Growth in volume of Euglena gracilis during the division cycle

Single-cell behavior and population heterogeneity: solving an inverse problem to compute the intrinsic physiological state functions

The dynamics of isogenic cell populations can be described by cell population balance models that account for phenotypic heterogeneity. To utilize the predictive power of these models, however, we must know the rates of single-cell reaction and division and the bivariate partition probability density function. These three intrinsic physiological state (IPS) functions can be obtained by solving an inverse problem that requires knowledge of the phenotypic distributions for the overall cell population, the dividing cell subpopulation and the newborn cell subpopulation. We present here a robust computational procedure that can accurately estimate the IPS functions for heterogeneous cell populations. A detailed parametric analysis shows how the accuracy of the inverse solution is affected by discretization parameters, the type of non-parametric estimators used, the qualitative characteristics of phenotypic distributions and the unknown partitioning probability density function. The effect of finite sampling and measurement errors on the accuracy of the recovered IPS functions is also assessed. Finally, we apply the procedure to estimate the IPS functions of an E. coli population carrying an IPTG-inducible genetic toggle network. This study completes the development of an integrated experimental and computational framework that can become a powerful tool for quantifying single-cell behavior using measurements from heterogeneous cell populations.

Synchronization of division in vitamin B12-starved Euglena gracilis

Vitamin B12 deficiency arrests cell division in Euglena gracilis. B12 starvation for short periods made it possible to induce synchronous growth by addition of the vitamin. Culture conditions were established to optimize replenishment synchrony. The DNA content of E. gracilis in steady state culture and vitamin B12 deficiency culture was measured by flow cytofluorometry and was consistent with colorimetric determinations. The cell volume and DNA distributions of E. gracilis in synchronous culture were analyzed and the sequential changes during the division cycle were computed. Synchronous culture permits more definitive studies of shifts in cell volume and DNA distributions, in which the biochemical events required for cell division are presumably synchronized.

Sequential changes in cell volume distribution during vitamin B12 starvation of Euglena gracilis

During vitamin B12 starvation of Euglena, a new peak appears in the cell volume distribution. Some cells are inhibited at a unique point in the cell cycle between the initiation of DNA synthesis and nuclear division. The mechanism of inhibition of other cells differs.

Growth and cell volume of Euglena gracilis in different media

It is necessary to propagate Euglena gracilis cells for several days after transfer from one medium to another to establish the steady state of balanced growth. Steady-state growth was established in minimal and in complex medium. Specific growth rates and cell volume distributions were computed for each culture medium. Mean cell volume of E. gracilis is not uniquely correlated with the specific growth rate.

An unusual mutant affecting the frequency of organelle mutation

A mutant of Euglena gracilis Z is described that spontaneously produces irreversibly bleached mutants at a high frequency. Bleaching only occurs during exponential growth under highly aerobic conditions. The kinetics for bleaching and growth of bleached and nonbleached cells suggest that mutation is induced by segregation of a cellular component that becomes limiting after several culture generations. The properties of spontaneous bleaching closely resemble the properties of induced organelle mutagenesis in E. gracilis and yeast.

Division cycle of Myxococcus xanthus. 3. Kinetics of cell growth and protein synthesis

The kinetics of cell growth and protein synthesis during the division cycle of Myxococcus xanthus was determined. The distribution of cell size for both septated and nonseptated bacteria was obtained by direct measurement of the lengths of 8,000 cells. The Collins-Richmond equation was modified to consider bacterial growth in two phases: growth and division. From the derived equation, the growth rate of individual cells was computed as a function of size. Nondividing cells (growth phase) comprised 91% of the population and took up 87% of the time of the division cycle. The absolute and specific growth rates of nondividing cells were observed to increase continually throughout the growth phase; the growth rate of dividing cells could not be determined accurately by this technique because of changes in the geometry of cells between the time of septation and physical separation. The rate of protein synthesis during the division cycle was measured by pulselabeling an exponential-phase culture with radio-active valine or arginine and then preparing the cells for quantitative autoradiography. By measuring the size of individual cells as well as the number of grains, the rate of protein synthesis as a function of cell size was obtained. Nondividing cells showed an increase in both the absolute and specific rates of protein synthesis throughout the growth phase; the specific rate of protein synthesis for dividing cells was low when compared to growthphase cells. Cell growth and protein synthesis are compared to the previously reported kinetics of deoxyribonucleic acid and ribonucleic acid synthesis during the division cycle.