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Aeromonas : Friend or Foe?
Elaine Mirkin and Joerg Graf
The medicinal use of leeches dates back to early civilization. In the 18th and 19th centuries, leeches were commonly used in France for bloodletting. In the early 1980s, the application of medicinal leeches was found to be useful to patients after reconstructive surgery in cases where the blood circulation was poor.
Yet 20% of these patients developed infections. Scientists attending the recent
international congress on invertebrate reproduction and development in South
Africa think they may have found a possible culprit.
Why is it that a microorganism can cause disease in one organism yet perform beneficial functions in another? This question can be addressed by studying Aeromonas, a bacterium that is a harmless symbiont in the digestive tract of the medicinal leech and also a pathogen of humans. The digestive-tract symbiosis is very simple involving only Aeromonas and the leech. This simplicity allows one to learn fundamental aspects about the mechanisms by which bacteria and animals interact and by comparison determine similarities and differences between mutualistic and pathogenic associations.
The digestive tract of animals is usually colonized by a complex community of microorganisms. In the human digestive tract, over 400 species of bacteria can be found and the total number of microbial cells exceeds 10-fold the number of human cells. The bacteria are thought to perform several beneficial functions including providing nutrients like vitamin K, preventing pathogenic bacteria from colonizing, and stimulating the immune system. It is clear that these microorganisms perform important roles for our health, yet we know very little about these bacteria and how they interact with our body. One major difficulty is the complexity of the microbial community.
One unusual aspect of the Aeromonas - leech symbiosis is that only Aeromonas is found in the digestive tract of the leech. What factors contribute to this unusual specificity? A study by Indergand and Graf, at the University of Berne, Switzerland, provided the first clue. They tested three different bacterial pathogens that were isolated from humans and found these bacteria could not grow as well inside the animal as the native symbiont. One pathogen was unable to grow in blood unless some of the blood's antimicrobial properties were inactivated. This treatment also permitted this bacterium to grow better inside the medicinal leech, demonstrating that antimicrobial properties of the ingested blood actually remained active inside the leech and contributed to the unusual species specificity. In contrast, the growth of the other two pathogens was inhibited only inside the animal, suggesting a modification of the ingested blood. The host and/or the symbiont may be responsible for these modifications, the nature of which is not known but it presents an exciting avenue for research.
If this association is indeed a mutualistic symbiosis, the bacteria should provide the host with a benefit. Three possibilities have been proposed. The bacteria may digest the ingested blood for the host. The bacteria may provide nutrients to the host. The bacteria may prevent other bacteria from growing in the host's digestive tract. All three hypotheses remain untested. One difficulty has been that it is very hard to demonstrate a nutritional defect because the animals can go for over nine months without feeding and still survive. It is now possible to breed the animals in captivity and to use juveniles that grow faster than adults to test if the presence or absence of bacterial symbionts makes a difference in the growth of the animal.
The medicinal use of leeches dates back to early civilization. In the 18th and 19th centuries, leeches were commonly used in France for bloodletting. In the early 1980s, the application of medicinal leeches was found to be useful to patients after reconstructive surgery in cases where the blood circulation was poor. However, due to the weakened immune system, infections occurred in 20% of those patients, presumably by the digestive tract symbiont. The infections can usually be prevented by a preemptive antibiotic treatment, but the occurrence of infections suggest Aeromonas is not always a friend as it has the potential to cause disease in humans. Aeromonas also causes blood infections and has been implicated in causing diarrhea in humans. This points to the ability of one bacterium to be both a mutualistic symbiont and a pathogen depending on the host.
The unusual specificity of the Aeromonas - leech symbiosis makes it an ideal model to study the digestive tract interaction of bacteria and animals. The simplicity allows one to investigate the interaction using molecular techniques to create mutants from symbiotic bacteria and then test how well the mutants can colonize the medicinal leech. This is the approach taken by Graf's research group in Berne, Switzerland. Bacteria that have lost the ability to colonize are called symbiosis mutants. These mutants can be further analyzed, for example, identification of the inactivated gene can provide information about what the bacteria needs in order to colonize the host. Basically each mutant is like an environmental probe telling us what property is required to survive and proliferate in the digestive tract. The genes that are important for symbiosis can be compared to those that are important in causing disease. These comparisons will help scientists learn more about the similarities and differences between benign and pathogenic associations.
This article is based on a talk given at the 9th International Congress on Invertebrate Reproduction and Development at the Rhodes University in Grahamstown, South Africa. Elaine Mirkin is a free-lance writer in Berne, Switzerland, and Joerg Graf is a group leader at the Institute for Infectious Diseases of the University of Berne. For further information please contact
joerg.graf@ifik.unibe.ch or visit our website
http://www.imm-km.unibe.ch/projekte/symbiosis/sym.html
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