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SUMMARY: The movement of the enormously enlarged filaments of Proleus vulgaris grown on penicillin agar ceased, or was slowed down, by screening the culture from the radiant heat of a microscope lamp. Organisms stimulated by heat after resting a short time in this way moved more rapidly than before. The increased activity was maintained for longer periods with longer rests, the increment in the period of activity increasing gradually as the resting period increased. In the condition of the test, rests of 60-120 sec. produced a maximal response. Repeated heat stimulation rapidly exhausts the capacity of the organisms to maintain a steady rate of movement. Under constant stimulation organisms maintain a fairly constant rate for some hours and then slow considerably as though they or their flagella were exhausted. A small decrease in radiant heat energy can induce a .revers81 in the direction of movement. Active flagella attached to immobile organisms react to heat stimuli in the same way. The effect of penicillin on the morphology of Proteus vulgaris has often been described. With concentrations between 3 and 10 units (u.)/ml. the filamentous forms of the microbe become very long and many of them become very distorted. In a previous article I and my co-workers (Fleming, Voureka, Kramer & Hughes, 1950) made observations on the motility of Pr. vulgaris grown on penicillin agar and examined by phase-contrast microscopy. We showed that when radiant heat from the microscope lamp was cut out by the insertion of a heat filter, the motility of the greatly enlarged filaments of Pr. vdguris ceased or slowed down, and that the time during which an organism could maintain its maximum activity depended on the period of rest which preceded such movement. These findings applied also to flagella belonging to filaments which were themselves motionless. The present paper records an extension of these observations. Technique. The organisms were grown in slide cultures as previously described. Cultures on agar containing 5 u./ml. of penicillin were incubated at room tem- perature. I was especially interested in the long filaments which were coiled like a watch-spring and whose only movement was a simple rotation. These could be watched for hours in the same field of the microscope. A sure way of obtaining these forms was to plant Pr. vulgaris in a thick streak across the cover-slip, and after this had dried to put two or three drops of penicillin agar at 50" on the cover-slip and immediately invert it on a slide. Along the edges of the streak there were always many 'watch-springs' to be found. The heat filter was that supplied by Kodak for their 'Retina' lantern; it absorbed only about 10 yo of the light but about 80 yo of the radiant heat. GMIV~ 30

Further Observations on the Motility of Proteus vulgaris Grown on Penicillin Agar