Surviving Between Hosts: Sporulation and Transmission

Abstract

To survive adverse conditions, some bacterial species are capable of developing into a cell type, the "spore," which exhibits minimal metabolic activity and remains viable in the presence of multiple environmental challenges. For some pathogenic bacteria, this developmental state serves as a means of survival during transmission from one host to another. Spores are the highly infectious form of these bacteria. Upon entrance into a host, specific signals facilitate germination into metabolically active replicating organisms, resulting in disease pathogenesis. In this article, we will review spore structure and function in well-studied pathogens of two genera, Bacillus and Clostridium, focusing on Bacillus anthracis and Clostridium difficile, and explore current data regarding the lifestyles of these bacteria outside the host and transmission from one host to another.

Fig. 1

Schematic representation of spores of B. subtilis (left), B. anthracis (middle) and C. difficile (right) illustrating the major spore structures. The inner-most compartment (housing the spore DNA), the core, is white. The inner membrane (the location of the germinant receptors in B. subtilis and B. anthracis, but not C. difficile) is the black oval surrounding the core. The grey region is the spore peptidoglycan and, surrounding that, the dark blue is the coat. Although an outer membrane is present during spore formation, it is most likely not present in the mature spore (72) and, therefore, not shown here. The sublayers of the coat are not indicated for simplicity. B. anthracis (but not the other two species), (65) possesses an additional outer layer, the exosporium, indicated by a red line (the exosporium basal layer) and protrusions extending from the basal layer surface (the hair-like projections, or nap). The region between the coat and the exosporium is the interspace. The C. difficile coat has a scalloped appearance in cross-section, as indicated in the representation.