In rod shaped organisms it is possible that the morphologically old pole cell retains the damaged components [4,5,7]. However, this poses a potential paradox. If old and new poles were primarily associated with aging, spherical organisms such as Staphylococcus or Micrococcus sp. that divide in different planes and do not have definite poles would be immune to aging [13]. The association of asymmetry with polarity therefore needs to be examined in phylogenetic and ecological contexts. Even in earlier studies, in a significant proportion of cells, slower growth and protein aggregation is not necessarily associated with old poles [5,7]. It is necessary to examine whether polarity is equally important in different strains of Escherichia coli. In the last five years since the demonstration of asymmetric division in E. coli [4] much research is focussed on the mechanisms of asymmetric segregation. Little progress is seen with regards to the eco-physiology and evolutionary angles of the problem. We address these issues by subjecting E. coli to selection under different sets of conditions and quantitatively examining the changes, if any, in cell division symmetry. We cultured three strains of E. coli (E. coli ATCC 113-3D, E. coli KL16 and E. coli 2563 obtained from National Collection of Industrial Microorganisms, NCL Pune, referred hereafter as strains 1, 2 and 3 respectively) continuously under two sets of nutritionally extreme (high and low nutrient concentrations) conditions. Using 10 mg/ml and 0.1 mg/ml glucose as the sole source of carbon and energy, agar cultures were serially transferred up to an estimated 2000 cell generations such that at the end of the prolonged subculturing we had nine cell lineages, three selected for either of the nutritional environments and three wild types. In order to keep the temporal variation in substrate concentration to a minimum, the cultures were transferred frequently to ensure that they did not experience prolonged stationary phase or starvation any time. We denote the strain selected under high caloric environment as H and the one selected under low caloric environment as L. The current medium condition for the experiment is denoted by small letters h and l for high and low caloric conditions respectively and the subscript denotes original strain (e.g. strain 1 selected under high concentration but currently being grown in low concentration is denoted as H1l). After an estimated 1000 and 2000 generations of selection each of the lineages was examined for growth rate, growth yield and symmetry of cell division time on both high and low nutrient media. Results Index of cell division time asymmetry For quantifying functional symmetry in cell division we defined an index of cell division time asymmetry based on the assumption that if the cell division was asymmetric, the daughter cell receiving older components will take longer to complete the next cell division [4,5,7]. Therefore an index of asymmetry was defined as the ratio of the difference in division time (sign ignored) of two sister cells as to the average division time of the two. The index returns a value of zero if both sister cells complete their next division cycle at the same time. If the division of the slower of the two cells takes twice as much as the faster one the index takes a value of 2/3 and if it the difference is three fold the index becomes 1. The index is independent of the absolute time of division and does not reflect absolute growth rate. For all the three wild types, the cell division time asymmetry was measured under high and low nutrient concentrations in four replicates. In all the 12 experiments the index of asymmetry was greater under high nutrient concentration as compared to low (Figure 1A–C) and 10 Bacterial Division Symmetry Figure 1. Glucose concentration and division time asymmetry. Effect of high and low glucose concentration (10 mg/ml and 0.1mg/ml respectively) on the mean index of division time asymmetry of all the wild strains is shown. Error bars represent s.e. Out of twelve replicates of three strains (A to C respectively), ten replicates showed significantly higher index of division time asymmetry under high glucose concentration than under low glucose concentration (see Table S1 for Mann Whitney results). This indicates plasticity in cell division time asymmetry and sensitivity to nutritional environment. doi:10.1371/journal.pone.0014516.g001 out of the 12 differences were statistically significant (Mann Whitney U-test, see Table S1 for statistical details). This indicates that E. coli cell division symmetry is responsive to current nutritional environment and greater symmetry is exhibited under nutritional limitation. For each of the strains the indices of cell division time asymmetry were calculated after selection for an estimated 1000 and 2000 generations. With strain 1 at 1000 generations, H1l had significantly higher asymmetry than W1l (p = 0.006) and L1l (p,0.0001), and L1l divided significantly more symmetrically than W1l (p = 0.001) (Figure 2A). L1h was significantly asymmetric than L1l similar to the wild type (p,0.0001). On the other hand, the asymmetry index of H1h was significantly greater than H1l only in a one tailed but not in two tailed test (one tailed p = 0.034). Further after 2000 PLoS ONE | www.plosone.org 2 January 2011 | Volume 6 | Issue 1 | e14516