Scientists used to think the number was around 300, but now researchers have found a bacterium that gets by with only 182 genes. The bacteria live inside tiny insects called psyllids. Researchers posit that the bacterium is in the evolutionary process of becoming a part of the insect.

Some of the smallest bacterial genomes are in bacteria that have what is called a symbiotic relationship with a host. The bacteria make something the host needs, and the host can take care of many of the bacteria’s biological needs.
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Johnson says most scientists believe that cells of higher organisms achieved their lofty status by enslaving bacteria that gave up a few too many genes. For example, our cells have something in them called mitochondria. Mitochondria are known as the power plants inside cells.

Johnson says mitochondria were once bacteria that lost too many genes.

“Of course, these events occurred a very, very long time ago,” she says, “and what I think is extremely exciting about this new research is that we might be seeing this happen in real time.”