The Digestive-Tract Symbiosis of Hirudo verbana, the Medicinal Leech
While the digestive tract of most animals is colonized by a very complex microbial community, the digestive tract of the medicinal leech is colonized by an unusually simple microbioal community. Only two bacteria colonize the crop of the leech: Aeromonas veronii biovar sobria and a bacterium that is related to Rikenella, a member of the Bacteroidetes. This simplicity helps us to investigate the interaction between the symbionts and the host and between the two symbionts.
Aeromonas veronii is an interesting bacterium on its own right, because it is also a causative agent of disease in humans. The spectrum of illnesses that A. veronii has been implicated in includes wound infections, diarrhea and septicemia. The increased usage of medicinal leeches in modern medicine and associated infections have led to an increased interest in the bacterial flora of the leech. Fortunately, these infections usually can be prevented by the administration of preemptive antibiotics. In patients with a normal immune response and sufficient blood flow to the site of attachment, the application of the medicinal leech is generally considered to be safe.
In addition to being a symbiont of the leech, A. veronii, can cause septicemia in mice. The ability of A. veronii to colonize different hosts in either a cooperative or pathogenic fashion and the specificity of the association with H. verbana provide us with an opportunity to address fundamental questions about cooperative and pathogenic digestive-tract associations.The host
The mdicinal leech is usually sold as H. medicinalis but a recent study by Dr. Mark Siddall's group revealed that the leeches commonly sold by the commercial suppliers are H. verbana. Prior to this discovery H. medicinalis was approved by the FDA as a medical device to treat venous congestion. The medicinal leech, H. verbana, is a segmented animal that is related to the earthworm. Leeches possess several evolutionary advanced features including compound eyes (one eye that is composed out of many individual eyes) and cephalization (possessing a head). Within the anterior (front) sucker are three jaws that cut the skin. During feeding, H. verbana secretes compounds with various functions including vasodilator (dilates blood vessels) and anticoagulant (prevents the blood from clotting) properties. In a single feeding event, the medicinal leech can consume over five times its body weight. The ingested blood is stored in the large crop of the digestive tract where also the extracellular symbionts are found. From the ingested blood, water and salts are removed. After such a large blood meal, the animal can go for over 6 months without feeding. Leeches are simultaneous hermaphrodites (both male and female at the same time). After an internal fertilization, the leech secretes a cocoon, that contains several individual eggs and is stripped off over the anterior end of the animal. It has been suggested that during this deposition the symbionts are transmitted from the parent to a nutritious fluid bathing the eggs in the cocoon. After the juveniles hatch from the egg, they remain for some time within the cocoon and consume the nutritious fluid, possibly acquiring the symbionts at this time. When the juvenile leeches leave the cocoon, they are fully pigmented and functional, but they cannot bite through mammalian skin. Thus it is thought that their first blood meal is from an amphibian.
The symbionts
Investigators in the 1940s and -50s isolated pure cultures of bacteria from the digestive tract of the medicinal leech and called them endosymbionts. Initial biochemical characterization suggested that the symbionts are Aeromonas hydrophila. Since these studies the Aeromonas taxonomy has undergone many changes and, in a study using biochemical tests and 16S rRNA sequencing, we showed that the symbiont is A. veronii. These studies relied on the ability to grow the bacteria away from the leech, but most bacteria cannot be grown in the laboratory. When we used an approach that did not rely on growing the bacteria, we discovered a second bacterium living in the crop of the leech. This bacterium was most closely related to Rikenella microfusus, a member of the Bacteroidetes. Bacteroidetes are common inhabitants of vertebrate guts and cannot grow in the presence of oxygen.The early investigators proposed several functions in which the symbionts may benefit the host.
1. The symbionts may digest the ingested blood. Until very recently, no host digestive enzymes were detected and Aeromonas is well known to produce numerous extracellular hydrolytic enzymes. However, despite the ability of the symbionts to lyse erythrocytes (beta-hemolysis), the ingested blood cells are stored intact for many months, suggesting that hemolytic ability of Aeromonas is inhibited. More recently, it was shown that the host releases proteases into the intestinum of the digestive tract where the actual digestion takes place.
2. The nutritious quality of blood is poor, for example it contains very low amount of the vitamine B12. Thus it has been suggested that the symbionts serve as a source of nutrients to the host.
3. Because it is so unusual to find a single species of bacteria in the digestive tract and early in vitro results suggested that Aeromonas can inhibit the proliferation of other bacteria, it has been proposed that the symbionts prevent the growth of non-symbiotic bacteria in the crop. However, there is little experimental evidence supporting any of these beneficial functions.
Factors contributing to the specificity of the symbiotic interaction
The specificity of this symbiosis is important for the biology of the animal and for the medical application of the leech. To date our research has revealed multiple factors that contribute to the unusual simplicity. (1) The complement system of the ingested vertebrate blood remains active and kills sensitive bacteria. (2) Leech hemocytes (macrophage-like cells) phagocytose sensitive bacteria. (3) Additional antimicrobial compounds appear to be released either by the leech or by the bacterial symbionts. But these compounds have not been identified. In addition, the native symbiotic flora may outcompete non-symbiotic bacteria for nutrients and space.
This symbiosis may be thought of as an interaction dependent upon the invertebrate host, the symbiotic bacteria and the vertebrate host that the leech feeds on.Current research topics include:
- Identification of factors that contribute to the specificity of the symbiosis.
- Identification of bacterial genes that are required to colonize the leech.
- Culturing the Rikenella-like symbiont.
- The use of 16S rRNA gene sequences to identify Aeromonas species.
- Characterize the symbiont present in the nephridia and bladders of the leech.
Below you will find various web pages that we maintain where you can find out more about the use of medicinal leeches, where to purchase them and labs that study beneficial symbioses between bacteria and animals.
Modern and ancient medical use of the medicinal leech
Contact information for leech farms
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