Phenotypic Plasticity and Effects of Selection on Cell Division Symmetry in Escherichia coli

Aging has been demonstrated in unicellular organisms and is presumably due to asymmetric distribution of damaged proteins and other components during cell division. Whether the asymmetry-induced aging is inevitable or an adaptive and adaptable response is debated. Although asymmetric division leads to aging and death of some cells, it increases the effective growth rate of the population as shown by theoretical and empirical studies. Mathematical models predict on the other hand, that if the cells divide symmetrically, cellular aging may be delayed or absent, growth rate will be reduced but growth yield will increase at optimum repair rates. Therefore in nutritionally dilute (oligotrophic) environments, where growth yield may be more critical for survival, symmetric division may get selected. These predictions have not been empirically tested so far. We report here that Escherichia coli grown in oligotrophic environments had greater morphological and functional symmetry in cell division. Both phenotypic plasticity and genetic selection appeared to shape cell division time asymmetry but plasticity was lost on prolonged selection. Lineages selected on high nutrient concentration showed greater frequency of presumably old or dead cells. Further, there was a negative correlation between cell division time asymmetry and growth yield but there was no significant correlation between asymmetry and growth rate. The results suggest that cellular aging driven by asymmetric division may not be hardwired but shows substantial plasticity as well as evolvability in response to the nutritional environment

Protein aggregation in E. coli : short term and long term effects of nutrient density.

During exponential growth some cells of E. coli undergo senescence mediated by asymmetric segregation of damaged components, particularly protein aggregates. We showed previously that functional cell division asymmetry in E. coli was responsive to the nutritional environment. Short term exposure as well as long term selection in low calorie environments led to greater cell division symmetry and decreased frequency of senescent cells as compared to high calorie environments. We show here that long term selection in low nutrient environment decreased protein aggregation as revealed by fluorescence microscopy and proportion of insoluble proteins. Across selection lines protein aggregation was correlated significantly positively with the RNA content, presumably indicating metabolic rate. This suggests that the effects of caloric restriction on cell division symmetry and aging in E. coli may work via altered protein handling mechanisms. The demonstrable effects of long term selection on protein aggregation suggest that protein aggregation is an evolvable phenomenon rather than being a passive inevitable process. The aggregated proteins progressively disappeared on facing starvation indicating degradation and recycling demonstrating that protein aggregation is a reversible process in E. coli

Biodegradation of Imidacloprid, the New Generation Neurotoxic Insecticide

ABSTRACT: Imidacloprid (1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine), a chloronicotinyl insecticide used to control biting and sucking insects, is very persistent in the soil with a half-life often greater than 100 days. Although a few soil metabolites have been reported in the literature, there are few reports of biodegradation of imidacloprid. Our objectives were to discover, isolate, and characterize microorganisms capable of degrading imidacloprid in soil. Two soil free stable enrichment cultures (NUS1, and NUS4) in minimal media were obtained that showed maximum degradation of Imidacloprid between 48 -72 hours after incubation. The degradation was indicated by growth of microorganisms in minimal media, where sole source carbon and nitrogen was Imidacloprid. The degradation product was characterized by High Performance Liquid Chromatography (HPLC), which was found to be 6-Chloronicotinic acid. The two isolates were thus found to metabolize Imidacloprid and were further characterized

Spatial distribution of intensity classes under the six selection nutrition combinations.

<p>Only cells with visible aggregates are included. Positions 0 to 0.5 indicate the position of the aggregate along the cell length, 0 representing the nearest pole and 0.5 representing the centre of the cell. Note that the pattern is similar in all the lines. Aggregates in class 1 can be observed anywhere along the length with a subpolar peak. Older aggregates are more frequently polar. Similarity across selection demonstrates that although there is a difference in the rate of formation of the aggregates, once formed their further fate and spatial dynamics may be unaffected by selection or standing nutrient concentration. A–F represents <i>Hh, Hl, Wh, Wl, Lh</i> and <i>Ll</i> respectively.</p

The effect of 4 day’s starvation on visible protein aggregates.

<p>(n = 100±20 each for data in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0107445#pone-0107445-g002" target="_blank">Fig. 2A to C</a>). A and B: The mean fluorescence intensities before (grey bars) and after (black bars) starvation for 4 days. Means calculated excluding (A) and including (B) aggregate free cells. All columns are significantly reduced in B but not reduced or significantly increased in A. This suggests that all aggregate classes are not degraded proportionately. Many of the larger ones could be relatively resistant to degradation while the smaller ones degrade fast, thereby increasing the mean excluding aggregate free cells after degradation. *denotes significant difference from Wh. $ denotes significance between corresponding starved and unstarved populations (at p<0.05). C: The time course of decrease in the mean fluorescence intensity with starvation: The intensities are expressed relative to the mean intensity at zero hours. p: represents mean excluding aggregate free cells and q: represents including them in continued stationary phase. r: represents averages excluding aggregate free cells and s: represents including aggregate free cells in distilled water. Error bars show standard error of the mean.</p

The progressively changing frequency distribution of fluorescence intensities on exposure to starvation.

<p>A: at the beginning of stationary phase B to E: 12, 24, 36, and 48 hours of starvation respectively. The intensities are classified at a higher resolution but the scale is matched with that in other figures. Note the transient bimodality created presumably due to faster degradation of smaller aggregate classes. Trends were similar but somewhat slower (not shown here) when cells were followed in the same medium after reaching stationary phase. The transient bimodality was consistently seen across both the conditions.</p

RNA and protein contents of cells.

<p>(A) Total RNA (ng/µl) (B) Total protein (µg/ml) and (C) The fraction of insoluble protein (IPF) content of the selection lines under high and low current nutrition. All are expressed relative to Wh. Black columns stand for KL16, dark grey for 2563 and light grey for MGAY. RNA contents differed over an order of magnitude in the <i>L</i> selected lines. *denote significant difference from <i>Wh</i>. $ denote significant difference between corresponding <i>h</i> and <i>l</i> conditions.</p

The distribution of intensity classes before and after starvation for 4 days.

<p>A–F represents <i>Hh, Hl, Wh, Wl, Lh</i> and <i>Ll</i> respectively. The suffix <i>s</i> stands for starvation. Although the distribution before starvation was different in all selection-concentration combinations, the distribution after starvation is similar. In all lines the aggregate free class increased significantly and class 1 reduced consistently. The trend in class 2 and 3 was less consistent.</p

Comparative superimposed DIC and fluorescence images of protein aggregates in cells immediately after entering the stationary phase.

<p>(A) and after starvation for 4 days (B). The scale bar represents 20 µ. Inset: some of the larger aggregates did not disappear after prolonged starvation (30 days) but altered the morphology showing diffused fluorescence sometimes even exceeding the cell boundary.</p