Size fractionation of exponentially growing Escherichia coli

Antimicrobial action of N-(n-dodecyl)diethanolamine on Escherichia coli: effects on enzymes and growing cultures

AIMS

This study investigates the effects of N-(n-dodecyl)diethanolamine (DDA) on enzymes and growing cells of Escherichia coli NCIMB 8277.

METHODS AND RESULTS

Enzyme activities in the presence of DDA were determined by measuring substrate-dependent oxygen consumption by whole cells, or of NADH formation or oxidation by cell extracts. Lysis of growing cells was followed by measuring changes in turbidity and cell count. DDA promptly arrested oxygen uptake on pyruvate and acetate, due to cofactor loss rather than to enzyme denaturation, since cell-free glyceraldehyde-3-phosphate and NADH dehydrogenases remained active. Formate and succinate oxidation by membrane-bound enzyme systems independent of cofactors was likewise unaffected. DDA lysed growing cells at rates related to drug concentration, pH, and the previous growth rate.

CONCLUSIONS

Loss of cellular enzyme activity following addition of DDA is due to cofactor leakage and not to enzyme denaturation. Whereas nongrowing cells remain intact in the presence of DDA, actively-growing organisms undergo lysis, consistent with autolysin action.

SIGNIFICANCE AND IMPACT OF THE STUDY

Cell lysis, not normally observed with membrane-active antimicrobials, also occurs with cetrimide, and may be dependent on the alkyl chain length in these compounds. The action on growing cells parallels that of penicillin and daptomycin, which bears a decanoyl residue that penetrates the cell membrane, causing leakage and membrane depolarization.

Biomass growth rate during the prokaryote cell cycle

The rate of biomass growth throughout the cell cycle of prokaryotes is important in the study of global regulation. Two limiting cases have generally been considered: the exponential model and the linear model. The exponential model is a logical expectation because protein, the main component of biomass of a bacterial cell, increases continuously during the cell cycle and therefore the means for synthesis of other cell components and metabolites also increases. In addition, during the cell cycle, ribosomes, the means of production of proteins, increase monotonically. As a consequence, the increase of all should be autocatalytic and the content of cell substance should be an exponential function of time. Two cellular components would not be expected to increase exponentially: the DNA and the cell envelope. The former because of the intermittent synthesis of the chromosome, and the latter because of changes in the surface-to-volume ratio with growth and division. In contrast to the exponential model, the linear model of Kubitschek postulates that the cell only increases its membrane transport capability over a brief period during the cell cycle, and, thus limited by transport, all cell components can increase only at a constant linear rate during most of the cell cycle. Other proposed models are intermediate and assume that the growth rate of the cell depends on some cell cycle event, such as the initiation of chromosome replication. The models have relevance to prokaryotes undergoing balanced growth; they may not be relevant to eukaryotic microbes or to eukaryotic cells in tissue culture that have endogenous rhythms or are controlled by protein growth factors. Logically, the models could possibly apply to a free-living cell that does not respond to environmental cues. Even under rigidly constant conditions, however, cells may try to respond to a stimulus that was periodic or regulatory under natural conditions, but is present at a constant level under the experimental culture condition. There are four classes of experiments that have been used to measure the accumulation of dry biomass or its components during the cell cycle of a bacterium, as typified by Escherichia coli. For the first class of experiments, the dimensions of living cells are measured under the microscope. So far, the experiments have been limited by the resolving power of the phase microscope, but adequate resolution should be possible with the confocal scanning light microscope or various video computer systems. Such experiments are called integral because augmentation of cell constituents is followed. The second class involves pulse-chase labeling of cells and then their separation into different phases of the cycle or age groups and measurement of the radioactivity per cell in the fractions. Such experiments are called differential in that the rate is measured directly instead of being deduced by comparing the total size at different times.(ABSTRACT TRUNCATED AT 400 WORDS)

Temporal programming of chloroplast and cytoplasmic ribosomal RNA transcription in the synchronous cell cycle of Chlamydomonas reinhardtii

Approximately 90% of the Chlamydomonas reinhardtii chloroplast and cytoplasmic rRNAs was transcribed in the nuclear G1 phase, which occurred during the light period under an alternating light-dark synchronization regime of 12 h each. The remaining 10% of chloroplast and cytoplasmic rRNAs was transcribed from its respective DNAs in the dark period, in the midst of an apparent turnover of a transcription appeared to be prokaryotic in sophistication. The transcription was not interrupted during chloroplast DNA synthesis which occurred during the light period. However, transcription of the nuclear DNA was repressed severely during the nuclear S phase in the dark period. The patterns of incorporation of 32P into chloroplast and cytoplasmic rRNA species in the cell cycle were similar to those of the actual rRNA synthesis as measured optically. However, the quantity of 32P incorporation per unit amount of rRNA synthesized varied considerably during the cell cycle, increasing in all rRNA's during the dark period. 32P incorporation data obtained from continuous and pulse 32P-labeling experiments also revealed a turnover of a small amount of both cytoplasmic and chloroplast rRNAs at the end of the S phase. The 32P incorporation into cytoplasmic and chloroplast rRNAs was well matched temporally with the 32P incorporation into their corresponding ribosomes, indicating that the newly synthesized rRNA molecules are utilized without delay throughout the cell cycle in the assembly of ribosomes.

The effect of gene concentration and relative gene dosage on gene output in Escherichia coli

The differential rate of synthesis of several Escherichia coli gene products was measured under conditions in which the average number of copies of the corresponding chromosomal gene had been changed by altering the replication velocity of the chromosome. The data show that in steady state exponential cultures the output of genes in a fully repressed, fully derepressed, or non-repressible state is proportional to the average number of copies of the gene per unit mass (gene: mass ratio) and does not depend on the number of copies of the gene relative to all other genes (gene: DNA ratio). In contrast, the output of a gene which was under regulation by endogenously generated effectors was independent of such changes in gene frequency. The relationship found between the number of copies of a gene per unit of cell mass and enzyme output provides a new method for determining the location of the chromosome origin and the direction of replication in bacteria.

Use of yeast populations fractionated by zonal centrifugation to study the cell cycle

Zonal centrifugation in a sucrose density gradient was used to separate yeast cells primarily by size and thus by age in the cell cycle. This approach provides an alternative to synchronous growth for examining the properties of cells at different stages in the cell cycle.

Division cycle of Myxococcus xanthus. II. Kinetics of stable and unstable ribonucleic acid synthesis

The kinetics of stable and unstable ribonucleic acid (RNA) synthesis during the division cycle of Myxococcus xanthus growing in a defined medium was determined. Under these conditions, M. xanthus contains one chromosome which is replicated during 80% of the cell cycle. Stable RNA synthesis was measured by pulselabeling an exponential-phase culture with radioactive uridine 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 stable RNA synthesis as a function of cell size was determined. Unstable RNA synthesis during the division cycle was determined by correlating the data for stable RNA synthesis with the relative amounts of stable and unstable RNA labeled during the short pulse. The data reported here demonstrate that: (i) cells synthesize both stable and unstable RNA throughout the division cycle; (ii) the rate of stable RNA synthesis increases in two discrete steps, corresponding to average ages of 0.15 and 0.75 generations; (iii) the rate of unstable RNA synthesis exhibits an initial rise, followed by a relatively constant rate of synthesis, and finally, a burst of unstable RNA synthesis prior to septum formation. The half-life of unstable RNA of M. xanthus, generation time of 390 min at 30 C, was 4 min. Comparison of the rates of stable and unstable RNA synthesis indicates noncoordinate RNA synthesis within the normal division cycle.

Synchronization of mouse L-cells by a velocity sedimentation technique

Differential velocity sedimentation at unit gravity has been used to separate an asynchronous population of mammalian cells into fractions synchronized in all phases of the cell cycle. Better enrichment was obtained for G(1) and S phases than for G(2)-M phase. Electronic cell volume measurements of the fractions indicated that the separation was primarily dependent on cell size, and an experimentally determined sedimentation coefficient agreed very well with its predicted value. Sources of dispersion in the separation (including the contribution of cell density heterogeneity) were quantitated and found to be insufficient to explain all of the observed dispersion. Both the limitations and the applications of the technique are discussed.

Synthesis of protein, ribonucleic acid, and ribosomes by individual bacterial cells in balanced growth

Relative rates of protein synthesis in individual cells were determined by allowing random populations to incorporate tritiated leucine for very short periods (pulses) and then examining autoradiographs of these cells to assess the amount of incorporation (grains per cell) as a function of cell size. Relative rates of ribonucleic acid (RNA) synthesis were determined in the same way by using tritiated uracil. Unless the uracil pulse was very short (less than 1/20 generation), the RNA labeled during the pulse was predominantly ribosomal. The rate of protein synthesis in individual cells is directly proportional to cell size. The rate of RNA synthesis also increases linearly with size in larger cells, but there appears to be a slight delay in RNA synthesis immediately after cell division. Total cellular content of protein, RNA, and ribosomes is directly proportional to cell size. Thus, we conclude that, in individual cells during the cell cycle (i) the average rate of protein synthesis per ribosome is constant and (ii) the increase in macromolecular mass of the cell is exponential with age.

Linear cell growth in Escherichia coli

Growth was studied in synchronous cultures of Escherichia coli, using three strains and several rates of cell division. Synchrony was obtained by the Mitchison-Vincent technique. Controls gave no discernible perturbation in growth or rate of cell division. In all cases, mean cell volumes increased linearly (rather than exponentially) during the cycle except possibly for a small period near the end of the cycle. Linear volume growth occurred in synchronous cultures established from cells of different sizes, and also for the first volume doubling of cells prevented from division by a shift up to a more rapid growth rate. As a model for linear kinetics, it is suggested that linear growth represents constant uptake of all major nutrient factors during the cycle, and that constant uptake in turn is established by the presence of a constant number of functional binding or accumulation sites for each growth factor during linear growth of the cell.

Purification of competent cells in the Bacillus subtilis transformation system

Transformed cells have been separated from nontransformed cells by centrifugation on a density gradient of Renografin-76. Separation was achieved both on a linear gradient and on a discontinuous gradient. Under optimal conditions, all of the cells in one band (median density, 1.110 g/ml) were transformants, whereas virtually all cells in the other (median density, 1.131) were nontransformants. In some instances, recentrifugation of the transformant band further enriched the transformant population. The transformed population can also be enriched by zonal centrifugation in a linear gradient of Ficoll. However, this technique is far less efficient than centrifugation in Renografin-76. Since the density of competent cells is identical to that of transformants, we conclude that the low density is a property of competent cells. The significance of this low density to the physiology of competent cells is discussed.