Common bacteria have "noses" that respond to smells, scientists said.
A study shows how certain bugs detect the presence of rivals by picking up whiffs of chemicals in the air.
The discovery means lowly bugs are now known to possess four of our familiar five senses.
They can "see" by responding to light, "feel" by reacting to physical touch, "taste" through direct contact with environmental chemicals, and "smell" by detecting airborne molecules. The only sense missing is hearing.
In laboratory tests, scientists found that two rival species of soil bacteria, Bacillus subtilis and Bacillus licheniformus, both reacted the same way to the smell of ammonia given off by the other.
Each bug began to generate biofilm, or "slime", whereby bacteria join together to colonise an area and push out any potential competitor.
The response decreased as the distance between the two bacterial colonies got bigger.
Ammonia is one of the simplest sources of nitrogen, a key nutrient for bacterial growth.
Professor Grant Burgess, director of the Dove Marine Laboratory at the University of Newcastle, said: "The sense of smell has been observed in many creatures, even yeasts and slime moulds, but our work shows for the first time that a sense of smell even exists in lowly bacteria.
"From an evolutionary perspective, we believe this may be the first example of how living creatures first learned to smell other living creatures.
"It is an early observation and much work is still to be done, but nevertheless, this is an important breakthrough which also shows how complex bacteria are and how they use a growing number of ways to communicate with each other.
"Bacterial infections kill millions of people every year, and discovering how your bacterial enemies communicate with each other is an important step in winning this war. This research provides clues to so far unknown ways of bacterial communication."
The research is published today in the Biotechnology Journal.
Dr Reindert Nijland, another member of the team, said although it was clear that the bugs responded to smell, what kind of "nose" they had was still unknown.
"The next step will be to identify the nose or sensor that actually does the smelling," he said.
Biofilm is a major source of infection caused by medical devices such as heart valves, artificial hips and breast implants.
It also costs the marine industry millions of pounds each year by slowing down ships and wasting fuel. On the other hand certain biofilms thrive on petroleum and can be used to clear up oil spills.
"Slime is important in medical and industrial settings, and the fact that the cells formed slime on exposure to ammonia has important implications for understanding how biofilms are formed," said Dr Nijland.