The effect of symbionts on the physiology of Hirudo medicinalis, the medicinal leech

At least two organs of Hirudo medicinalis, the medicinal leech, are colonized by bacterial symbionts, the crop of the digestive tract and the bladders of the excretory system. The digestive tract is usually colonized by a pure culture of Aeromonas veronii biovar sobria. This unusual specificity eases the experimental investigation of this symbiosis. Using a colonization assay, we have shown that the symbionts proliferate rapidly for the initial 12 h and persist at a density of 5x10(7) CFU/ml for at least 7 days in the crop of H. medicinalis. From the bladders of H. medicinalis, an Ochrobactrum species can be cultured that is related to Brucella, an intracellular pathogen, and to Rhizobium, an intracellular symbiont. The effect of the symbionts on the host was investigated in two studies that fed the animals antibiotic-containing blood. These experiments suggested that bacteria are responsible for 56% of the increase in the secretion of ammonia and for 60% of the increase in the oxygen consumption of the animal after feeding. While it is clear that the bacteria effect the physiology of the animal, an effect on the growth rate and reproductive fitness of the animals remains to be demonstrated. These hypotheses can be tested because we can breed the animals in the laboratory and raise the juveniles.