Abstract
The strict anaerobe Clostridium difficile is the most common cause of nosocomial diarrhea, and the oxygen-resistant spores that it forms have a central role in the infectious cycle. The late stages of sporulation require the mother cell regulatory protein σK. In Bacillus subtilis, the onset of σK activity requires both excision of a prophage-like element (skinBs) inserted in the sigK gene and proteolytical removal of an inhibitory pro-sequence. Importantly, the rearrangement is restricted to the mother cell because the skinBs recombinase is produced specifically in this cell. In C. difficile, σK lacks a pro-sequence but a skinCd element is present. The product of the skinCd gene CD1231 shares similarity with large serine recombinases. We show that CD1231 is necessary for sporulation and skinCd excision. However, contrary to B. subtilis, expression of CD1231 is observed in vegetative cells and in both sporangial compartments. Nevertheless, we show that skinCd excision is under the control of mother cell regulatory proteins σE and SpoIIID. We then demonstrate that σE and SpoIIID control the expression of the skinCd gene CD1234, and that this gene is required for sporulation and skinCd excision. CD1231 and CD1234 appear to interact and both proteins are required for skinCd excision while only CD1231 is necessary for skinCd integration. Thus, CD1234 is a recombination directionality factor that delays and restricts skinCd excision to the terminal mother cell. Finally, while the skinCd element is not essential for sporulation, deletion of skinCd results in premature activity of σK and in spores with altered surface layers. Thus, skinCd excision is a key element controlling the onset of σK activity and the fidelity of spore development.
Highlights
Endosporulation is an ancient bacterial cell differentiation process allowing the conversion of a vegetative cell into a mature spore through a series of morphological steps [1, 2]
Clostridium difficile, a major cause of antibiotic-associated diarrhea, produces resistant spores that facilitate its persistence in the environment including hospitals
A less tight connection between the forespore and mother cell lines of gene expression is observed in C. difficile compared to Bacillus subtilis especially at the level of the late sigma factor, σK
Summary
Endosporulation is an ancient bacterial cell differentiation process allowing the conversion of a vegetative cell into a mature spore through a series of morphological steps [1, 2]. Vegetative forms multiply and produce two toxins, TcdA and TcdB, which are the main virulence factors [6]. These toxins cause enterocyte lysis and inflammation leading to diarrhea, colitis, pseudomembranous colitis or more severe symptoms including bowel perforation, sepsis and death. C. difficile forms spores in the gut that are essential for transmission of this strict anaerobe and contribute to the establishment of reservoirs in the environment including the host and hospital settings [7, 8]
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