SALT EFFECTS IN BACTERIAL GROWTH 



III. SALT EFFECTS IN RELATION TO THE LAG PERIOD AND 

 VELOCITY OF GROWTH' 



J. M. SHERMAN, G. E. HOLM and W. R. ALBUS 



From the Research Lahoralories of the Dairy Division, United States Department of 

 Agriculture, Washington, D. C. 



Received for publication February IS, 1922 



In a previous paper of this series (Holm and Sherman, 1921), 

 it has been shown that certain neutral salts, in proper concen- 

 trations, accelerate the growth of Bad. coli, as measured by the 

 time required to produce visible turbidity, the time required to 

 reduce methylene blue, and the rate of acid production in the 

 presence of a fermentable carbohydrate. 



In the present work we have extended these experiments by 

 the use of the plate count in an effort to throw more light on 

 the mechainism of the salt action. Since our previous experi- 

 ments have shown that the salt effect is magnified on the acid 

 side of the region of optimum growth (Sherman and Holm, 1922), 

 we have used in the present work media adjusted to a reaction 

 of pH 5.4. All of the counts here reported represent the average 

 of tripUcate plates on extract-pepton agar incubated for three 

 days at 33°C. Further details of the experiments are given in 

 the appendix. 



From figures 1 and 2, plotted from the data obtained in ex- 

 periments 1 and 2 in which the growth of Bad. coli in 1 per cent 

 pepton, 1 per cent pepton plus 0.2 M NaCl, and 1 per cent pepton 

 plus 0.1 j\I Na2S04 was measured, it is seen that the accelerating 

 action of the salts is due to an increase in the velocity of growth 

 of the organisms. In other words, the period of logarithmic 

 growth is shortened since the number of bacteria present in the 



' P^iblished with the permission of the Secretar>' of Agriculture. 



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