Structural Characterization of Spores of Diverse Human, Animal, and Environmental Origin and Comparison to Spores.

is an often-lethal bacterium causing human and animal disease. Crucial to the infectious cycle of is its ability to produce spores, which can germinate into toxin-producing vegetative bacteria under favorable conditions. However, structural details of the spore are lacking. Here, we used a range of electron microscopy techniques together with superresolution optical microscopy to characterize the spore morphology with an emphasis on the exosporium. The spore is made up of multiple layers with the exosporium presenting as a smooth balloon-like structure that is open at the spore poles. Focusing on the outer spore layers, we compared the morphologies of spores derived from different strains and determined that there is some variation between the spores, most notably with spores of some strains having tubular appendages. Since is a close relative of , their spores were compared by electron microscopy and their exosporia were found to be distinctly different from each other. This study therefore provides new structural details of the spore and offers insights into the physical structure of the exosporium across clostridial species. is a significant pathogen with mortality rates approaching 100%. It is the bacterial spore that is critical in initiating infection and disease. An understanding of spore structures as well as spore morphology across a range of strains may lead to a better understanding of infection and disease. However, the structural characteristics of the spores are limited. In this work, we have addressed this lack of detail and characterized the spore morphology. The use of traditional and advanced microscopy techniques has provided detailed new observations of spore structural features, which serve as a reference point for structural studies of spores from other bacterial species.