Osmotic Regulation of the Growth Rate 631 



jor contribution to the differential growth observed after five 

 hours. 



Comparisons between solutes were made on an osmolal 

 basis. The basal medium was assumed to make a constant con- 

 tribution to the total osmolality and not to interact appreciably 

 with the added solutes; the osmolality of the medium was con- 

 sidered to be that produced by the added solutes ( 3 ) . The media 

 were prepared on a molal basis, i.e., moles of solute to be tested 

 per 1000 g of solvent. The calculations of osmolal concentrations 

 were made using the data given by Heilbrun (3) and by Harris 

 (2). 



RESULTS 



EflFect of NaCl on the Growth Rate 



The growth rate of Vibrio MB 22 was regulated by the 

 amount of NaCl added to the basal medium; the total growth 

 yield was independent of this concentration ( Fig. la ) . No growth 

 occurred in the basal medium without the addition of NaCl. That 

 this result was the reflection of the differential death of the in- 

 oculum cells resulting from osmotic shock was considered a pos- 

 sibility. However, when inoculum cells were resuspended for 30 

 minutes at 32 C in the basal salt solution (0.05 M MgCl2, 0.005 M 

 K.SO4) containing respectively 0.05 M, 0.1 M, and 0.14 M NaCl, 

 no differences among the viable counts in the suspensions were ob- 

 served; the effect of osmotic shock was probably minimal. The 

 maintenance of these cells in the hypotonic solutions was prob- 

 ably largely due to the Mg"^^ content of the basal medium (7, 9). 

 A second possibility was that the cells varied in their ability to 

 grow at lower salt concentrations; however, the viable count of 

 cells in the inoculum was equivalent in agar media containing 0.4 

 M, 0.1 M, or 0.05 M NaCl. The colonies growing in any particular 

 concentration were equal in size; the colonies developed at 

 least twenty-four hours earlier in the medium containing 0.4 M 

 NaCl than in that containing 0.05 M NaCl. No adaptation of the 

 cells to lower NaCl concentrations was observed; inocula grown 

 in 0.05 M NaCl responded to NaCl in a manner indistinguishable 

 from the usual inoculum ( Figure lb ) . From these observations it 



