No endospore formation confirmed in members of the phylum Proteobacteria
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. ImportanceEndospore formation is a mechanism that allows bacteria to generate resilient dormant spores under harsh environmental conditions. Although this process has been traditionally restricted to the largely Gram-positive bacteria of the phylum Firmicutes, recent studies have also described endospores in some Proteobacteria. High complexity of endosporulation, reflected in extensive morphological transformations governed by hundreds of conserved genes, hinders its facile acquisition via horizontal gene transfer. Therefore, ability of distantly related bacteria to produce endospores would imply an ancient nature of this mechanism and potentially a pivotal role in species diversification and outer membrane biogenesis.