<i>Pseudosulfitobacter pseudonitzschiae</i> hitchhikes on gliding colonies of <i>Cellulophaga lytica</i>

Interspecies interactions shape microbial communities; this is central for microbial ecology. <i>Cellulophaga lytica</i> PlyA2 is a marine flavobacterium, which glides over surfaces and forms ordered, structurally coloured colonies, which display angle-dependent reflection of light. <i>Pseudosulfitobacter pseudonitzschiae</i> SW is an apparently nonmotile, nonstructurally coloured marine bacterium. Here, we aim to understand the interaction of both strains at cellular, genomic, optical, and proteomic levels. Cocultivation on agar showed that <i>P. pseudonitzschiae</i> uses gliding <i>C. lytica</i> to spread by microbial hitchhiking in which <i>Pseudosulfitobacter</i> appears to "surf" on basal layers of motile <i>Cellulophaga</i>. This dispersal mechanism was found to be often beneficial for <i>P. pseudonitzschiae,</i> which could maximally expand its population up to 350-fold relative to monoculture. Coculture was often of limited benefit for <i>C. lytica</i>, only in extended cultivation on rich medium was the presence of <i>P. pseudonitzschiae</i> detrimental to its viability. The proteome of <i>P. pseudonitzschiae</i> was strongly impacted by the association with <i>C. lytica</i>. Quorum-sensing signalling, potential exchange of amino acids, vitamins, and other metabolites are likely mediating this hitchhiking interaction. In contrast, <i>C. lytica</i> made minimal adjustments to its proteome composition in coculture. Supported by optical analysis, <i>P. pseudonitzschiae</i> patterned <i>C. lytica</i> by changing how groups of the latter organised to reflect light. Our results underscore the unusual, dynamic interplay between two bacterial species and provide insights on the mechanisms underlying this relationship.