Bacteria sniff each other out

Bacteria have a sense of smell, which they may use to sniff out competitors and food sources, according to new research published this week in Biotechnology Journal.

Colonies of Bacillus licheniformis, which detect their neighbors by "smelling" ammonia Image: Reindert Nijland

A study led by Reindert Nijland, now at the University Medical Center in Utretcht, The Netherlands, found that Bacillus bacteria can sense each other's presence through the air by sensing ammonia production.

"This is basic science that's really, really interesting because if bacteria can really smell, that's something unexpected," Nijland told The Scientist.

Although researchers had known that bacteria could sense the presence of ammonia, "this is the first time it was shown that a gas is sensed for the purpose of regulating social behavior," said Jörg Stülke, a microbiologist at the University of Göttingen in Germany, who did not participate in the study.

Nijland, then a post-doctoral fellow in the lab of Grant Burgess at the Dove Marine Laboratory at Newcastle University in the UK, was originally trying to figure out how different growth media affected the biofilm-forming abilities of Bacillus subtilis and B. licheniformis. Trying to save valuable lab space, Nijland set up several different experiments in the two Bacillus species on the same 96-well microtiter plate. On the left side of the plate, Nijland grew his bacteria in a nutrient-rich broth, while growing the same bacteria on the right side in media that encouraged them to form the sticky, slimy matrix of sugars and other compounds known as a biofilm. He assumed that the bacteria would not affect each other since they were physically separated.

As Nijland continued his experiments, he noticed something strange. The Bacillus species grown in biofilm-friendly media nearest the wells with the bacteria grown in nutrient-rich media formed larger biofilms that had a darker red pigment. As the bacteria in the biofilm-promoting media got farther away from the bacteria in the nutrient-rich media, the characteristic red pigmentation of Bacillus biofilms began to fade, indicating lower biofilm formation.

Since the bacteria were not physically connected to each other, Nijland could only conclude one thing: The Bacillus must be sensing the presence of the nearby bacteria through the air. Those bacteria growing in nutrient-rich media seemed to be producing some sort of signal that could be sensed by bacteria growing in a biofilm-promoting media. The bacteria must be responding to an airborne volatile compound, which meant the bacteria had fulfilled the textbook definition for olfaction. Only animals and other "higher" eukaryotes were thought to have a sense of smell, but Nijland's work showed that bacteria also have an olfactory sense. The problem was: what compound were the bacteria sensing?

Nijland began testing all types of volatile chemicals, "basically anything that we had that was smelly," he said. Nijland identified his volatile compound after placing purified ammonia in one row of wells on the microtiter plate and then growing B. licheniformis in biofilm-friendly media in the row of wells next to the purified ammonia. The B. licheniformis in the adjacent wells formed red pigmented biofilms, whereas the B. licheniformis growing in separate plates without ammonia did not form biofilms.

"Ammonia is the simplest available nitrogen source," Nijland said. "All organisms need nitrogen to produce their proteins." The ammonia is thought to signal both the presence of nutrients and the presence of other bacteria, since the biofilms Bacillus species produce in response to ammonia contain antibiotics that can kill competing bacteria. And the ability to "smell" ammonia "gives bacteria a way to sense nutrients where nutrients are and then migrate towards them," he said.

Knowing more about how biofilms form might also lead to better ways to kill off these notoriously hardy and persistent infections, Nijland added.

Nijland, R. & Burgess, J. G. "Bacterial olfaction," Biotechnology Journal, doi:10.1002/biot.201000174, 2010.

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