By Janelle Weaver
Bacteria don't have noses, but they detect some odors in a similar way to animals, according to a study to be published on Wednesday. The pungent gas ammonia causes bacteria to form slimy colonies called biofilms, and smelling it may help the microbes to locate food and avoid competitors.
The study offers the first evidence that bacteria respond to odors--produced when volatile chemicals evaporate--and is perhaps the earliest evolutionary example of olfaction, says Reindert Nijland, a microbiologist at the University Medical Centre Utrecht in the Netherlands. Nijland and Grant Burgess, a marine microbiologist at the University of Newcastle, UK, are publishing their findings in the Biotechnology Journal.
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Ammonia is an important source of nutrients for bacteria because it contains nitrogen, which is used to make proteins and nucleic acids. The ability to detect it in the air would allow bacterial colonies to direct their growth towards areas with lots of resources, helping them to compete against other organisms.
Nijland and Burgess did not set out to identify bacterial responses to airborne compounds. Instead, they were studying biofilm formation in the soil-dwelling bacterium Bacillus licheniformis. They filled each half of a 96-well array with a different type of culture medium--on the right, a medium designed to promote the formation of biofilms, and on the left, one rich in general nutrients. The bacteria in the broth on the right formed reddish-brown biofilms, but they flourished best in the columns closest to the middle, indicating that the growth was helped by the nutrient-rich broth on the left.
The duo quickly homed in on ammonia as the crucial signal for biofilm induction. They found that growth media containing ammonium sulphate, which bacteria metabolize and convert to ammonia, caused slimy aggregates to form in nearby wells. When they put ten columns of bacteria-filled wells next to columns containing an aqueous solution of ammonia, they noticed a similar pattern, suggesting the microbes sense and react to the volatile chemical, Nijland says.
A volatile debate
It is well known that bacteria respond to gases, such as carbon monoxide and oxygen, says Patrick Hallenbeck, a microbiologist at the University of Montreal in Quebec, Canada. For instance, they migrate to the concentration of oxygen that is optimal for growth. And volatile compounds, such as fatty acid methyl esters, control the expression of genes that, in some cases, determine a bacteria's virulence. "I wouldn't buy the argument that it is the first time someone has shown that bacteria can sense a gas in the environment," says Hallenbeck.
But Nijland counters that oxygen and carbon monoxide don't have odours, whereas ammonia does--so their work does show an example of bacterial olfaction. "It doesn't make sense to smell oxygen because it's always around you. You smell things that give important information," he says.
Pete Greenberg, a microbiologist at the University of Washington in Seattle, argues that the authors may be measuring an effect of low environmental acidity rather than ammonia per se. Before bacteria can detect the compound drifting from adjacent wells, it must first dissolve into their liquid surroundings. But aqueous ammonia has a high pH, or low acidity, so it causes the pH of the medium to change--an environmental stressor that can influence biofilm formation. Nijland and Burgess's findings are "no great surprise", Greenberg says. "They decided they discovered bacteria olfaction, when it is really chemical sensing, in my opinion."
Nijland acknowledges that ammonia influences the pH of the solution but says they observed the effect with low concentrations of ammonia that did not change pH a lot. The medium that the bacteria were grown in was also buffered to prevent large changes in pH. "But I can't fully exclude that pH has something to do with it," he says.
"Other researchers might criticize the use of the term 'olfaction'," Nijland adds. "If you define it as sensing a volatile molecule, that is exactly what we observed." He hopes that the results will lead to strategies for combating biofilms, which resist antibiotics and make disease-causing bacteria more dangerous.
