The Digestive Tract Symbiosis of Hirudo medicinalis

In contrast to the digestive tract communities residing in most animals (Savage 1977; Demaio et al. 1996; Lilburn et al. 1999), the digestive tract of Hirudo medicinalis appears to be colonized by a relatively simple microbial community (Lehmensick 1941; Busing et al. 1953; Graf 2000). Only one symbiont, an Aeromonas sp., was consistently cultured in several investigations and culture independent approaches suggest the presence of only a few additional taxa (described below). This apparent simplicity is intriguing and suggests that several mechanisms are involved in establishing and maintaining this simplicity.

The biology of the leech is important for identifying possible factors that may contribute to the specificity of this symbiosis and I want to begin by briefly introducing this fascinating parasite that was used extensively for bloodletting in the 1800s (the biology of H. medicinalis is reviewed in the monograph by Sawyer (1986)). The medicinal leech feeds exclusively on blood. In a single blood meal, H. medicinalis can consume over five times its body weight. During the feeding, the leech releases powerful anticoagulants and vasodilators. Their activity is so potent that the blood continues to flow for about 15 min from the wound after the engorged leech falls off. This "blood-letting" ability has been utilized for centuries and the heavy collection in conjunction with habitat destruction has led to the near extinction of the medicinal leech in Western Europe (Graf 2000; Carter 2001). The medicinal use of leeches has made a recent revival to rescue tissue with venous congestion after microvascular plastic surgery (Henderson et al. 1983; Whitlock et al. 1983; de Chalain 1996). Interestingly for microbiologists, in up to 30% of these patients, wound infections with the digestive tract symbiont, Aeromonas sp., were reported (Whitlock et al. 1983; Dickson et al. 1984; Abrutyn 1988; de Chalain 1996;

Sartor et al. 2002). Currently, these infections are prevented with a preemptive antibiotic treatment.

The large blood meal is stored in the crop where water and salts are absorbed from the intraluminal fluid (ingested blood, Fig. 1). Within 48 h, approximately 50% of the gained weight is lost, due to the loss of water from the crop (Sawyer 1986). In addition, ions are also absorbed from the intraluminal until it is isoosmotic with the leech hemolymph (approximately 20 milliosmole). The erythrocytes are stored for months, apparently intact, in the crop despite of the presence of ^-hemolytic bacteria. Whether or not a portion of the erythrocytes is lysed or punctured in the crop to release nutrients to the resident symbionts has not been determined. The physical breakdown of all of the erythrocytes occurs in the intestinum. The animals can survive for over 9 months between blood meals. This long time between feeding events is likely to be an important selection pressure on the symbionts.

Fig. 1. Drawing of a ventrally dissected H. medicinalis. The crop houses the ingested blood. A. veronii biovar sobria is found in the crop and intestinum (modified from Graf 2000)

Diagram Hirudo

Fig. 1. Drawing of a ventrally dissected H. medicinalis. The crop houses the ingested blood. A. veronii biovar sobria is found in the crop and intestinum (modified from Graf 2000)

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