488 Marine Microbiology 



responded to added Na^ in a Na"^ deficient medium, it was of in- 

 terest to know if this residual requirement for Na"^ for maximum 

 rate and extent of growth could be eliminated by training. Three 

 cultures were selected for study. They were representative of the 

 tliree types of mutants ol:)tained - those which did not reduce 

 nitrate, those which chd slightly and those which did strongly. 

 These were each transferred serially into flasks of chemically 

 defined medium containing no added Na"^. The inoculum at each 

 transfer was also used to inoculate a series of flasks of the same 

 medium containing increasing concentrations of Na"^ to keep a 

 check on the response of the organism to Na^ after each serial 

 transfer. The results, Table 6, show that by the eighth transfer 

 all three cultures grew to the same ultimate extent in the pres- 

 ence and in the absence of added Na+. Two of the isolates still 

 responded sHghtly to the addition of O.OIM Na"^ in the early stages 

 of growth but the third, C, did not. 



TABLE 6 



Results of Experiments Designed to Train Three Mutants to Grow as 

 Well in the Absence of Na+ as in its Presence 



* Evelyn colorimeter readings, 660 m/i filter. 



The responses of two organisms of terrestrial origin to Na"^, 

 Pseudomonas aeruginosa 9027 and a strain of E. coli, were also 

 determined in a chemically defined medium. Maximum growth 

 of these organisms occurred in the absence of Na' and no stimu- 

 lation of the rate of growth of the organisms was observed on 

 addition of Na^. 



