Some Biochemical Differences 



427 



amined populations of marine and fresh water strains from the 

 point of view of the numbers of variants which were present 

 enabhng the strains to grow in media of unfamiHar sahnities. 

 Their findings are represented schematically in Figure 1. The 

 marine types showed three classes of beliaviour: (1) the whole 

 population tolerated from zero to about 4% NaCl (these popula- 

 tions had small numbers of variants tolerating very high salt 

 concentration); (2) an intermediate type in which only a small 

 proportion of the population (c 0.1%) tolerated no NaCl; (3) 

 an exigent type in which no variants present were able to grow 

 without NaCl. The exigent type was not successfully "trained" 

 to a fresh water habit; naturally, the others "adapted" at once. 

 The fresh water type was readily trained to behave like the 

 tolerant marine type; by repeated subculture of the tolerant ma- 

 rine type in 10% NaCl an exceptionally tolerant strain was ob- 

 tained in which 100 per cent of the viable population grew within 

 a salinity range of 0.25 to 11% NaCl. The exigent marine type 

 differed from the others in that the chloride ion concentration, 

 rather than the sodium ion concentration, conditioned the via- 



lOr 



TOLERANT 



INTERMEDIATE 

 EXIGENT S.W. 

 F.W. TYPE 



{s.w. 



TYPES 



4 6 8 10 12 14 



%NaCI IN COUNTING MEDIUM 



Fig. 1. Salt relations of Desulfovibrio desulfuricans. (After Littlewood & 



Postgate, (11)). 



