22 I. INTRODUCTION TO VIRUSES 



and of dysentery bacillus as well as the strain of coli, which was very 

 sensitive to the phage as above mentioned, from which it had been pro- 

 duced, while the activity of the phage sample proved to vary strikingly 

 among these strains of bacteria, and the ratio of the number of plaques 

 produced by a given quantity of the sample was found to be as follows : 



1 : 0.4 : 0.001 = coli : typhoid : dysentery. 



Therefore, if the typhoid or the dysentery bacillus strain was 

 used instead of the coli strain, the minimal quantity required for pro- 

 ducing a plaque would be estimated as 10/4 X 6.8 X 10"'^ g. with the ty- 

 phoid, and 1,000 X 6.8 X 10-^^ g. with the dysentery bacillus. Hence, it 

 would be said that the phage sample which could reveal the high acti- 

 vity on the coli strain was able to act on the dysentery strain merely 

 as a very weak phage, only one particle out of 1,000 being capable of 

 acting as a phage (44). 



Plaques would never show uniform properties, but differ strikingly 

 in size and shape even when the plaques were produced on an agar 

 plate with the same phage sample and with the same strain of bacteria, 

 showing that each phage particle had each individuality. The above 

 mentioned difference in the plaque number due to the difference in the 

 host bacteria may be attributed to this individuality, but it appears 

 possible that more than one particle may be required for the produc- 

 tion of one plaque if phage is not strong enough or host cells are not 

 sensitive enough to the phage so as a single particle can infect a bac- 

 terial cell. 



In addition to the function as a virus, some phage particles exhibit 

 a faculty of depriving the bacteria of their viability. In a study on 

 this faculty, we have found that many particles are necessary, in order 

 to deprive the viability, to combine with a single bacterial cell when 

 the cell is rather resistant, although a single paricle seems sufficient for 

 a sensitive cell (45) (46). It has been reported that also many particles 

 are needed to infect a single cell if the phage is inactivated to a proper 

 degree by ultraviolet irradiation, wherein the higher the irradiation the 

 larger number of particles are required (47) (48). Moreover, the phage 

 particles treated by ultraviolet irradiation to be reduced inactive, is 

 found to hasten the lysis of the heavily irradiated E. coli, which are 

 already inclined to autolysis (49). Similar evidence has been presented 

 also with influenza virus ; the virus particles irradiated by ultraviolet 

 light or heated to 50°C can act as the virus, if great many particles 

 affect a small number of host cells (50). Further, it has been claimed 

 that the infectivity of mouse-pox virus is enhanced remarkably when 

 it is added with a vast number of the virus particles inactivated by 

 ultraviolet irradiation (51). 



In view of these facts it may safely be concluded that either when 



