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Why even microbes will commit suicide

Rocky Mountain Laboratories, NIAID, NIH

Professors at the Microbial Evolutionary Ecology at the University of Z├╝rich investigated the suicidal behavior of E. coli cells (above).

By Jennifer Viegas
Discovery News

A study on suicidal E. coli sheds light on why organisms throughout the animal kingdom, big and small, sometimes decide to do themselves in.

The good news is that suicide appears to be comparatively rare in larger animals, but more common among microscopic life forms, such as microbes, according to the study, published in the latest Proceedings of the Royal Society B.

Natural selection -- the process by which organisms that adapt to their environment tend to survive and produce more offspring -- favors suicide when the death guarantees the survival of relatives and the individual is less likely to reproduce in future.

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An example of this, according to study co-author Rolf Kümmerli, is when "a parent saves his children out of a burning house. This is beneficial because the rescued relatives share many of the genes with the suicidal helper." Many people are driven to save their kids and close loved ones, no matter what.

Other forms of suicide among humans, such as bombers on a kamikaze mission, likely have nothing to do with natural selection, and instead reflect the by-product of something else. In the case of the bomber, that could be the individual's environment and life’s experiences. Depression or other forms of mental illness, however, could be inherited.

Kümmerli, a professor in the department of Microbial Evolutionary Ecology at the University of Zürich, and colleagues Dominik Refardt and Tobias Bergmiller investigated the suicidal behavior of E. coli. Some cells of this common bacteria will kill themselves in the presence of bacteria-killing parasitic viruses.

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Rolf Kummerli

A 250-fold magnification of two E. coli colonies. The phage could eat holes into the colony of a susceptible strain (left bottom), whereas colonies of the suicidal strain remain unharmed (upper right).

Kümmerli explained to Discovery News that when a protein of an E. coli cell senses viral attack, it becomes activated and, with other proteins, triggers drainage of membrane holes of the bacterial cell. It’s as though the cell biochemically stabs itself.

"Consequently, vital cell liquid and components pour out into the environment, which leads to cell death," he said. "The dead cell is presumably like an empty perforated sack."

Even among lowly microbes, such behavior would seem to go against survival and procreation mechanisms. Behaviors that benefit others, at the expense of the individual, however, can emerge when multiple relatives are saved. They can also emerge to benefit unrelated others when the cost of suicide is low.

In the case of E. coli cells, they would likely die from the viral attack anyway, and their death prevents parasite transmission to nearby other E. coli cells.

Suicide is also well documented in social insects that tend to live in large populations, such as ants and bees. Some ants will even explode themselves to prevent intruders from attacking their relatives.

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Suicides among non-human mammals and other larger animals are mostly anecdotal, but they do tend to once again apply to social species, such as dogs and dolphins. Dolphin trainer Ric O'Barry, for example, claimed that he watched the famous TV star dolphin, Flipper, take her own life out of sheer depression due to confinement in captivity. O'Barry later became an animal activist.

Gaining a better understanding of the drivers behind suicide could lead to life-saving benefits. Researchers in future might be able to coax harmful bacteria, viruses and other microbial organisms to kill themselves, potentially saving human and other animal lives.

Stuart West, a professor of evolutionary biology at the University of Oxford, commended the new research, saying the researchers "show here that if the costs of suicide are low (the individual is unlikely to reproduce anyway), then relatedness doesn't have to be very high, although it does have to be above zero."