Ingested blood contributes to the specificity of the symbiosis of Aeromonas veronii biovar sobria and Hirudo medicinalis, the medicinal leech.

Hirudo medicinalis, the medicinal leech, usually carries in its digestive tract a pure culture of Aeromonas veronii bv. sobria. Such specificity is unusual for digestive tracts that are normally colonized by a complex microbial consortium. Important questions for the symbiotic interaction and for the medical application after microvascular surgery are whether other bacteria can proliferate or at least persist in the digestive tract of H. medicinalis and what factors contribute to the reported specificity. Using a colonization assay, we were able to compare experimentally the ability of clinical isolates and of a symbiotic strain to colonize H. medicinalis. The symbiotic A. veronii bv. sobria strain proliferated well and persisted for at least 7 days inside the digestive tract. In contrast, the proliferation of Pseudomonas aeruginosa and Staphylococcus aureus was inhibited inside the animal compared to growth in the in vitro control, indicating that the ingested blood was modified within the digestive tract. However, both strains were able to persist in the digestive tract for at least 7 days. For an Escherichia coli strain, the viable counts decreased approximately 1, 000-fold within 42 h. The decrease of viable E. coli could be prevented by interfering with the activation of the membrane-attack complex of the complement system that is present in blood. This suggests that the membrane-attack complex remained active inside H. medicinalis and prevented the proliferation of sensitive bacteria. Thus, antimicrobial properties of the ingested vertebrate blood contribute to the specificity of the A. veronii-H. medicinalis symbiosis, in addition to modifications of the blood inside the digestive tract of H. medicinalis.