The same kind of evolutionary mechanism that explains why people tend to cross an intersection in an orderly fashion, or why a flock of birds can turn in the air with such balletic grace, may help explain why Salmonella bacteria are able to swarm together to better resist antibiotics.
That’s one of the implications of a new study, just published in the Proceedings of the National Academy of Sciences (PNAS), from biologist Rasika Harshey.
“There are certain fairly complex group behaviors, which confer an advantage on the group, that result from individuals following pretty simple rules,” says Harshey, a professor of molecular genetics and microbiology.
In the case of Salmonella, the swarming behavior emerges when the bacteria are packed together on a surface that an individual, isolated bacterium can’t traverse. By clinging and moving together in a swarm, they’re able to move far enough away from the antibiotics to give at least some of the individuals the chance to survive.
Until recently, says Harshey, the increased resistance of swarming Salmonella to antibiotics was assumed to depend not on motion, but on the capacity of the individual bacteria to alter themselves, down to the genetic level, in response to the threat.
“The bacteria who swarm on the harder surfaces, which are the most difficult, do have to differentiate,” she says. “They slow down the process of cell division. They up-regulate the genes responsible for producing flagella. They get longer. They can even signal each other to produce wetting agents which lower the surface tension of the environment.”
Harshey discovered recently, however, that those physical changes in the bacteria don't explain their added resistance to antibiotics. Most of the resistance, she says, is not even a result of swarming behavior, but simply a result of high cell density.
"It turns out that if you create a high density with non-swarmers," she says, "they’re resistant too.”
Swarming does confer extra resistance, however, and Harshey found that it results simply from the collective ability of the Salmonella to move. The faster they move, the more of them survive.
Like people at a crosswalk, who are able to walk in concert simply by following simple rules—for example: follow the person in front of you, stay at a certain distance—the bacteria don’t need to alter themselves in order to work together. They just need to follow the rules.
One question that begs further exploration, says Harshey, is how to explain why some of the swarming bacteria are willing to move in the presence of antibiotics, even though their position in the swarm may cause them to be killed by the antibiotics. Is it altruism? Are they sacrificing themselves in order to give bacteria with a shared genetic legacy a better chance to live? Or is it simply what’s called “selfish herd” behavior, with all of them trying to get away but some getting caught in the wrong place?
“We see the group behavior,” says Harshey, “and we see some of the effects of it. But we don’t always know what the underlying rules are or what evolutionary process allowed the rules to be selected for. That’s what I’m trying to understand.”
Comments