Generation Time

by Joachim Gruber

The generation time of a single cell is the duration TG of a cell cycle, measured as the time between two consecutive mitoses.

In a bacteria population the single-cell generation time varies during the development of the population. Each phase of this evolution has its own single-cell generation time TG or its specific generation time distribution TG, min ... TG, max. When giving the single-cell generation time one has to specify the phase of the development in which one has measured it.

One distinguishes the following phases of the development (Stephen T. Abedon, Important words and concepts (3/10/01) from Chapter 6, Black, J. G. (1999). Microbiology. Principles and Explorations. 4th ed., Prentice Hall, Ohio State University, Mansfield, OH 44903, USA):

  1. lag phase A, no cell diviaion. This lag in division is associated with a physiological adaptation to the new environment, by the cells, prior to their resumption of division. That is, cells may increase in size during this time, but simply do not undergo binary fission.
  2. log phase B during which binary fission occurs. This phase of growth is called logarithmic or exponential because the rate of increase in cell number is a multiplicative function of cell number. The generation time of the population (sensu stricto) is the time it takes for the population in phase B to double in number of bacteria.
  3. Continuous culture phase C.
  4. Decline (die-off, kill) phase.  Typically this die-off occurs exponentially in time. This death occurs because vegetative cells can survive exposure to harsh conditions (few nutrients, too many toxins, antibiotics) for only so long.
Microbiological literature reports the kill kinetics of Borrelia burgdorferi. The borrelia population decreases in these experiments exponentially with a half life  TS = 10 ... 12 hours dependent amog others from the antibiotic, its time of action, Bb species and Bb genotype. (Abb. 1, Heinemann M et al.). As is explained in parentheses that follow are of similar size.

(Here is an explanation for it:

In principle, the population can be killed with the same rate as it is cell dividing (1.), with a smaller rate (2.) or a larger rate (3.):

  1. If the cell wall antibiotic
  2. TS = TG.
  3. If the cell wall antibiotic does not kill each cell that divides, some  cells survive and thus the killing (in the presence of the antibiotc) is slower than the cell division in the absence of the antibiotic. In other words: TS is the upper limit of the Bb generation time TG (i.e. TS > TG). The Bb generation time is then smaller than the above mentioned 10 ... 12 hours. This is consistent with the culture experiments by e.g. Pollack et al.
  4. Pollack RJ, Telford SR 3rd und Spielman A, Standardization of medium for culturing Lyme disease spirochetes, J Clin Microbiol 1993,May;31(5):1251-5.
  5. If the cell wall antibiotic kills the cell that is dividing plus other cells, killing (in the presence of the antibiotic) is faster than cell division (in the absence of the antibiotic):

  6. TS < TG.
    Interestingly, it has been observed that in the presence of the antibiotic up to twice as many cells die per unit time as would divide in the absence of the antibiotic. In other words: 1/2 TG < TS < TG.
On the whole,  TS and TG have roughly the same size.)

Some related basics in pharmakodynamics.

version: March 1, 2004
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