Taking a microscale look at symbiotic interactions-and why it matters.

In 1881 the first significant work on earthworm behavior and ecology was published (1). It was Charles Darwin´s last scientific book and is still a fascinating and rewarding read. In PNAS, Geier et al. (2) use an earthworm taken directly from nature to present a method that allows in situ visualizations of micrometer-scale physical and chemical interactions between the worm and all its symbionts and parasites. In between the two publications lies the realization that animals evolved in a microbial world (3), that symbiosis appears as a general principle in eukaryotic evolution (4), and that animals function as metaorganisms (5⇓–7). Microbial symbionts influence virtually all aspects of eukaryote biology (3, 8), but what maintains the balance between symbionts and their hosts? Also, where exactly in the animal body do these interactions take place? Surely the answer lies in a precise and spatially controlled cell-to-cell communication between the partners (9).Geier et al. (2) report a technique that might help to answer these questions, potentially allowing us to discover the “vocabulary of symbiosis,” i.e., the metabolites exchanged between the host and the symbionts in the context of the local spatial organization of the animal. The method termed “CHEMHIST” combines microcomputed tomography (µCT) and matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). It allows metabolite imaging in a defined part of a … [↵][1]1Email: tbosch{at}zoologie.uni-kiel.de. [1]: #xref-corresp-1-1