Unique growth and morphology properties of Clade 5 strains revealed by single-cell time-lapse microscopy.

is a gastrointestinal pathogen of both humans and agricultural animals and thus a major One Health threat. The species consists of five main clades, with Clade 5 currently undergoing speciation from Clades 1-4. Clade 5 strains are highly prevalent in agricultural animals and can cause zoonotic infections, suggesting that these strains have evolved phenotypes that distinguish them from Clade 1-4 strains. Here, we compare the growth properties of Clade 5 strains to those of Clade 1-4 strains using anaerobic time-lapse microscopy coupled with automated image analysis. Our analyses indicate that Clade 5 strains grow faster and are more likely to form long chains of cells than Clade 1-4 strains. Using comparative genomic and CRISPRi analyses, we show that the chaining phenotype of Clade 5 strains is driven by the orientation of the invertible switch sequence, with chaining strains exhibiting a bias to the -ON state. Interestingly, Clade 5 strains with a bias towards the -ON state shifted to a largely -OFF state during murine infection, suggesting that the -OFF state is under positive selection during infection. Collectively, our data reveal that Clade 5 strains have distinct growth properties, which may allow them to inhabit diverse ecological niches.