Largest bacterium ever discovered has an unexpectedly complex cell

A newly discovered bacterium, Thiomargarita magnifica, challenges the definition of a microbe: its filament-like single cell is up to 2 centimetres long. T. magnifica achieves its unprecedented size by having unique cellular features: two membrane sacs. One is filled with its genetic material; the other, which is much larger, helps to keep its cellular contents pressed up against its outer cell wall so that the molecules it needs can diffuse in and out. Researchers have dubbed these sacs ‘pepins’ — inspired by the pips in fruit — and note that they blur the line between single-celled prokaryotes and eukaryotes (the group that includes humans), which pack their DNA into a nucleus.