An introduction to Symbionts: What exactly is it?

Symbiosis is the co-existence of two different organisms that live in close physical association with each other.

Symbiosis is the co-existence of two different organisms that live in close physical association with each other. These relationships can be mutually beneficial or one sided, and the absolute definition is controversial among scientists. While some think symbiosis should only refer to mutualistic interactions, others argue that all close interactions should apply, including parasitism.

Ecto-symbionts  are species’ that live on another, such as mistletoe growing on trees and endo-symbionts are species that live within another. Not all associations are so intimate however, and symbiosis can apply to species that live closely but are not physically attached.

For example, this shrimp and the goby fish spend most of their lives in the same burrow, helping each other out. The goby guards the burrow with its superior eyesight, as the shrimp digs way at his hole and in return removes dirt and parasites from the goby’s body.

Why they’re important

Co-existence of species can be found almost wherever you look. Every plant in your garden has flowered thanks to the symbiosis of plants and pollinating insects such as bumblebees.

Some symbionts live within another animal (endo-symbionts), such as the bacterium Zoamastogopera, found in the stomach of termites, which aids in the digestion of cellulose (which is quite important when your diet solely consists of wood).

Similarly, you are probably familiar with various yoghurt adverts telling you about the ‘good bacteria’ in your digestive system, which essentially perform the same task of helping with digestion and energy uptake of fermenting carbohydrates.

So, symbionts are important throughout the food-chain, usually being unrecognised and unnoticed in our digestive systems.

Endo-symbionts often have more complicated relationships with their hosts than in appears. Wolbachia is a bacteria species that has been discussed at length in science due to their manipulative nature. Wolbachia can survive in a huge variety of tissues, but one in particular is almost always targeted.

Sometimes referred to as “gonad-chomping bacteria,” Wolbachia usually infects the host’s gonads, where it helps promote its own spread by manipulating  cells to ensure that infected females produce more infected eggs.

There are various different processes that the bacteria use to increase their infectivity, some strains taking different approaches depending what species they are in. It’s scarily impressive how intelligent a bacterium can be.

Like most endo-symbionts, Wolbachia can be inherited from parents to offspring. Symbiotic bacteria are almost always inherited by the mother, because of the large amount of space in her egg cells in comparison to the father’s sperm, which is already jam-packed with a nucleus with very little room to spare. 

Questions of benefits

This is also why mitochondria are only ever inherited from the mother. This has made evolutionary partnerships fairly straightforward, as every offspring inherits a singly strain of the microbe and they can co-exist mutually beneficially, or at least harmlessly.

However, it has been discovered by researchers in Japan that the sperm cells of the green rice leafhopper contain several copies of Rickettsia bacteria, meaning that if these infected sperm fertilise eggs, they can pass their copies of Rickettsia to the next generation.

This may appear to be harmless, as the presence of bacteria within cells is so common, but it can cause problems is the mother and father leafhopper are both infected with Rickettsia or different strains.

It is not known whether the leafhoppers gain any actual benefit from their symbionts, but they do not suffer any apparent loss. However, if two different bacterial strains develop within the co-infected offspring, conflict can arise within the host, causing harm or disease.

The adaptations that allow one Rickettsia strain to beat another may allow them to cause disease, meaning that the endo-symbiont flips from being a harmless (potentially beneficial) bacterium into a harmful one.

Everything is in a delicate balance. Upset the natural occurrence of bacteria within your gut, and you can get a nasty opportunistic infection. Just because a bacterium is labelled as “good” and “helpful,” it doesn’t mean that they are also useful elsewhere, or in abnormally high numbers.

We have a lot to thank bacteria for, but there’s also a great deal to be wary of.


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Image: jurvetson / Flickr