28 F. M. BURNET and DORA LUSH 



is chiefly a widening of the ring of lysis. Phage C made its appearance after the 

 phage had been under study for some months, in the form of certain plaques iden- 

 tical with the normal ones except for the absence of the central growth of bacteria 

 (Fig. I). The plaques when isolated bred true, and phage C has maintained this 

 characteristic since. 



At the time of its appearance, no large plaque staphylococcal phage other than 

 C had been isolated in the laboratory, so that the possibility of contamination can 

 be ruled out. The two phages are serologically identical (Burnet and Lush 19-35 ) 

 and the further relations to be discussed below will make it clear that there is a 

 true genetic relation between them : C must be regarded as a mutant of C. 



The Type of Resistant Cultures Produced by the Two Phages. 



There is no difficulty in obtaining resistant cultures after the action of these 

 two phages, if two plates are spread with a young culture of SF, and when this 

 has dried a few drops of C and C spread on respective plates, after incubation the 

 first will show a thin line of lysis marking the edge of the area on which the phage 

 was spread (see Fig. 2), but over the rest of the area there is a confluent sheet of 

 apparently normal culture. This central growth, however, is completely resistant 

 to phage C and retains its character on subculture : it will be referred to as SF/C. 

 The plate spread with C shows the usual appearance obtained after lysis with a 

 moderately strong phage. There is an area of complete clearing on which are 

 scattered a few well-developed resistant colonies and an occasional semilysed 

 "nibbled" colony (Fig. 3). The intact colonies breed true and retain their resist- 

 ance : they will be referred to as SF/C. 



The properties of SF, SF/C, and SF/C may now be compared briefly. All 

 show a similar type of growth on broth or agar, and the colonies are indistinguish- 

 able from one another by inspection. The effect of various phages lysing SF on the 

 two variants is shown in Table 1, which shows both the lytic effect on agar and the 

 absorptive power of killed bacteria for the phages. 



Table 1. 



Characteristics of Initial and Phayc-resistant Cultures. 



Absorption of phages C and C 

 by dead cultures. 



Lytic action of phages shown Phage C. Phage C 



Culture. ABC C D Au I 2 20 200 2 20 200 



SF + + + + + + 100% 60% 80%, 40% 



SF/C + + -- + + 000 00 



SF/C + --- + + 000 000 

 + = active lysis on agar, — = no lysis. 



Absorption of phages by bacteria was tested according to the technique pre- 

 viously described (Burnet and Lush 1935). serial dilutions of heat -killed broth 

 cultures being added to small constant amounts of phage, and the mixtures plated 



