Pathogenic bacteria train their defence in lakes and oceans

A lot of bacteria have defence mechanisms to survive a predator. Peter Mathisen at Umeå University has studied predation resistance among bacteria in lake- and ocean water. He has found some astonishing links between the aquatic environment and the spreading of diseases such as tularaemia. Bacteria seem to have similar mechanisms to defend themselves against small protozoa in aquatic environments as well as against the immune system in mammals. The results are important for assessments of aquatic environments at risk of spreading pathogenic bacteria.

It is well known that toxic algae species are favoured by eutrophication. Peter Mathisen proves that the redundant nutrients also can lead to an increase in the predation resistance among the bacteria. Photo: Andre maslennikov/Azote.

In one single liter of lake- or ocean water there are approximately one billion bacteria. Each bacteria Is so small that you just barely can see it even through a microscope, but bacteria play an important role in the cycling of nutrients in the environment. When algae or other organisms die they will be decomposed by bacteria. In a way, bacteria are to be regarded as the sewage plants of the waters.

Survives inside the host

But as we all know, some bacteria are pathogenic, and can cause diseases in humans and other mammals. The pathogenic bacteria have special ways of defending themselves against predators – they can survive inside the cells of the host animal and thereby spread the disease. An example of such a bacteria is Francisella tularensis, which causes the disease tularaemia among mammals. Francisella tularensis is not only found within the infected cells of mammals, it can also be found in lake water.

Thrilling link

When a mammal is infected by a pathogenic bacteria the immune system is triggered, and so called macrophages mobilise to engulf the infecting bacteria. Some bacteria are resistant against the macrophage’s attempts to dissolve the bacteria, and can survive, replicate and cause the disease in the animal. There is a thrilling link between pathogenic bacteria that infect landliving mammals, the aquatic environment and its inhabitants and the environmental status of our lakes and sea areas. Peter Mathisen points out important connections that haven’t earlier been known, and that will have a high relevance for risk assessment concerning the spreading of pathogenic bacteria worldwide.

Similar to aquatic bacteria

Peter Mathisen (right) discussing results with his opponent, professor T. Frede Thingstad, during the dissertation. Photo: Agneta Andersson.

The survival strategy of Francisella tularensis is surprisingly similar to the strategy found of other aquatic bacteria. In the aquatic environment protozoa – tiny, single-celled organisms – are the enemies of the bacteria, and live on eating the even smaller bacteria. Normally the bacteria die when engulfed by the protozoa, but some particularly resistant types of bacteria can survive inside the protozoa.

There are different forms of protozoa, for example flagellates and ciliates. The protozoa have the ability to suppress the bacteria population, and just like the bacteria they are found in large amounts in lake- and sea waters.


To be able to defend yourself

A lot of organisms have developed mechanisms to defend themselves against predation. Plants might be toxic to animals, camouflage coloured animals can hide from predators, and fish schools make the attacker confused. In the tiny world of bacteria and protozoa there are also a load of different defence mechanisms, even if they are a lot more difficult to study.

Some examples are bacteria that aggregate, or bacteria that grow in size, making the engulfing by protozoa more difficult. Or, as in the case of Francisella tularensis, the have found ways to survive even when eaten by a protozoa or a human macrophage. A lot of these predation resistant bacteria are also pathogenic, with the ability to cause diseases in humans and other mammals.

A protozoa has engulfed a large amount of bacteria. Photo: Johanna Thelaus/FOI.

Predation affects the bacteria community
In experiments, Peter Mathisen exposed common marine bacteria from the Gulf of Bothnia to a predator, the little ciliate Tetrahymena pyroformis. In the laboratory the survival and growth of the bacteria was studied in the presence of different densities of the ciliate, as well as with different nutrient conditions. The nutrient experiments were performed to get a picture of how the defence mechanisms change in eutrophication situations.

The results showed clearly that the bacteria community changed when exposed to the predator. What might have seemed as a quite homogenous mixture of bacteria proved to be a very diverse system, with many different strategies to survive. Some of the bacteria had a high resilience from the start, while others clearly developed the predation resistance when exposed to the ciliate. In nature bacteria are exposed to varying degrees of predation, and the study shows that the bacteria community seems to have the ability to increase the resilience when the risk for predation increases.

Close link to pathogens
It seems as if the resistance that bacteria develop against protozoa also works against human macrophages. An increasing occurrence of predation resistant bacteria in eutrophic waters will therefore lead to an increased risk of spreading of pathogenic bacteria. Results presented in Peter Mathisens thesis indicate that aquatic environments act as “gyms” for bacteria, where the presence of predators train their defence against being killed and eaten up. The mechanisms are astonishingly similar when it comes to surviving a protozoa or a human macrophage.

Eutrophication leads to increased predation resistance

Many ocean areas, among them the Baltic, are subjected to eutrophication. It is well known that the nutrient redundancy affects the food web negatively, and that inedible, toxic algae species are favoured. A less known effect is that the growth of inedible bacteria is promoted by the nutrients. The bacteria make use of the redundant nutrients, and the protozoa, living on the bacteria, increase as well. Thereby the predation pressure on the bacteria will increase as a result of the eutrophication. The increased predation pressure leads to an increase in the predation resistance among the bacteria.

Important for risk assessment

In other words, changes in the nutrient status of an aquatic environment could lead to an increase in the spreading of pathogenic bacteria. Climate change could have the same effect. The results presented by Peter Mathisen in his thesis are therefore important for assessment of aquatic environments at risk for spreading pathogenic bacteria. The aquatic environments seem to be far more important in this respect than was earlier known.

Text: Kristina Viklkund


Mathisen, P. 2017. Environmental factors selecting for predation resistant and potentially pathogenic bacteria in aquatic environments. Umeå University.

Page Editor: Kristina Viklund

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