Studies on the Stabilily oj the Na^ R( quirement 485 



about the same Na"^ level as the imadapted culture would grow 

 after long incubation in the chemically defined medium, it was 

 of interest to know how the Na"^ requirement of the adapted and 

 unadapted cultures compared. The results, Table 2, show that 

 the adapted culture, like the unadapted organism, still required 

 Na"^ when tested in the synthetic medium, though the adapted 

 one did grow a little more quickly and at a slightly lower Na* con- 

 centration than the unadapted parent. 



We also found that the organisms which eventually grew 

 after long incubation in the chemically defined medium at a 

 level of 0.03M Na' also grew without training when transfeiTcd 

 to the trypticase medium prepared without added Na"^. 



When the synthetic medium used in this study was solidi- 

 fied with 1.5 per cent agar, the chief source of Na^ contamination 

 was the agar added. Such a medium contained about 0.007M 

 Na". The plating of heavy suspensions of the marine bacteria (2 

 to 4 X 10^ organisms) on this medium failed to produce colonies 

 unless Na* was added. Efforts were therefore made to induce 

 mutation to growth on this medium by ultraviolet irradiation. 

 The results of one such experiment are recorded in Table 3. Ir- 

 radiated suspensions of organism B-16 plated both immediately 

 and after a period of incubation under conditions designed ta 

 produce a limited number of cell divisions, when plated on the 

 chemically-defined medium without added Na^ gave rise to a very 

 limited number of colonies after three weeks of incubation. 



Because we were endeavouring to produce organisms which 



TABLE 3 



Induction of Mutants of Marine Bacterium B-16 by Ultra-violet 



Irradiation 



