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Stewart et al. (2005) [4]. Polar aging occurs in bacterial cells with apparent morphological symmetry. |
Cells expressed YFP under Pl promoter and lac repressor, cultured on agarose pad slide. Inheritance of new and old poles was recorded. Determined if pole age affects growth rate and viability. |
Cells were inoculated onto a microscope cavity slide sealed with LB-agarose at 30°C. Cells were tracked on a single plane and video along with images were taken using Metamorph software. Tracked 9 generations for 94 microcolonies. No explicit stress condition was applied. |
Cell doubling rates at each generation were averaged per cell position in lineage, forming a bifurcating average tree. Old pole and new pole growth rates were compared by pairwise two-tailed T-test. Control datasets were analyzed to test whether pole age and growth rate values show random distribution. |
Pole age and growth rate values show nonrandom distribution. Cells inheriting the new pole have an increased growth rate. Dead cells show greater inheritance of old pole. Old pole cells produce less biomass in their offspring than new pole cells. |
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Lindner et al. (2008) [12]. Old poles inherit a greater proportion of protein aggregates. Protein aggregation decreases growth rate independently of old-pole location. |
Tracked protein aggregation by chaperone IbpA-YFP, which tags inclusion bodies. Tested the distribution of protein aggregates (inclusion bodies) between poles. |
Cells were inoculated onto a microscope cavity slide sealed with LB-agarose at 37°C. Cells were tracked on a single plane; images were taken by Metamorph. Tracked 9 generations for 12 microcolonies. Streptomycin was used as a stress condition. |
Growth rates were calculated by exponential fit to cell length increase as a function of time. Growth rates were normalized to the generation means. Old pole and new pole growth rates were compared by t-test for normally distributed, unpaired data. Equal variance was determined by F test. |
Young-pole offspring grow faster and old-pole offspring grow more slowly than mother cell. Inclusion bodies (protein aggregates) form at midcell, get stuck in newly formed poles, then stay as pole ages. Inclusion bodies slow growth rates independently of polar location. |
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Winkler et al. (2010) [13]. Heat shock aggregates proteins and increases polar age asymmetry; allows loss of protein aggregates via old-pole cells, and increase of new-cell growth. Nucleoid occlusion (inhibition of septation) drives protein aggregates to the poles. |
Cells containing thermolabile proteins linked to YFP under ara or lac promoter were heat shocked and then placed on a nutrient-dense agarose pad. Cells and protein aggregates were followed as cells divided |
Cells were cultured on LB-agarose pad slide at 30°C. During time-lapse experiments, images were captured manually. Growth rates were calculated for each division into two daughter cells. Tracked 4 generations (colonies of 30 cells). |
Growth rates were measured for old-pole and new-pole cells cultured continuously for 4 doublings. Mean and standard deviation (SD) were presented. |
Under heat stress, old-pole cells consistently inherit polar aggregates. Growth rates decline in aggregate-filled old-pole cells over 4 generations. Cells inheriting new poles with no aggregates show increased growth rates (“rejuvenation”). |
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Lele et al. (2011) [7]. Nutritionally dilute environment selects for cell division symmetry (increased growth yield). |
Three strains (113-3D, KL16, 2563) were serially cultured on agar for 2000 generations. Conditions were high glucose (10 mg/ml) or low glucose (0.1 mg/ml). |
After 1000 and 2000 generations on agar, growth rate was tested in liquid culture. Cell divisions were observed on agarose pads. Tracked 4–5 generations. |
Index of division time asymmetry was calculated using non-parametric Mann Whitney test. Cell lengths were compared by pairwise t-test, one-tailed. Correlation analysis was performed using nonparametric Kendall’s tau test. |
Higher glucose concentration increased cell division asymmetry. In all strains, growth yield was negatively correlated with cell division asymmetry. No conditions showed division asymmetry associated with old poles. |
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Rang et al. (2012) [9]. For cells not expressing fluorescent reporters, polar aging (division asymmetry) requires a protein damage agent such as streptomycin. |
Strain K-12 MG1655 was cultured by method of Stewart et al. (2005). Inheritance of new and old poles was recorded. Determined if pole age affects growth rate and viability. |
Cells were inoculated onto a microscope cavity slide sealed with LB-agarose at 37°C. Conditions included 0,1,2, or 3 μg/ml of streptomycin. Cell growth into microcolonies was captured by time-lapse photography. Tracked 8–9 generations (colonies up to size 400). |
Doubling rates of mother and two daughter cells were analyzed with a best-fit linear regression. Three parameters were calculated: the doubling time of the fittest, most damage-free cell; the asymmetry coefficient; and the amount of damage that a cell incurs per unit time. |
Cells cultured without streptomycin show no division asymmetry, and zero damage accumulation. The rate of damage accumulation increases with streptomycin concentration (from 0 to 3 μg/ml). |
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Lloyd-Price et al. (2012) [14]. Individual protein aggregates migrate toward a cell pole, with strong bias for the old pole. |
Cells expressed a MS2 coat protein fused to a GFP along with a RNA target plasmid. Tracked the migration of individual RNA-MS2-GFP complexes within cells. |
Cells were induced for MS2 and cultured on sealed LB-agarose at 37°C supplemented with antibiotics and arabinose inducer. After 1 h, cells were tracked for 2 h with images obtained every minute. Cell division time was 1.5 h (tracked 4 generations). |
The degree of biased polar segregation of aggregates was analyzed using the model of biased binomial partitioning of RNA-MS2-GFP complexes. |
As cells elongate, RNA-MS2-GFP complexes migrate toward a pole. Complex migration shows strong bias for the old pole. |
