94 J. T. HOLDEN 
mesenteroides, on the other hand, the pool increased in size throughout the growth phase 
and was maintained at a high level for at least 15 h beyond the period of active growth 
after which it declined in size and variety. S. faecalis maintained a relatively constant 
pool throughout the period of active growth and while there were some small losses 
thereafter, a large and varied pool was retained by the cells even 3 days after active 
growth of the culture had ceased. In gram-negative bacteria equally inconstant obser- 
vations have been encountered. MuruyAMA!™ observed a large increase with age 
in the pool of a pseudomonad, while MANDELSTAM™® reported a marked and regular 
decline in E. colt. It is likely that the occurrence and retention of an amino acid in the 
eal 
ES ad 
Wa a 15 
_ ae, 
ek ied ean: 
Figs. 11-16. Free amino acid pool of Leuco. mesenteroides P-60, grown for varying times in a de- 
fined medium containing acid hydrolyzed casein. Incubation times were as follows: Fig. 11, 12 h; 
Fig. 12, 16h; Fig. 13, 22 h; Fig. 14, 39 h; Fig. 15, 68 h; Fig. 16, 100 h. Other details as in Figs. 5-10. 
pool is the result of a balance in the rates of a considerable number of reactions which 
influence its concentration and that the component reactions differ among organisms 
and for the same organism at different phases of growth, thus accounting for these 
divergent patterns of response. 
It should be noted that a number of investigations§’, 125, 159 have shown that cells 
from early exponential-phase cultures have less cell wall substance than cells harvested 
from stationary-phase cultures and, therefore, that they are more susceptible to 
damage by unfavorable physical and osmotic forces. This property was not widely 
appreciated at the time most of the studies cited here were performed. Thus, there is 
considerable possibility that some artifactual changes in pool composition mistakenly 
attributed to culture age have been described. Some of the changes, however, are 
unlikely to have originated in this way. It is of considerable interest that cells taken 
at different times from populations dividing exponentially in a medium containing 
References p. 105/108 
