Here is a story of a pathobiont returning to the sea.

It started with polar bear feces collected off the arctic ice: diversity dominated by Psychrobacter. Did these ride the bear or come from the ice? Find out here in our new BioRxiv preprint, in which graduate student Daphne Welter delves into the genus with comparative genomics of 85 strains, each of which she grew in 24 different conditions. Psychrobacter have a remarkable breadth of temperatures at which they grow. She observes two ecotypes – those that can grow at mammalian body temperature and those that can’t – they have lost the high end.

Welter finds that growth at high temperature is basal in the Pyschrobacter clade, and the closest relatives, Moraxella, are pathobionts. Psychrophily is derived. The bear scat has both kinds. Daphne tests which can colonize a germfree mouse: Growth at 37 is required but not sufficient to colonize the germfree mouse. Growth at 37 is lost in the more derived members of the genus. But they still like meat: they grow best on amino acids, and they are known to spoil frozen food.

So it looks like Psychrobacter rode warm bodied hosts back into the sea, and is in the process of adapting to the habitat of its erstwhile hosts.