Intracellular membranes of bacterial endospores are reservoirs for spore core membrane expansion during spore germination

Abstract

Bacterial endospores are formed by certain bacteria, such as Bacillus subtilis or the pathogenic Bacillus anthracis and Clostridioides difficile, to allow survival in environmental conditions which are lethal to vegetative bacteria. The spores possess a particular architecture and molecular inventory which endow them with a remarkable resistance against desiccation, heat and radiation. Another remarkable spore feature is their rapid return to vegetative growth during spore germination and outgrowth. The underlying processes of this latter physiological and morphological transformation involve a number of different events, some of which are mechanistically not entirely understood. One of these events is the expansion of the central spore core, which contains the DNA, RNA and most spore enzymes. To date, it has been unclear how the ~1.3- to 1.6-fold expansion of the core membrane surface area that accompanies core expansion takes place, since this occurs in the absence of significant if any ATP synthesis. In the current work, we demonstrate the presence of intracellular membrane structures in spores located just below the core membrane. During spore germination these internal core membranes disappear when the core size increases, suggesting that they are integrated into the core membrane to allow core expansion. These intracellular membranes are most probably present as more or less compressed vesicles or tubules within the dormant spore core. Investigations of spores from different species suggest that these intracellular membrane structures below the core membrane are a general feature of endospore forming bacteria.

Killing of bacterial spores by dodecylamine and its effects on spore inner membrane properties

“…Presumably the spore cortex and to some extent the coat ‘compresses’ the layers below it to some extent so that the IM’s potential expansion by taking up the significant amount of membrane vesicles just below the IM surface (Laue et al . 2018) is largely, albeit not completely blocked. Indeed, previous work (Sunde et al .…”

“…6a,b, white arrowheads) which recently were visualized just below the IM in dormant spores of all species looked at, so far (Laue et al . 2018). These specific membranes appear to fuse with the IM upon spore germination, thus allowing an ~30% increase in IM surface area in germination without new membrane synthesis.…”

“…The white arrowheads indicate membranes below the IM as described previously (Laue et al . 2018). Scale bars in panels a–e are 200 nm, and in panel f is 100 nm.…”

“…Thin section electron microscopy on spores was performed by using microwave‐assisted chemical fixation and LR White embedding, as described previously (Laue et al . 2018).…”

Fluoride movement into and out ofBacillusspores and growing cells and effects of fluoride accumulation on spore properties

“…; Laue et al . ). Specifically, lipids in the IM environment are essentially immobile, and it seems likely that this would also affect IM protein function, but how is not known.…”