tailieunhanh - Salt effects in bacterial growth

 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 mechbanism of the salt action. Since our previous experiments 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 . All of the counts here reported represent the average of triplicate plates on extract-pepton agar incubated for three days at 33°C. Further details of the experiments are given in the appendix. | SALT EFFECTS IN BACTERIAL GROWTH III. SALT EFFECTS IN RELATION TO THE LAG PERIOD AND VELOCITY OF GROWTH1 J. M. SHERMAN G. E. HOLM AND w. R. ALBƯS From the Research Laboratories of the Dairy Division United States Department of Agriculture Washington D. c. Received for publication February 18 1922 In a previous paper of this series Holm and Sherman 1921 it has been shown that certain neutral salts in proper concentrations accelerate the growth of Bact. coli as measured by the time requừed 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 mechanism of the salt action. Since our previous experiments 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 . All of the counts here reported represent the average of triplicate 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 experiments 1 and 2 in which the growth of Bad. coli in 1 per cent pepton 1 per cent pepton plus M NaCl and 1 per cent pepton plus M NazSCh 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 1 Published with the permission of the Secretary of Agriculture. 583 SALT EFFECTS IN BACTERIAL GROWTH 585 culture at the breaking point of the growth curve is approximately the same in all cases. It also appears that the salts have the effect of shortening the latent period previous to rapid growth. This difference is marked in the case of the