Growing colony of E. coli bacteria. Image: UC San Diego |
When a bacterial cell divides into two daughter cells and those two cells
divide into four more daughters, then eight, 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 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.
“Aging in organisms is often caused by the accumulation of non-genetic
damage, such as proteins that become oxidized over time,” says 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,” says 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.
“We ran computer models and found that giving one daughter more the damage
and the other less always wins from an evolutionary perspective,” says Chao. “It’s analogous to diversifying your portfolio. If you could invest $1 million
at 8%, would that provide you with more money than splitting the money and
investing $500,000 at 6% and $500,000 at 10%?”
“After one year it makes no difference,” he adds. “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%. 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,” says 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.”