Pole Age Affects Cell Size and the Timing of Cell Division in Methylobacterium extorquens AM1

Abstract

A number of recent experiments at the single-cell level have shown that genetically identical bacteria that live in homogeneous environments often show a substantial degree of phenotypic variation between cells. Often, this variation is attributed to stochastic aspects of biology-the fact that many biological processes involve small numbers of molecules and are thus inherently variable. However, not all variation between cells needs to be stochastic in nature; one deterministic process that could be important for cell variability in some bacterial species is the age of the cell poles. Working with the alphaproteobacterium Methylobacterium extorquens, we monitored individuals in clonally growing populations over several divisions and determined the pole age, cell size, and interdivision intervals of individual cells. We observed the high levels of variation in cell size and the timing of cell division that have been reported before. A substantial fraction of this variation could be explained by each cell's pole age and the pole age of its mother: cell size increased with increasing pole age, and the interval between cell divisions decreased. A theoretical model predicted that populations governed by such processes will quickly reach a stable distribution of different age and size classes. These results show that the pole age distribution in bacterial populations can contribute substantially to cellular individuality. In addition, they raise questions about functional differences between cells of different ages and the coupling of cell division to cell size.In several species of alphaproteobacteria, the two cells emerging from division have been shown to be systematically different. First, experiments with Caulobacter crescentus and Sinorhizobium meliloti have shown differences in protein localization between cells with new and old poles (13,29). Second, experiments with several species of alphaproteobacteria have shown that cells with "new" poles are smaller than cells with "old" poles (13), (12). Third, in C. crescentus, cells with increasingly old poles show signs of aging, while new pole cells are rejuvenated (2, 3). Here, we used the alphaproteobacterium Methylobacterium extorquens to investigate whether the age of a cell's poles has a systematic effect on its size and on the timing of cell division.M. extorquens is a facultative methylotroph that is ubiquitous in aquatic and terrestrial environments (15) and is often associated with plants (8). M. extorquens strain AM1 is a genetic model system (31) that shows substantial variation in cell size and interdivision intervals (26, 27) between genetically identical cells. Motivated by these reports on phenotypic variation, we asked three questions about cell size asymmetry and the timing of cell division. First, does cell size increase with pole age? Previous experiments with alphaproteobacteria have only distinguished between "old" and "new" poles (12, 13). If we analyze "old" poles of different ages, do we see a systematic effect of pole age on cell size? Secon...

The Histidine Kinase PdhS Controls Cell Cycle Progression of the Pathogenic Alphaproteobacterium Brucella abortus

“…The morphological asymmetry and the regulatory network involving the RR DivK and CtrA are conserved among several alphaproteobacteria (2,3,15,33). Interestingly, the symbiotic bacterium Sinorhizobium meliloti contains a PdhS homolog named CbrA (12,13).…”

A functional perspective on phenotypic heterogeneity in microorganisms

“…Such age dependence has been demonstrated for a number of cellular traits in budding yeast (Saccharomyces cerevisiae) 26,27 as well as in bacterial systems. For example, in the alphaproteobacterium Methylobacterium extorquens, both cell size and the timing of cell division depend on cellular age 28 . In Mycobacterium tuberculosis, cellular age has been found to influence survival following exposure to antibiotics, although the direction of this effect is inconsistent between studies 29 .…”

Repair rather than segregation of damage is the optimal unicellular aging strategy

“…Since then, the evidence for aging in unicellular organisms in the sense of reduced growth rate of the old-pole cell has become less clear: some further studies supported a limited degree of aging in the bacteria E. coli [ 17 ], Bacillus subtilis [ 18 ], Mycobacterium spp. [ 19 ], and the diatom Ditylum brightwellii [ 20 ], while others found no evidence of aging in E. coli [ 19 , 21 , 22 ] and other bacteria [ 22 ] or the unicellular eukaryotic alga Euglena gracilis [ 23 ]. Chao and co-workers [ 24 , 25 ] pointed out that age, in the sense of the damaged fraction of cells, could reach a steady state in growing cells where damage accumulation would be balanced by damage dilution such that ‘age’ would not increase with time, and that this was the case in the studies of [ 16 , 21 ].…”