Which bacterium was the first to have its genome sequenced?

Haemophilus influenzae was the first bacterium to have its entire genome sequenced. The landmark sequence, completed using whole-genome shotgun sequencing, was published in 1995.

The project studied strain Rd KW20, whose genome contains 1,830,138 base pairs arranged in a single circular chromosome. Researchers identified 1,604 protein-coding genes, along with genes for transfer RNA and other RNA molecules. Its relatively small genome made H. influenzae a practical early target for complete sequencing.

The achievement was significant because it demonstrated that scientists could sequence a complete genome without mapping every region first. Craig Venter and colleagues at The Institute for Genomic Research carried out the work, drawing on DNA resources associated with Hamilton Smith. The bacterium became the first free-living organism—not merely the first bacterium—to have its full genome published.

A common mix-up is Escherichia coli, which is often used in genetics and molecular biology but was sequenced later. Viruses and organelles had smaller genomes sequenced earlier, so the wording “first genome ever sequenced” would be broader and less precise.