Packing of Cells in a Multicellular Magnetotactic Bacterium.

Multicellularity has independently evolved more than twenty-five times in Earth’s history, giving rise to a remarkable diversity of organismal architectures. Among these, multicellular magnetotactic bacteria (MMBs) of the genus Magnetoglobus present a striking case: they form spherical consortia composed of tens of elongated cells arranged in a monolayer. Each cell is slightly elongated, which constrains its packing geometry. We investigate the relationship between the geometry of a cell, the emergence of nematic defects, and the chirality of the spherical collective. Using high-resolution imaging and quantitative shape analysis, we characterize the packing patterns and explore how topological defects may act to coordinate the growth of individuals and facilitate the observed multicellular life cycle of the consortium. Our findings provide physical insight into the general principles that underlie the evolution of spatial order in microbial collectives.