In Salmonella Attack, Taking One for the Team

To make you sick, some salmonella bacteria make the ultimate sacrifice.

A tainted piece of lettuce or egg might carry thousands of salmonella bacteria. But even that many salmonella cannot muscle out the helpful bacteria that live day in and day out in the intestines.

So, about 15 percent of the salmonella go on a suicide mission, invading the intestinal walls. There, the immune system handily wipes them out. But that also sets off a wider immune response that, while attacking the salmonella within the gut, also wipes out many other micro-organisms.

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“This inflammation removes many of the competitors, so the second group which waited outside can proliferate,” said Martin Ackermann, a professor of environmental sciences at the Swiss Federal Institute of Technology in Zurich and an author of a scientific paper describing the findings in the Aug. 21 issue of the journal Nature.

The research, which combined bacterial biology with the mathematical models of game theory, illustrates how even simple organisms appear to cooperate for the greater good of their species.

The suicidal salmonella are genetically identical to their more reserved kin; a random process within individual bacteria appears to cause some of them to sign up for the suicide mission.

The researchers showed that this self-sacrifice works if it benefits members of the species alone. If a mutation created a salmonella strain that always opted out of the suicide attacks, these salmonella would benefit in the short term within the gut, but they would have trouble spreading to the next victim.