Which bacterium is the largest ever discovered, with cells visible to the naked eye?

Thiomargarita magnifica is the largest bacterium ever discovered, with individual cells visible to the naked eye.

Scientists announced the species in 2022 after finding it in mangrove sediments in Guadeloupe. Its threadlike cells can reach about 1 centimetre in length, making them roughly 5,000 times larger than many typical bacteria. The species name magnifica refers to its remarkable size.

Its giant size challenges the usual idea that bacteria are always microscopic and structurally simple. Thiomargarita magnifica has a large central vacuole and DNA-containing compartments called pepins, features that help it organize its unusually large cell. These structures are not the same as the nucleus of a eukaryotic cell.

A related bacterium, Thiomargarita namibiensis, was previously considered the largest known bacterium, but T. magnifica is substantially longer. The species is still being studied, including how its cellular organization supports its size.