Olfaction in Bacteria: Sensing Other Species Through Smell

Some may find it hard to believe that scientists are ascribing a sense of smell to a simplistic unicellular organism, but this theory is supported by a study published on August 11th 2010 in a Biotechnology Journal. Researchers have found that bacteria do indeed have the ability to smell volatile molecules, namely ammonia, in addition to the previously established ability to detect oxygen. The discovery was accidental - as is often the case with events that break through the established set of ideas, revealing with yet another bit of astonishing information a world that is even more fascinating than we thought.
Scientists were in the process of studying how bacteria produce biofilm, a slimy substance excreted by the colonies, enabling them to stick to surfaces, each other and block other "competing" microorganisms that are in close physical proximity. Interestingly, biofilm is more than just a glue-like chemical shield; it's also a kind of an organizational matrix, complete with channels for nutrients and water transport to and from, serving to order the bacterial colony into something that resembles a multicellular organism.

Bacterial Olfaction Centers on the Presence of Ammonia

Researchers started out with a microtiter plate coated with two different types of biofilm-promoting substances. The left half was more nutrient-dense and hence resulted in more biofilm production compared to the right. The study took on a new direction when a gradation was noticed in the amount of biofilm produced by bacteria on the right. The closer they were to the middle of the plate (and thus, closer to the more favorable conditions of the left side), the more biofilm was produced.
The two colonies were separated in a way that insured there could be no physical contact between them. In light of this only one explanation was possible: the colony to the right was responding to something in the air that somehow commented on the conditions to the left – the presence of higher nutrient level as well as the other colony. But what?
After much retesting with many different strains of bacteria and equal results the culprit became apparent – the nutrient-rich medium used to coat the left side of the plate also happened to result in the feasting microorganisms producing ammonia molecules which those on the right could detect or “smell”. And the closer the right-sided organisms were to those on the left, the greater the ammonia concentration that they smelled, and as a result, the greater their biofilm production.

Bacteria can Smell – But How?

The question in all of this that yet remains to be answered is through which of its inner mechanisms is a unicellular organism apparently able to sniff out molecules in its environment? The readership is perhaps at this moment producing their own gradation in the extent to which they will be following further developments toward the answer, unless of course the technicalities of the process happen to be just as astounding as the apparent fact that it exists.

To the initiated such investigations can't help but be astounding whatever the particularities: A mere half-digested thought run over memories of high-school biology discussion on the complexities of the olfactory system testifies that such a system is made up of many different types of cells spread out between the complex organism's nose or olfactory organ and brain, in light of which it seems ludicrous to assume that it's all a prime example of nature's tendency for overkill in the design of its creatures.
If all it takes then is a single of the cells involved (provided that it's the correct type) to replace much of this biochemical machinery why would all of these other, apparently unnecessary structures and complexities have evolved? The answer may be that complex olfactory structures are needed to navigate the more involved world of complex organisms.
At this higher level of function it is no longer enough to detect a single and simple nutrition-related olfactory signal but a myriad of highly nuanced smells carrying valuable information for the creature's nose to detect and brain to process, about other similarly complex as well as lesser beings, directly and indirectly relevant to its existence. Viewed in this light, a complex olfactory system is necessary only when airborne molecules whose detection is of crucial importance to the organism happen to be complex themselves in their number, variety and required analysis of meaning.

Sources:

• Nijland, R. and Burgess, J. G. (2010), Bacterial olfaction. Biotechnology Journal, 5: 974–977. doi: 10.1002/biot.201000174
• Vokshoor, Amir. Anatomy of Olfactory System. Medscape Reference Online.