FIGURE 3.

Decision tree to characterize the mechanism underlying positive interactions between two microorganisms. The dotted squares contain preliminary questions to establish whether this is a case of positive interaction or not, or supplementary questions to establish its type. In the case of bidirectional interactions, the decision tree can be read twice: the first reading aims to characterize the effect of microorganism A on B (plain arrows), and the second aims to characterize the effect of B on A (dotted arrows). The sequential questions are designed to establish whether the interaction:

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  is direct or indirect,
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  engages a sharing of public goods or is due to physicochemical changes in the medium,
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  is reliant on secreted primary metabolites or not.