the cells continued to reverse periodically for 3 to 9 periods which, however, switched to a value of about 6 to 8 s, independent of the frequency of preceding stimulation. This endogeneous oscillation was most distinct when the stimulation period either equalled the endogeneous period or was twice or half of its length. During the endogeneous oscillation, the responsiveness to an attractant stimulus showed a pronounced phase-dependence. These results point to the oscillation of a signal in the sensory pathway which, different from our former assumption, seems to be not self-sustained but has to be set going by external stimulation. Received: 14 January 1998 / Revised version: 9 April 1998 / Accepted: 17 May 1998