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| Bacteria multiply in pairs |
When a bacterial
cell divides into two daughter cells and those two cells divide into four more
daughters, then 8, then 16 and so on, the result, biologists have long assumed,
is an eternally youthful population of bacteria. Bacteria, in other words,
don't age -- at least not in the same way all other organisms do.
But a study
conducted by evolutionary biologists at the University of California, San Diego
questions that longstanding paradigm. In a paper published in the November 8
issue of the journal Current Biology, they 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. An advance copy of the study appears this week in the journal's
early online edition.
"Aging in
organisms is often caused by the accumulation of non-genetic damage, such as
proteins that become oxidized over time," said Lin Chao, a professor of
biology at UC San Diego who headed the study. "So for a single celled
organism that has acquired damage that cannot be repaired, which of the two
alternatives is better -- to split the cellular damage in equal amounts between
the two daughters or to give one daughter all of the damage and the other
none?"
The UC San Diego
biologists' answer -- that bacteria appear to give more of the cellular damage
to one daughter, the one that has "aged," and less to the other,
which the biologists term "rejuvenation" -- resulted from a computer
analysis Chao and colleagues Camilla Rang and Annie Peng conducted on two
experimental studies. Those studies, published in 2005 and 2010, attempted
unsuccessfully to resolve the question of whether bacteria aged. While the 2005
study showed evidence of aging in bacteria, the 2010 study, which used a more
sophisticated experimental apparatus and acquired more data than the previous
one, suggested that they did not age.
"We analyzed
the data from both papers with our computer models and discovered that they
were really demonstrating the same thing," said Chao. "In a bacterial
population, aging and rejuvenation goes on simultaneously, so depending on how
you measure it, you can be misled to believe that there is no aging."
In a separate
study, the UC San Diego biologists filmed populations of E. coli bacteria dividing over hundreds of generations and
confirmed that the sausage-shaped bacteria divided each time into daughter
cells that grew elongated at different rates -- suggesting that one daughter
cell was getting all or most of the cellular damage from its mother while the
other was getting little or none. Click this link to watch the time-lapse film
of one bacterium dividing over 10 generations into 1,000 bacteria in a period
of five hours and see if you can see any differences.
"We ran
computer models and found that giving one daughter more the damage and the
other less always wins from an evolutionary perspective," said Chao.
"It's analogous to diversifying your portfolio. If you could invest $1
million at 8 percent, would that provide you with more money than splitting the
money and investing $500,000 at 6 percent and $500,000 at 10 percent?"
"After one
year it makes no difference," he added. "But after two years,
splitting the money into the two accounts earns you more and more money because
of the compounding effect of the 10 percent. It turns out that bacteria do the
same thing. They give one daughter a fresh start, which is the higher
interest-bearing account and the other daughter gets more of the damage."
Although E. coli
bacteria appear to divide
precisely down the middle into two daughter cells, the discovery that the two
daughters eventually grow to different lengths suggests that bacteria do not
divide as symmetrically as most biologists have come to believe, but that their
division is really "asymmetrical" within the cell.
"There
must be an active transport system within the bacterial cell that puts the
non-genetic damage into one of the daughter cells," said Chao. "We
think evolution drove this asymmetry. If bacteria were symmetrical, there would
be no aging. But because you have this asymmetry, one daughter by having more damage
has aged, while the other daughter gets a rejuvenated start with less
damage."
Story Source:
The above story is reprinted from materials provided by University of California - San Diego. The original article was written by Kim McDonald.Note: Materials may be edited for content and length. For further information, please contact the source cited above.
Journal Reference:
- Camilla U. Rang, Annie Y. Peng, Lin Chao. Temporal Dynamics of Bacterial Aging and Rejuvenation. Current Biology, 27 October 2011 DOI: 10.1016/j.cub.2011.09.018
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