Do bacteria age?
We grow up with the idea that all that is alive will eventually die. But what about bacteria? When a single microbe divides into two daughter cells, does the original cell then still exist? And when the daughter cells in turn divide into four, then eight, 16, 32,… and so on, the whole population remains young. Or put in a different way, the aging effects are cancelled out by division. (Micro)biologists have long believed bacteria remain young as long as they can multiply. Bacteria simply don’t age, or do they?
If bacteria age and a division results in 1 older and 1 younger cell, any growing population of cells will contain both young and older cells. Approximately half of any population will be young (youngest) the rest will be older. Is it possible to see a difference in growth speed when looking at individual cells in such a population?
Simply looking at colony size will not be sufficient. Colony size differences as much as a factor 2 are expected. At the start of such an experiment, some bacteria will start just after division and some just before (nearly a 2-fold difference).
Also one can question whether old and new cells are different enough in growth rate to cause visible size differences in colonies. In fact, even a very old bacterium that is capable to divide once (even when this is slightly slower), will produce a young cell. The young cell will have an optimum growth rate and will divide quickly producing a new young cell etc. As long as a cell can divide once, the visual colony will contain mainly young cells. Over time, old cells exponentially decrease in frequency. In any healthy population 1 in 2 cells are young (first division cells). 1 in 4 are only 1 division older.
Time Lapse microscopy was used to study the earliest divisions of a bacterium when forming a microcolony. The video below of a growing colony of E. coli bacteria (Credits: University of California – San Diego) perhaps hold the answer to our question. It illustrates that a bacterium dividing in 2 and then 4 etc. does not result into an internal youthful population of bacteria. Instead, the E. Coli bacteria shown in this movie (dividing in 5 into approximately 1000 bacteria) split into 1 aging and one youthful cell. The researchers conclude that not only do bacteria age, but that their ability to age allows bacteria to improve the evolutionary fitness of their population by diversifying their reproductive investment between older and more youthful daughters.
New technological developments and automation allows to study and monitor 1000s of individual microcolonies.
(Credits: University of California – San Diego, youtube)