Psychrobacter’s Evolution From Host to Environment

You may have heard that biology is the “science of exceptions” and this story originating from polar bear poop is yet another example of this saying. It’s a story of host-associated bacteria that has evolved to return to the environment. This discovery is contrary to what’s observed for many bacteria: that evolution of host-associated bacteria began with free-living microbes.

The main character of this story? Psychrobacter.

Some like it Hot. Some like it Cold.

Daphne Welter, a graduate student in Ruth Ley’s lab at the Max Planck Institute for Developmental Biology, began this study by examining 24 physical characteristics (temperature and salinity preferences, for example) of 85 Psychrobacter strains.

Based on their characteristics, the team divided the strains into two types of Psychrobacter: those that can grow between 4°C and 37°C, and those that could only grow between 4°C and 25°C. The former group which the team called the “flexible ecotype” encompasses temperatures from the environment and mammalian hosts, while the latter group, or the “restricted ecotype” could only grow in non-host environmental temperatures.

Now that the team had a grasp on the physical characteristics of these strains, they looked a bit deeper and inspected their genomes. The flexible ecotype strains had small genomes, contained more transposons, and were more likely to be bile salt resistant. In terms of evolutionary relatedness, the flexible ecotype strains are interspersed within the restricted ecotype strains, suggesting that flexible ecotype strains are either readapting to the animal hosts or that they’ve just retained these host-associated genes from their ancestral strains. The restricted ecotypes contain more lipid metabolism and biofilm genes, and genes involved in cold stress adaptation.

While the polar bear poop contained both the flexible and restricted ecotypes of Psychrobacter, only the flexible ecotype could colonize germ-free mice.

Psychrobacter has been found in the skin and guts of marine mammals, the guts and throats of birds and fish, sea water, sea ice, marine sediment, permafrost, and frozen foods. While some cause disease in mammals, this is rare.

Psychrobacter’s sister: Moraxella

This brings us to a set of microorganisms closely related to Psychrobacter: the Moraxella species. Based on their sequences, Moraxella and Psychrobacter share a common ancestor. It appears that Psychrobacter species branched off from Moraxella species in the phylogenetic tree. Moraxella, also a pathobiont commonly isolated from host mucosal tissue, can only grow at 37°C.

This connection to the warm loving Moraxella is further evidence that Psychrobacter began as a host-associated bacterium that has transformed to one that is environmentally associated.

From free-living to host-associated and back

A previous analysis of genomes associated with different environments finds that genomes are typically enriched in genes needed for either survival in the animal host or in the environment, but not both. Genomes specialized for mammals are usually incompatible with other environments. And, it is this transition that has accounted for specialized functions of many host-associated bacteria. Many gut-associated bacteria also don’t have environmental reservoirs since the selective pressures between the gut and the environment are different from one other. Though there are exceptions (V. cholerae and Yersinia enterocolitica, for example).

For Psychrobacter, the story is different. These bacteria, once host-associated, have begun to lose some of these host-associated genes, making way for genes that aid in environmental survival. This tradeoff gives rise to species that are cold-loving but can’t thrive at mammalian host body temperatures. These cold-loving Psychrobacter are just another example of how biology is peppered with departures from the norm.

Featured image from Pixabay.