We continue the Microbial Advent Calendar with Leeches, and of course, they have bacteria to help them digest blood! Before continuing any further we have to take a mandatory Wes Anderson’s Life Aquatic with Steve Zissou stop. Please enjoy the gif on your right.
Now we can proceed. Like many other monophagous (pompous way to say that they only eat one thing) animals, leeches depend on symbionts to supplement their diet with nutrients they can’t suck out of their host.
Leeches have a specialized organ associated with the esophagus called the mycetomes (so far no relations to any mycete of any kind). These organs vary greatly in size, shape, and number, depending on the host species of leech. The associated symbionts also vary, some species are associated with Alphaproteobacteria while others are associated with different species of Gammaproteobacteria or Alteromonadales.
Many papers have described the organs and associated bacteria. These two papers, here and here, give an idea of what the mycetome organ looks like and the different types of bacteria associated with leeches. On the more exotic side, this paper describes the association between Psychromonas bacteria and deep sea leeches!
Finally, trying to understanding the exact role of the bacteria in the association this paper, here, reconstructed and examined the genomes of four different species of endosymbionts. They found that the bacterial genomes are reduced, suggesting a strict association between leech and symbiont. The study also describes the metabolic convergence of the different genomes showing that, although the symbiotic association between host and bacteria might have happened in separate events, the bacteria independently specialized to complement a critical host need: Vitamin B!