Ciliates, just like humans, are colonized by a vast diversity of bacteria. Some ciliates and their bacterial symbionts have become friends for life, as researchers from the Max Planck Institute for Marine Microbiology in Bremen demonstrated by comparing a group of these single-celled ciliates and their bacterial partners from the Caribbean and the Mediterranean Seas. The bacteria provide their ciliate hosts with nutrition by oxidizing sulfur. Surprisingly, they found that this partnership originated once, from a single ciliate ancestor and a single bacterial ancestor, although a whole ocean separates the sampling sites.