Spores for thought: do Clostridium spores have a specific cell release mechanism?

A recently published IFR paper in Food Microbiology describes evidence for the role of the exosporium and spore coats in germination and cell emergence in C. sporogenes.

Jason Brunt

Here, Dr Jason Brunt, an IFR scientist working in Professor Mike Peck’s research group within the GHFS Research Programme and the Food Safety Centre, blogs about the research behind this new paper.

Clostridium botulinum is a dangerous pathogen that forms the highly potent botulinum toxin, which when ingested causes botulism, a deadly neuroparalytic disease. The closely related Clostridium sporogenes is a significant cause of waste through food spoilage, is occasionally pathogenic, and is often regarded as the non-toxigenic equivalent of C. botulinum. All Clostridium species form highly heat resistant endospores, enabling these bacteria to survive adverse conditions. Subsequently, spores may germinate, giving rise to vegetative cells that multiply and lead to adverse events such as toxin production or food spoilage.

The structure of clostridial spores generally comprises an exosporium, spore coat, outer membrane, cortex, inner membrane and spore core (Figure 1 below). However, the structure and content of the outermost layers, in particular the exosporium, vary for individual species and strains. Although the exact role of the exosporium is not completely understood, studies suggest that it may play a role in germination, outgrowth and attachment. While the spore coat provides protection for the spore, the exosporium is the first point of contact of the spore with its environment. When conditions in the environment become favourable, the dormancy of bacterial spores is broken, germination occurs and cell multiplication recommences. The precise mechanism of how the cell is released from the spore and whether this is a pre-programmed event remains unknown.

Figure 1

Our recent paper published in Food Microbiology is the first to address this gap in scientific knowledge by focussing specifically on the cell release mechanisms of C. sporogenes. To understand more about these processes, electron microscopy was used to visualise structural changes during germination, emergence and outgrowth.

Interestingly, we observed disruption in the coat of the germinating spore, plus an aperture in the exosporium that aligned with this break in the spore coat. Electron microscopy was used to investigate further the structure and function of this aperture. Our images revealed that C. sporogenes spores possessed one of two terminal structures at one end of the exosporium; either an aperture or a lipped terminal protrusion (termed a sporiduct). We then went on to show that the newly formed cell emerged through these structures (False coloured image below).

C. sporogenes spores possess one of two terminal structures at one end of the exosporium; either an aperture or a lipped terminal protrusion (termed a sporiduct). We then went on to the newly formed cell emerges through these structures. Image by Kathryn Cross, IFR

The findings presented in Food Microbiology establish the role of the exosporium and spore coat in germination and cell emergence in C. sporogenes. Evidence suggested that rupture of the spore coats occurs adjacent to the opening in the exosporium. It is hypothesised that this polarity is genetically pre-determined in the dormant spore. Our long term aim is to formulate detailed strategies to interrupt these processes. This would be of great benefit to the food industry to help control these pathogenic clostridia.

Written by Dr Jason Brunt, May 2015.

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