A cyclic swarming assay produces genetically and phenotypically distinctive variants of Enterobacter sp. SM3

Introduction Swarming motility, the collective migration of bacteria across semi-solid surfaces, is often associated with cell elongation and increased flagella number or strength. Enterobacter sp. SM3 is a strain originally isolated from murine gut and previously shown to mitigate colitis in mice. Methods To investigate phenotypic and genomic correlates of enhanced surface motility, we derived hyper-swarming variants of SM3 through serial passage from a single clonal isolate, generating five independent lineages collectively termed “Swarmium.” Results Optical microscopy revealed that Swarmium populations contained cells that were two to three times longer than the parental strain. Motility analysis showed similar swimming speed and tumble duration, but longer run times and reduced tumble angles. Whole-genome sequencing (mean coverage 634×; range 571–845×) identified population-level sequence variants affecting loci annotated for functions including flagellar regulation (rcsB), cell division (ftsL), translation machinery (ribosomal RNA operons and EF-Tu), and Type VI secretion-associated loci (including PAAR-adjacent regions and DUF6531-containing proteins). Discussion These findings identify candidate genes associated with the hyper-swarming phenotype, although their functional significance remains unclear. This study identifies genetic changes in populations selected based on swarm expansion rate and provides useful leads for future functional and mechanistic investigations of bacterial swarm motility.