138 THE BACTERIOPHAGE AND ITS BEHAVIOR 



the 12 filtrates. Place them in the incubator at 30°C.* After incubation 

 for 24 hours it will be found, as a usual thing, that some of the suspen- 

 sions are limpid, indicating thus the presence of a very active bacterio- 

 phage. Other tubes are but slightly less clouded than the control sus- 

 pension without added filtrate. Others are as turbid as the control, 

 sometimes even more so.f Filter these cloudy, or turbid, suspensions 

 through candles. Add 1 cc. of each of the filtrates to a tube containing 

 10 cc. of a suspension (250 million per cubic centimeter) of Shiga bacilli, 

 and immediately spread 0.05 cc. of the mixture upon a plate or an agar 

 slant. After incubation, some of these agar cultures will appear sterile; 

 others will show confluent or isolated plaques. In some tubes the 

 plaques will be large; in others small, even pin-point in size. 



This experiment shows that different bacteriophage races, although 

 active for a single strain of bacteria, present a whole range of potencies. 

 Some of the races cause a prompt and complete dissolution of heavy sus- 

 pensions; others can be detected only by the formation of minute plaques 

 upon the agar. Between these extremes are all intermediate degrees of 

 activity. Indeed, it is quite possible that there are still weaker races 

 which escape detection because of an insufficiently dehcate technic. 



But, it may be said, we know that the bacteriophage principle is 

 formed of corpuscles. May it not be that the differences in activity 

 as manifested by different filtrates are due, not to a qualitative differ- 

 ence among the corpuscles, but rather to a difference in the number of 

 corpuscles present within a given volume of the different filtrates? Is 

 it not possible that the very active filtrates contain a large number of 

 corpuscles, while those which are weak contain but few? 



Two observations already recorded suffice to show that there is indeed 

 a qualitative difference among the corpuscles. We have seen that as a 

 matter of fact bacteriophagy may be complete in some instances if but 

 a single very active corpuscle is introduced into the bacterial suspension. 

 And yet in other cases, in the suspensions which remain turbid, there 

 may be a great many corpuscles, as shown by the fact that a single drop 



* At first^i^ I stated that the temperature should be 37°C. More recently 

 Hauduroy has suggested that it is preferable to allow the tubes to remain at room 

 temperature. Taking into consideration the results obtained in all of the experi- 

 ments performed it would seem that with weak races of bacteriophage the results 

 are best when the temperature is held at 30°C. 



t In general, it appears that the feces of animals contain a more active bac- 

 teriophage in summer than in winter, and that the fecal bacteriophage is more 

 active in hot countries than in cold regions. We will return to this subject in 

 Part III. 



