Mark E. Siddall
Curator of Annelida and Protozoa
American Museum of Natural History
siddall at amnh.org

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Bacterial Symbioses in Leeches

Gut flora of medicinal leeches
Our collaborations with Joerg Graf at University of Connecticut have continued resulting in a comparative analysis of the gut flora of European medicinal leech species, confirming the presence of a single culturable species of Aeromonas in each alongside a single unculturable species in Bacteroidetes. We are presently bringing to completion a more detailed analysis of the symbiotic crop flora of a greater diversity of leeches from across 3 continents.

The phylogenetic placement of culturable bacteria from the gut of medicinal leeches demonstrates the diversity of Aeromonas species found in different species of leech, as well as the diversity of unrelated Bacteroidetes as cosymbionts.

Leech esophageal endosymbionts
Invertebrate taxa that utilize nutritionally poor diets often are reliant on obligate bacterial endosymbionts. Vertebrate blood is severely deficient in essential B vitamins and some amino acids (Wigglesworth, 1939) and a broad array of invertebrates that feed on blood throughout their life cycle harbor bacterial symbionts providing some of these essential nutrients to their hosts.

Reichenow was the first to recognize the presence of bacterial symbionts in the so-called "esophageal glands" of placobdellid leeches. These glands, now referred to somewhat inappropriately as mycetomes are paired sac-like caeca, each opening into the esophageal lumen about mid-way between the base of the muscular proboscis and the beginning of the first pair of crop caeca (where blood is stored for digestion between successive feedings). This arrangement (see right) is characteristic of all leeches in the Placobdella group.

Our characterizations of the symbionts demonstrated that they are alphaproteobacvteria, closely related to the nitogen fixing rhizobiales.