Elsevier

Current Opinion in Microbiology

Volume 65, February 2022, Pages 138-144
Current Opinion in Microbiology

Second messenger signaling in Clostridioides difficile

https://doi.org/10.1016/j.mib.2021.11.006Get rights and content

Highlights

  • •

    Metabolism of c-di-GMP, c-di-AMP, and pGpp has been confirmed in Clostridioides difficile.

  • •

    c-di-GMP signaling is accomplished via two classes of riboswitch and indirect control of the transcription factor SigD.

  • •

    c-diAMP signaling involves protein effectors, including a transcriptional regulator, a sensor kinase, and a potassium transport protein.

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    The mechanisms of pGpp signaling in this organism are unknown.

  • •

    Nucleotide signaling feedback loops are common and thus far have been identified for c-di-GMP in C. difficile.

Small, diffusible second messenger molecules transmit information about extracellular conditions to intracellular machinery in order to influence transcription, translation, and metabolism. The enteropathogenic bacterium Clostridioides difficile coordinates its response to a dynamic and hostile environment via nucleotide second messengers. While riboswitch-mediated cyclic diguanylate regulation has been extensively characterized in C. difficile, signaling by cyclic diadenylate and by guanosine alarmones has only recently been confirmed in this organism. This review summarizes the current knowledge of how nucleotide second messenger signaling regulates physiological processes in C. difficile.

Introduction

Small molecule second messengers carry information about extracellular conditions through the cytoplasm. Rapidly synthesized or released in response to ‘first messengers’ perceived at the cell envelope, these signals allow bacteria to adopt dramatically different behaviors and morphologies in response to favorable or unfavorable environments. Second messengers must also be rapidly degraded or sequestered when conditions change. The intestinal pathogen Clostridioides difficile transits through a broad range of conditions within the gastrointestinal tract between ingestion from a contaminated environment and successful colonization of the large intestine. C. difficile has a dimorphic lifestyle. As a vegetative cell it can exhibit flagellar motility or form sessile biofilms, and secretes protein toxins that cause diarrheal disease [1]. C. difficile can also differentiate into extremely resilient spores that spread C. difficile infection (CDI) to new hosts [2]. These drastic transitions between motility, sessility, and sporulation are regulated by a complex and incompletely understood combination of environmental cues that are dynamically affected by the host diet, commensal microbiota, and antibiotic exposure [3,4]. As CDI is a highly drug-resistant infection with a high recurrence rate, there is a great deal of interest in how this organism perceives and responds to extracellular signals in order to optimize its survival and proliferation within the host [5].

Unlike eukaryotic cells, bacteria do not sequester and release calcium ions in response to extracellular stimuli or utilize inositol phosphates derived from membrane lipids as second messengers [6,7]. The responsiveness of bacterial cells to their surroundings depends largely on nucleotide signals.

Section snippets

Cyclic mononucleotides

Cyclic mononucleotides are ubiquitous signals in eukaryotes and are utilized by some, but not all, prokaryotes [8,9]. cAMP and cGMP are synthesized from ATP and GTP, respectively, by cyclases and hydrolyzed to produce AMP and GMP by phosphodiesterase (PDE) enzymes (Table 1) [8, 9, 10]. The C. difficile genome does not encode homologs of any characterized adenyl or guanyl cyclases and there is currently no evidence that this organism synthesizes or responds to cyclic mononucleotides (Table 2) [11

c-di-GMP

In diverse bacterial clades, c-di-GMP promotes aggregation and biofilm formation and discourages flagellar motility, as well as regulates virulence factor production and responses to environmental perturbations [13]. These common phenotypes are achieved by a variety of mechanisms utilizing protein and ribonucleotide effectors. c-di-GMP-binding receptor proteins include transcriptional regulators, ATPases, sensor histidine kinases and response regulators, synthetic enzymes, and regulators of

(pp)pGpp

The bacterial ‘alarmone’ second messengers pppGpp and ppGpp, together (p)ppGpp, are synthesized in response to nutrient limitation and extracellular stress [51]. Conserved synthetase enzymes from the RSH and SAS families catalyze the formation of (p)ppGpp by transferring a pyrophosphate from the 5′ hydroxyl of ATP to the 3′ hydroxyl of either GTP or GDP [52,53•]. RSH family enzymes are bifunctional, containing (p)pGpp hydrolysis domains [52]. In cells where (p)ppGpp has accumulated above a

Conclusions

More than any other class of biomolecule, nucleotides embody information. Long nucleotide polymers contain an organism’s genetic heritage, while mononucleotide and di-nucleotide signals dictate responses to environmental fluctuations. This is especially true for C. difficile, which uses nucleotide signals to regulate extreme morphological and behavioral transitions (Figure 2). While the c-di-GMP metabolic infrastructure and riboswitch effectors have been extensively studied, regulatory

Conflict of interest statement

Nothing declared.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

Many excellent and relevant publications were not discussed due to space constraints. The author has been supported by N.I.H. awardsK22 AI118929-01 andR15 GM126527-01A1 and byPulse Biosciences. The funders had no role in the preparation of this manuscript. Thanks to Dr Astha Pokhrel for critical reading of this manuscript.

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