Aging E. coli ?? Most people don't really think about bacteria aging. One "old" bacterial cell can divide to form two "young" ones, and (for nearly all species) it's impossible to tell which was the "original" cell. Both daughter cells function normally, and each continues to divide as long as nutrients are available--in this sense, the bacteria are essentially "immortal!"¹ However, cellular senescence does occur in bacteria²: in stationary phase, when nutrients are limited and cells are dividing slowly or not at all, individual cells can lose their ability to function over time.

Stationary phase begins as a fast-growing bacterial culture (e.g., in a rich laboratory culture medium) begins to exhaust the available nutrients. Cell division slows and eventually ceases, and the number of cells in the culture stabilizes (see the left panel below). The bacteria reduce their metabolic rate and activate a variety of genes to enable them to survive nutrient limitation and cope with stresses such as heat and oxidation that they might encounter before the return of nutrients³. Soon, the cell number begins to decrease, marking what was once referred to as "death phase". More recently, however, we have realized that the cell number will eventually stabilize again and remain stable indefinitely, a state termed "long-term stationary phase" (right panel below). Although the number of cells changes little, this is actually a dynamic state in which some cells die, others can divide and mutants better equipped for survival can be selected⁴.

In long-term stationary phase, E. coli behaves in many ways much like aging cells of higher organisms. During this time, metabolism is very limited, and a cell can survive only if it can maintain its macromolecules, including existing proteins, in a functional state. Oxidative damage and other environmental stresses are major causes of cell death, and stress protection is critical. Cellular functions decline over time, as does the cell's ability to recover successfully even when nutrients become available. Understanding successful long-term survival in E. coli can thus lead to enhanced understanding of successful aging at the cellular level in all organisms.

Next: PCM in E. coli>>

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¹Or maybe not. An intriguing article by Stewart et al. (PLoS Biology 3:e45, 2005) suggests the possibility of measuring reproductive aging in bacteria.

²For intriguing reviews discussing bacteria as a model system for aging, see: Nyström, T. 2002. Aging in bacteria. Curr. Opin. Microbiol. 5:596-601; Nyström, T. 2003. The free-radical hypothesis of aging goes prokaryotic. Cell. Mol. Life Sci. 60:1333-1341.

³Reviewed in: Siegele, D. A., and R. Kolter. 1992. Life after log. J. Bacteriol. 174:345-348.

⁴Finkel, S. E., and R. Kolter. 1999. Evolution of microbial diversity during prolonged starvation. Proc. Natl. Acad. Sci. USA 96:4023-4027.