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How big bacterial species can be together

The amazing stability of large and diverse bacterial communities can be explained by a model that emphasizes the requirements of microbial foods.

In the bodies of humans and other animals, microbes form complex but highly stable ecosystems. Researchers have been puzzled by this stability, but now two physicists have used a mathematical model to show that the complexity and stability of such communities can arise from the way in which bacteria consume resources.

The study suggests that the puzzling characteristics of microbial ecosystems can be explained by focusing on the resources that bacteria use, rather than factors such as how bacteria cooperate or compete.

Biologists have wondered how bacterial communities can be so stable, given their profound complexity. For example, several hundred species coexist stably in the human intestine, although each species has the potential for rapid exponential growth, which could unbalance the community.

Researchers have also puzzled the unpredictable composition of bacterial communities and why two different communities growing in almost identical environments often end up containing a radically different mix of species.

Explanations for these and other characteristics of microbial communities require a change of perspective, argues Akshit Goyal of the National Center for Biological Sciences in India and Sergei Maslov of the University of Illinois at Urbana-Champaign.

Rather than focusing on competition or cooperation between species or their responses to environmental changes, the researchers suggest looking for the molecular resources that bacteria harvest and how bacterial activity changes. the set of resources available over time.

Several biologists have clearly defended this view, and now Goyal and Maslov present a model that supports it.

The researchers developed a mathematical model of a bacterial community that consisted initially of a single species capable of exploiting a single available resource, such as a particular molecule.

When bacteria use a resource, they digest it and then release the waste products. These products can be useful for other bacteria, which can arrive at random at any time.

Each time a new species joins the community, it survives by consuming the waste products of others. In turn, the new species produces new waste products, so the amount of resources grows.

Goyal and Maslov assumed that each species can use only one resource and that its digestion produces two secondary resources.

They also assumed that new species occasionally enter the community from the outside and then survive only if they can exploit a resource more effectively than any other species already present.

Surprisingly, their simulations showed that the community became more stable as its diversity increased. In principle, if a new species uses a resource more effectively than a species already present, then that older species would become extinct, killing any species dependent on its waste products or the products of its dependent species.

But the researchers found that such "avalanches" of extinction rarely occur in a mature and diverse community. Over time, it is increasingly less likely that a new species enters the community will be more efficient in collecting their resources than the one that is already present. Diversity and stability naturally grow together, as biologists see in real microbial ecosystems.

The model also naturally explains why multiple communities growing in identical conditions can become very different.

The final mix of species depends significantly on the historical sequence of species that enter the community, the so-called assembly process.

A species that enters only persists if the resource it needs is present at the time and at levels sufficient for growth, which depends on the previous presence of several other species.

"Our model is very simple," Maslov says, "but we believe that it lays the foundation for a mathematical understanding of how nutrient flow affects the diversity, stability and reproducibility of species composition in microbial communities." . 19659002] "I really like this job," says ecologist Álvaro Sánchez of Yale University. "The use of simple assembly models for large communities is very necessary, since most microbiomes are highly complex, often thousands of species."

The authors note that their current model is based on several simplifying assumptions, for example, that a community microbe begins with access to a single resource, and each species consumes exactly one resource and produces two waste products.

However, preliminary evidence suggests that the overall result is not very sensitive to these details, and researchers have set out to explore more complex models to further test these ideas.

This research is published in Physical Review Letters.

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