"Exploring 5 Successful Microbial Symbioses"
[from 'Technology Networks']
Our environment is filled with a multitude of microbes, some harmful others not. However, some organisms have taken coexisting a step further and developed relationships, or symbioses, with microbial partners. Symbiosis was first defined in 1879 by de Bary as “the living together of unlike organisms” [De Bary, A. (1879) Die Erscheinung der Symbiose. Verlag von Karl J. Trubner, Strassburg] and may take the form of a simple one host - one symbiont arrangement, or more complex webs of inter-connecting relationships, such as that seen in the guts of many animals. Symbiotic relationships can be beneficial to both organisms (mutualism), beneficial to one whilst the other is unaffected (commensalism) or benefit one whilst the other is harmed (parasitism). Whatever the relationship types though, for both parties to survive, there is often a close relationship between the host’s immune system and the microbial symbiont.
1. Lighting up the depths with bioluminescence
Nocturnal Hawaiian bobtail squid (Euprymna scolopes) spend their days buried in the sand but come nightfall they emerge to feed. To predatory fish below they would be easily detectable as a dark shape against a bright sky, especially on moonlit nights, making them vulnerable. However, they have overcome this obstacle by evolving light organs filled with the bioluminescent bacteria Vibrio fischeri, which offer them camouflage. The squid are not born with the bacteria and as juveniles must colonize their light organs from free bacteria in the sea water. Strangely, work has shown that the adult squid expel around 90 % of their symbionts from their light organ daily, termed “venting”. This does however provide an inoculum with which newly hatched squid can populate their light organs. But what’s in it for the bacteria? Well analysis has shown that the environment in which the bacteria exist in the light organ is rich in amino acids produced by the host suggesting one reason why it would benefit them to stay.
2. Food for thought
The human gut, as with the gut of many animals, has a complicated interconnecting relationship with a host of bacterial species which make up the gut microbiome. We rely upon them to help breakdown our food and in return they receive a ready nutrient source. Everyone these days has heard about the importance of having healthy gut bacteria and it’s difficult to avoid the multitude of adverts for “probiotic this” or “boost your gut bacteria with that”. A lack of diversity or bias to certain species of gut bacteria have been associated with increases of some conditions, diseases and obesity. An unbalanced diet may be the source of the problem, but when we undergo treatments, like antibiotic therapies to kill off pathogenic bacteria, as an unwanted side effect, our beneficial symbionts may also be killed off. For many people, their gut flora will recover over time naturally given the correct diet, but for some unlucky individuals this is not the case. This is particularly problematic for sufferers of Clostridium difficile infection where a vicious cycle of antibiotic treatment to kill the infection and re-infection due to a lack of healthy bacteria can lead to long term issues. To restore a healthy balance of symbionts, scientists are increasingly turning to fecal transplantation to give them a head start. Results have been promising so far with up to 90 % effectiveness in the treatment of recurrent C. difficile infection. There are still many aspects to consider, such as the possibility of passing on unwanted infectious disease, the best method of transplantation, and monitoring which are being addressed by experts.
3. Symbiosis - related genes don’t always stay put
For the legume family, nitrogen-fixing bacteria (rhizobia) that exist in the plant’s root nodules are of key importance to enable survival in nitrogen-poor soils. Approximately 70 % of legume species play host to these symbionts that convert atmospheric nitrogen to usable ammonium compounds. In return, rhizobia receive photosynthates from the host plant. The genes that encode the nitrogen fixing abilities of rhizobia are often located on plasmids or symbiotic islands so, much like antibiotic resistance genes, they can be horizontally transferred between bacterial strains. Acquired genetic material, such as symbiotic gene sets, can be identified at the genome level as they will appear phylogenetically divergent from the core genome of the bacteria. The ability to transfer genes and their corresponding functions has important consequences for both bacterial and legume species, enabling symbionts that may have previously been incompatible to local legume species to acquire the necessary genes to form a uccessful relationship.
4. Parasitoid wasps deliver a double whammy
Parasitoid wasps lay their eggs in the caterpillars of moths and butterflies. However, it is their symbiotic relationship with polydnaviruses that make their life cycle possible. The viral DNA integrates itself into the wasp’s genome but only replicates in part of the wasp’s ovary. When the wasp injects its eggs into the caterpillar, viral DNA is therefore delivered to the caterpillar too. The virus modulates the host’s immune system, enabling the eggs to grow, hatch and mature inside the host. Recent evidence shows that the polydnaviruses have even more far reaching impacts. The virus induces the caterpillar to reduce salivary glucose oxidase levels, resulting in a reduced immune response from the plant on which the caterpillar is feeding. Consequently, caterpillar growth is increased which benefits growth of the parasitoid offspring and therefore persistence of the virus into the next generation.
5. Controlling parasites by manipulating their symbionts
Many bloodsucking insects rely on bacterial symbionts for essential nutrients to complement their unbalanced diets. Meanwhile many are themselves hosts to disease-causing microbes that they transmit when moving between infected and uninfected individuals to obtain their blood meals. An example of this is seen in the Triatominae bugs (also called kissing bugs and assassin bugs), which transmit Trypanosoma cruzi, the protozoa that causes Chagas disease between infected individuals. There is currently no vaccine for Chagas disease which, if left untreated, can be lifelong and life threatening. Triatominae bugs have been found to contain Rhodococcus species in their gut where they synthesise vital group B vitamins. The Rhodococcus symbionts are absent in the nymph stage and are acquired from other insects via coprophagy. This has led researchers to explore an innovative approach for controlling the pathogenic T. Cruzi to assist disease control measures. Scientists have created genetically modified Rhodococcus that carry genes encoding antitrypanosome factors, such as cecropin A. Early results show great promise in disease control but further work is required to evaluate the impact and safety of introducing exogenous genes into the symbiont population.
