You can say about a live form is smart when it can, based on some present clues predict what will happen in the future. Intelligence is often associated with the brain and neuronal system, but new research has shown that single cell organism – like bacteria also have the ability to learn.

A bacteria has the ability of associative learning through evolutionary changes in their complex networks of interacting genes and proteins. That means that a single bacteria can't predict what might happen given some conditions, but if some hundred generations are expose to same kind of conditions, then future bacteria will know what to expect in that given circumstance.

A experiment that reflects those conclusions was made by Saeed Tavazoie, a molecular biologist at Princeton University with a bacteria called E.coli. When that microorganism enters the human body the medium gets warmer and the bacteria switches to a less efficient low-oxygen mode, to prepare for the lack of oxygen it will encounter.
To prove its theory Saeed Tavazoie and his team, grew the microbes in controlled conditions and increased temperature without decreasing oxygen level. The result? In a few hundred generations microbes stopped associating the temperature rise with low oxygen levels.

Researchers hope to use the result of this experiment to keep under control infectious diseases.