Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.07.23.219022v1?rss=1 Authors: Beskrovnaya, P., Fakih, D., Sexton, D. L., Xing, S., Golmohammadzadeh, M., Morneau, I., Guadarrama Bello, D., Nanci, A., Huan, T., Tocheva, E. Abstract: Endospore formation is used by members of the phylum Firmicutes to withstand extreme environmental conditions. Several recent studies have documented endospore formation in species outside of Firmicutes, particularly in Rhodobacter johrii and Serratia marcescens, members of the phylum Proteobacteria. Here, we aimed to investigate endospore formation in these two species by using advanced imaging and analytical approaches. Examination of the phase-bright structures observed in R. johrii and S. marcescens using cryo-electron tomography failed to identify endospores or stages of endospore formation. We determined that the phase-bright objects in R. johrii cells were triacylglycerol storage granules and those in S. marcescens were aggregates of cellular debris. In addition, R. johrii and S. marcescens containing phase-bright objects do not possess phenotypic and genetic features of endospores, including enhanced resistance to heat, presence of dipicolinic acid, or the presence of many of the genes associated with endospore formation. Our results support the hypothesis that endospore formation is restricted to the phylum Firmicutes. Copy rights belong to original authors. Visit the link for more info