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

When sensing the presence of other species, bacteria meet the textbook definition for olfaction



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.

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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The molecules in the majority form the superstructure while the minority remain disordered. (Credit: Image courtesy of University of Liverpool)

University of Liverpool, Great Britain   —  Research at the University of Liverpool has found how mirror-image molecules gain control over each other and dictate the physical state of superstructures.

The research team studied ‘chiral’ or ‘different-handed’ molecules which are distinguishable by their inability to be superimposed onto their mirror image. Such molecules are common – proteins use just one mirror form of amino acids and DNA, one form of sugars. Chirality leads to profound differences in the way a molecule functions – for example, drugs such as thalidomide can have positive effects on a patient but can prove harmful in their mirror image form.

Molecules can also assemble in large numbers and form ‘superstructures’ such as snowflakes which are created from large numbers of water molecules. When chiral molecules assemble they can create ‘handed’ superstructures; for example left-handed molecules can assemble together to make a left-handed superstructure. The Liverpool team studied this process in detail by assembling molecules at flat surfaces and using imaging techniques to map the formation of superstructures at nanoscale level.

Before now, scientists have not known whether, in systems containing both left-handed and right-handed molecules, one mirror-form of a molecule could take supremacy over its opposite number in the creation of superstructures, dictating their physical state and chemical and biological properties.

The research found that when equal numbers of mirror-molecules are present at the surface, they organize into separate left and right-handed superstructures, each with distinctly different properties. Crucially, a small imbalance in the population leads to a dramatic difference and only the molecules in the majority assemble into its superstructure, while the minority is left disordered at the surface and unable to create advanced molecular matter.

Professor Rasmita Raval from the University’s Surface Science Research Centre said: “We were surprised at these results. All perceived wisdom was that the left and right-handed molecules would simply create their respective superstructures in quantities that reflected the molecular ratio – that is, we would observe proportional representation. Instead, what we obtained was a kind of ‘molecular democracy’ that worked on a ‘first-past-the-post’ system where the majority population wrested chiral control of the superstructures and the minority was left disorganized.”

Theoretical modeling carried out by the University of Eindhoven in the Netherlands found that this behavior arises from the effects of entropy, or disorder, which leads the chiral molecules in the majority to preferentially organize into their superstructure.

The work has important implications in the pharmaceuticals industry and could lead to the development of surface processes to enable separation of drugs and products that are currently difficult to purify. The research also introduces the possibility that assembly processes at surfaces may naturally have led to the evolution of proteins and DNA – the molecules of life – containing just one mirror form of amino acids and sugars.

The research, in collaboration with the University of Eindhoven, is published in Nature Chemistry.



Story Source:

Adapted from materials provided by University of Liverpool.

Journal Reference:

Sam Haq, Ning Liu, Vincent Humblot, A. P. J. Jansen & Rasmita Raval. Drastic symmetry breaking in supramolecular organization of enantiomerically unbalanced monolayers at surfaces. Nature Chemistry, 2009; 1 (5): 409 DOI: 10.1038/nchem.295