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Bacteria that divided their metabolic labor (left colony) grew faster than
bacterial cells that produced all amino acids on their own (right colony).
What bacteria teach us about division of labor
Bacteria grow faster if they feed each other. When they divide their metabolic labor, they grow faster than bacterial cells that produce all their amino acids on their own.
Division of labor is more efficient than a struggle through life without help from others – and this is also true for microorganisms.
Researchers in Experimental Ecology and Evolution at the Max Planck Institute for Chemical Ecology, along with colleagues at the Friedrich Schiller University in Jena, Germany, came to this conclusion when they performed experiments with microbes.
The scientists worked with bacteria that were deficient in certain amino acids and depended on a partner to provide the missing nutrient.
Bacterial strains that complemented each other’s need by providing a missing required amino acid, showed a fitness increase of about 20%, relative to a non-deficient strain without a partner.
This result helps to explain why cooperation is such a widespread model of success in nature.
The results are published in The ISME Journal, 28 November 2013.
Ecology and evolution: close relatives
Each life form on our planet has to adapt to its environment as well as it can. Apart from adapting to climate conditions and food supply, each species must get along with other organisms in its' habitat.
Over the course of evolution, species adapt continuously to each other and to their environment by changing their genetic features. Which is why we see cold resistant species living at the poles and heat resistant species living in the deserts. Nutritional needs and metabolic regulation underlie the principle of evolution — and microbes follow the same rules.
"No matter where you look: Microbial communities can be found in almost every habitat you can think of,” says Christian Kost, leader of the research group Experimental Ecology and Evolution at the Max Planck Institute for Chemical Ecology in Jena, Germany.
Microbes often live in symbiosis with higher organisms, but they also cooperate with each other in order to optimize resources available to them.
A look at the genome of cooperating bacterial strains shows that some of them are unable to perform all of their vital metabolic functions on their own. Instead, they rely on cooperative partners, other organisms that provide nutrients they cannot produce anymore and which inhabit their environment.
However, the result of such cooperation is a risky — if one partner is lost, the other dies as well.
Can such a dependency be a trait selected for and maintained in a bacterial population? Is this idea compatible with Darwin’s theory of the “survival of the fittest”? If so, cooperating partners should perform as well, if not better, than microbes without a partner.
Synthetic Ecology: simulating ecological limits in a test tube
Naturally evolved, symbiotic bacterial communities do not do well in laboratory situations. Therefore, Max Planck scientists used a synthetic model: Escherichia coli bacteria. These bacteria were genetically modified so that one strain was unable to produce the amino acid tryptophan, but produced all other amino acids in high concentrations. When this strain grew in culture with another strain unable to produce the amino acid arginine, both strains were able to feed each other.
Amazingly, the co-culture experiment showed that growth of amino acid modified bacteria increased by 20% when compared to normal bacterial strains able to produce all essential amino acids by themselves.
The inability of the bacterial strain to produce an essential amino acid had a positive effect on its growth — when a partner was present to compensate for that loss. In explanation: specialized production of one needed amino acid made the bacterial cells more efficient, resulting in faster growth.
Interestingly, the two cooperating, amino acid exchanging strains even outcompeted a self-sustaining normal (wild-type) bacterial strain.
The research results from Christian Kost’s lab illustrate why symbiotic relationships with bacteria are so prevalent.
In the course of evolution, an association may get so close that the mutual partners merge into a new, multicellular organism.
Cross-feeding interactions, in which bacterial cells exchange costly metabolites to the benefit of both interacting partners, are very common in the microbial world. However, it generally remains unclear what maintains this type of interaction in the presence of non-cooperating types. We investigate this problem using synthetic cross-feeding interactions: by simply deleting two metabolic genes from the genome of Escherichia coli, we generated genotypes that require amino acids to grow and release other amino acids into the environment. Surprisingly, in a vast majority of cases, cocultures of two cross-feeding strains showed an increased Darwinian fitness (that is, rate of growth) relative to prototrophic wild type cells—even in direct competition. This unexpected growth advantage was due to a division of metabolic labour: the fitness cost of overproducing amino acids was less than the benefit of not having to produce others when they were provided by their partner. Moreover, frequency-dependent selection maintained cross-feeding consortia and limited exploitation by non-cooperating competitors. Together, our synthetic study approach reveals ecological principles that can help explain the widespread occurrence of obligate metabolic cross-feeding interactions in nature.
The ISME Journal , (28 November 2013) | doi:10.1038/ismej.2013.211
Samay Pande, Holger Merker, Katrin Bohl, Michael Reichelt, Stefan Schuster, Luís F de Figueiredo, Christoph Kaleta and Christian Kost
The research project was funded by the Volkswagen Foundation, the Jena School for Microbial Communication, the Fundação Calouste Gulbenkian and the Fundação para a Ciência e a Tecnologia as well as Siemens SA Portugal. [JWK/AO]
Pande, S., Merker, H., Bohl, K., Reichelt, M., Schuster, S., de Figueiredo, L., Kaleta, C., Kost, C. (2013). Fitness and stability of obligate cross-feeding interactions that emerge upon gene loss in bacteria. The ISME Journal. Advance online publication 28 November 2013; doi: 10.1038/ismej.2013.211 The ISME Journal