Bacteria make a lot of smells, mostly ones that we'd rather not think about. The hundreds of volatile compounds that bacterial cultures produce can signal many things, although I'm probably one of very few people who associate the smell of warm E. coli with pleasant lab memories rather than some kind of a hygiene disaster. Different bacteria have unique volatile traces, important for microbiological diagnosis in the olden days and for the crafting of microbe based foods like wine, beer, and cheese (some especially stinky species of which I'm smelling in the photo). Until very recently, however, the smells that bacteria make weren't known to have much of a biochemical function.
Last year, a cool paper showed one of the first examples of "bacterial olfaction," with a species of Bacillus increasing formation of a sticky biofilm in response to volatile ammonia produced by bacteria in a neighboring well of a culture plate. While "olfaction" might not be the right word for this kind of chemical response, it is fascinating that bacteria can respond to chemical signals sent at a distance. Bacteria have many mechanisms that sense chemicals floating in solution around them, including nutrients, toxins, and signals from neighboring bacteria, and now it seems that this kind of signaling goes beyond the borders of the petri dish.
Ten years ago, researchers identified that some kind of long-distance communication was boosting the growth and antibiotic resistance of E. coli through a mysterious mechanism not related to these better-understood soluble signals. A very recent paper found through clever genetic and biochemical experiments that the bacteria are again "smelling" ammonia, which changes the permeability of the cell membrane, preventing antibiotics from getting inside and killing the cell, a finding that may impact our understanding of non-inherited antibiotic resistance (PDF).
Smells are part of how we understand the world around us, and volatile chemicals have been identified as modes of intra- and inter-species (or even inter-kingdom) communication between plants, fungi, insects, and now, bacteria. Research like this will allow us to "listen in" and perhaps even participate in the smelly conversations between different organisms.