Which bacterium was the first free-living organism to have its genome completely sequenced, in 1995?

Haemophilus influenzae was the first free-living organism to have its complete genome sequenced, with the result published in 1995.

The genome was sequenced by a team led by J. Craig Venter at The Institute for Genomic Research, or TIGR. The project used whole-genome shotgun sequencing, in which many randomly selected DNA fragments are read and then assembled computationally. This approach became a major influence on later genome projects.

The H. influenzae genome contains about 1.83 million base pairs and approximately 1,740 predicted protein-coding genes. Its relatively compact genome made the bacterium a practical early target for complete sequencing. The achievement marked a transition from studying individual genes to examining an organism’s full genetic complement.

A common mix-up is Mycoplasma genitalium, whose genome was sequenced in 1995 as well, but H. influenzae was the first free-living organism completed. Viruses had been sequenced earlier, yet they are not free-living cellular organisms.