Friday, March 17, 2017

Cooperation shapes the spatial patterns of bacterial organization

Cooperative bacterial strains colonizing a surface
Bacteria colonize surfaces in all environments. That could be the surfaces of soil aggregates, of rocks in a stream bed, of plant leaves, of animal skin, or that could be the surface of your showerhead... On such surfaces microbes establish complex communities ('biofilms') that can contain many different interacting species. These various species are usually not randomly distributed in the biofilm, but rather organized depending on their environmental preferences (for example some like well-aerated areas, others not so much...) and on the type of interactions that they have with each other. This can result in complex patterns of organization that manifest at the microscopic scale and up to the millimeter scale. Such patterns are not trivial, as they can sustain microbial activity and functions that would not be possible in a well-mixed environment, which has importance for biotechnology applications as well.

In a new study published this month, we examined the role of cooperation in shaping spatial patterns of bacterial organization on wet surfaces. The paper is available online and is entitled 'Cooperation in carbon source degradation shapes spatial self-organization of microbial consortia on hydrated surfaces'. Our idea was that a feeding dependency between two partners would directly control their distribution in space, hence imposing a specific pattern. We used a simple model system made of two bacterial strains that could grow using the chemical compound toluene (a hydrocarbon), but only when they were working together as a 'team' (a bacterial consortium in the jargon). In the illustration below you can see the sequential degradation of toluene by the two bacterial strains (represented with cyan and magenta colors). The photo shows growth on an agar plate with toluene as the sole source of carbon: the strains only form a colony (a few mm wide) when they are spotted together on the agar surface. This way, we force them to cooperate in order to degrade the compound and use it for their cellular growth. If we replace toluene by another compound that both bacteria can use independently, in that case cooperation does not take place - instead, the two bacteria compete for the limited resources available to them.

Pseudomonas putida (Pp) strains F4 and F107 cooperate to degrade toluene and grow as a colony on a surface

To visualize the patterns that the bacteria would form, we tagged them with different fluorescent proteins (a green and a red fluorescent protein) that allow us to discriminate them with a fluorescence microscope (see photo below). Using this system, we showed that forced cooperation in toluene degradation results in a form of mutualistic interaction between the strains, that is a 'win-win' situation where both partners benefit. In this scenario, the two bacterial strains tend to self-organize on the surface that they colonize, producing alternate strands distant by only tens of microns. This is dictated by the nature of their interaction: they rely on the diffusion of toluene byproducts from one partner strain to another, therefore they cannot efficiently cooperate if they are too distant from each other. This distinct pattern of organization vanishes when we abolish cooperation and replace it by a competition scenario (in this case, we feed the bacteria with benzoate, a compound that both strains can use independently). By contrast, competition induces a clear segregation between the two bacterial strains, which end up being separated by millimeters in certain areas.

Patterns of bacterial strains competing or cooperating as they grow on a surface.

The effects of feeding dependency and of diffusion illustrated in this study with a an artificial system also exist in natural habitats. This allows us to make some predictions: for example we would expect to see more spatial organization in environments where steep gradients of one limiting nutrient source are found. Our work is also a step towards a better design of artificial microbial 'landscapes': we need to understand the first principles guiding microbial assembly in order to better control it.


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