Bacterial composition and growth phase.

The events that follow the exponential phase of bacterial growth (Streptecoccus faecalis 9790) in a well buffered medium are determined by the nature of the growth-limiting nutrient (l-4). Termination of the exponential phase by the depletion of a “growth essential” ammo acid may be followed by either lysis or a phase of decreasing growth rate. The nature of this postexponential response seems to depend on the functions of the depleted amino acid. When the latter is an essential cell wall component (e.g. lysine or n-alanine) lysis tends to follow depletion, whereas in other cases there is substantial further growth which is characterized by the formation of additional cell wall substance. Thus, depletion of valme is followed by a 40 to 50% increase in the dry weight of the culture during the next 15 hours. In the case of threonine depletion, the postexponential gain in dry weight can exceed lOO%, and requires more than 40 hours for completion. Each of the other “cytoplasm-essential,” but not “wall-essential,” aminoacids gives rise to its own distinctive postdepletion\pattern, generally intermediate between those of valine and threonine, but in a few cases intermediate between the pattern of valine and that of lysine (which is characterized by lysis) . As one approach to the understanding of these individual differences, it was desirable to obtain a picture of cellular composition more complete than that furnished by the analytical studies which had revealed the prominent role of wall synthesis in the postexponential process (2, 4). The present report deals with some aspects of the composition of cells in the exponential growth phase (“log cells”) and of cells representing the stage of nearly maximum postexponential growth after the depletion from the medium of threonine (“threonine cells”). The latter were chosen because the postdepletion change is greatest in the case of threonine depletion (4).