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Good bacteria more useful when transmitted from the mother

Good bacteria are more useful if transmitted directly from one generation to the next instead of other ways of transmission. The results are valid for bacteria in animals as well as in plants according to a study from the universities of Lund and Oxford.
A squid, Euprymna.
An example of useful bacteria is bioluminescent bacteria that make it possible for the squid to send out light so that its' body contour is difficult to see. It works as protection against predators. Photo: C. Frazee

Useful bacteria are found in animals and plants. Some bacteria help to defend against parasites, while others are good for the nutrition by producing amino acids and vitamins or contributing to photosynthesis. Until now, no one has known why some useful bacteria are vital for the host, whereas others are not.

Charlie Cornwallis, assistant professor at the Department of Biology at Lund University in Sweden, is one of the researchers that have mapped out the symbiosis between the host organism and the bacteria. Together with colleagues from Oxford University he has studied 106 symbiotic relationships between bacteria and hosts like insects, worms, plants and fungi.

The results show that the mode of transmission to a great extent decides how important the bacteria are for the host organism. Bacteria transmitted from one generation to the next are more important for the host than bacteria that have been picked up in other ways. If bacteria transmitted between generations are removed it will do double as much harm to the host’s health compared to when bacteria picked up from the environment are removed.

The results also show that bacteria that are good for the host’s nutrition in general are more important than bacteria that, for example, help to protect against parasites. The ”nutrition bacteria” are beneficial for the host all the time, while the ”parasite bacteria” are useful when there are parasites around.

When the mode of transmission is combined with function the difference in importance is significant. The host’s health and fertility deteriorate three times as much when ”nutrition bacteria” are removed compared to ”parasite bacteria”.

”Put together our results show that host organisms have most use for and rely most on useful bacteria that are transmitted from one generation to the next and that help the host’s nutrition. This seems to be the general pattern when we look at all the different organisms that are included in our study,” says Charlie Cornwallis.

Bacteria that are transmitted between generations also profit if the host is fertile and in good health. Such a host is likely to have more offspring and is therefore more able to transfer the bacteria to another generation. Bacteria transmitted via the environment have less to gain from a healthy host.

”The significance of symbiotic relationships is the key to understand why the evolution has taken certain leaps within some species but not others,” says Charlie Cornwallis.

The results have been published in an online article in Nature Communications.

Jan Olsson

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