Surface layer protein A variant of Clostridium difficile PCR-ribotype 027

S-layers: The Proteinaceous Multifunctional Armors of Gram-Positive Pathogens

S-layers are self-assembled crystalline 2D lattices enclosing the cell envelopes of several bacteria and archaea. Despite their abundance, the landscape of S-layer structure and function remains a land of wonder. By virtue of their location, bacterial S-layers have been hypothesized to add structural stability to the cell envelope. In addition, S-layers are implicated in mediating cell-environment and cell-host interactions playing a key role in adhesion, cell growth, and division. Significant strides in the understanding of these bacterial cell envelope components were made possible by recent studies that have provided structural and functional insights on the critical S-layer and S-layer-associated proteins (SLPs and SLAPs), highlighting their roles in pathogenicity and their potential as therapeutic or vaccine targets. In this mini-review, we revisit the sequence-structure-function relationships of S-layers, SLPs, and SLAPs in Gram-positive pathogens, focusing on the best-studied classes, Bacilli (Bacillus anthracis) and Clostridia (Clostridioides difficile). We delineate the domains and their architectures in archetypal S-layer proteins across Gram-positive genera and reconcile them with experimental findings. Similarly, we highlight a few key "flavors" of SLPs displayed by Gram-positive pathogens to assemble and support the bacterial S-layers. Together, these findings indicate that S-layers are excellent candidates for translational research (developing diagnostics, antibacterial therapeutics, and vaccines) since they display the three crucial characteristics: accessible location at the cell surface, abundance, and unique lineage-specific signatures.

The host immune response to Clostridium difficile infection

Clostridium difficile infection (CDI) is the most common infectious cause of healthcare-acquired diarrhoea. Outcomes of C. difficile colonization are varied, from asymptomatic carriage to fulminant colitis and death, due in part to the interplay between the pathogenic virulence factors of the bacterium and the counteractive immune responses of the host. Secreted toxins A and B are the major virulence factors of C. difficile and induce a profound inflammatory response by intoxicating intestinal epithelial cells causing proinflammatory cytokine release. Host cell necrosis, vascular permeability and neutrophil infiltration lead to an elevated white cell count, profuse diarrhoea and in severe cases, dehydration, hypoalbuminaemia and toxic megacolon. Other bacterial virulence factors, including surface layer proteins and flagella proteins, are detected by host cell surface signal molecules that trigger downstream cell-mediated immune pathways. Human studies have identified a role for serum and faecal immunoglobulin levels in protection from disease, but the recent development of a mouse model of CDI has enabled studies into the precise molecular interactions that trigger the immune response during infection. Key effector molecules have been identified that can drive towards a protective anti-inflammatory response or a damaging proinflammatory response. The limitations of current antimicrobial therapies for CDI have led to the development of both active and passive immunotherapies, none of which have, as yet been formally approved for CDI. However, recent advances in our understanding of the molecular basis of host immune protection against CDI may provide an exciting opportunity for novel therapeutic developments in the future.

Comparative proteomic analysis of Clostridium difficile isolates of varying virulence

The soluble proteome of three Clostridium difficile strains of varying pathogenic potential, designated B-1, Tra 5/5 and 027 SM, were compared using differential in-gel electrophoresis in which the proteins of each strain were labelled with CyDyes. This enabled visual inspection of the 2D profiles of strains and identification of differentially expressed proteins using image analysis software. Unlabelled protein reference maps of the predominant proteins were then generated for each strain using 2D gel electrophoresis followed by protein sequencing of each spot using a Reflectron matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer. Increased coverage of the proteome was achieved using 1D gel electrophoresis in a bottom-up approach using LC-MS/MS of 1 cm gel slices. A total of 888 different proteins were detected by comparative analysis of isolates grown in parallel for 64 h on blood agar plates. Of these, only 38 % were shared between all isolates. One hundred and ten proteins were identified as showing ≥2-fold difference in expression between strains. Differential expression was shown in a number of potential virulence and colonization factors. Toxin B was detected in the more virulent strains B-1 and 027 SM, but not in the lower virulent strain Tra 5/5, despite all strains possessing an intact pathogenicity locus. The S-layer protein (Cwp2) was identified in strains 027 SM and Tra 5/5 but not strain B-1, and differences in the post-translational modification of SlpA were noted for strain B-1. The variant S-layer profile of strain B-1 was confirmed by genomic comparison, which showed a 58 kb insertion in the S-layer operon of strain B-1. Differential post-translation modification events were also noted in flagellar proteins, thought to be due to differential glycosylation. This study highlights genomic and proteomic variation of different Clostridium difficile strains and suggests a number of factors may play a role in mediating the varying virulence of these different strains.

Molecular basis of early stages of Clostridium difficile infection: germination and colonization

Clostridium difficile infections (CDIs) occur when antibiotic therapy disrupts the gastrointestinal flora, favoring infected C. difficile spores to germinate, outgrow, colonize and produce toxins. During CDI, C. difficile vegetative cells initiate the process of sporulation allowing a fraction of the spores to remain adhered to the intestinal surfaces. These spores, which are unaffected by antibiotic therapy commonly used for CDIs, then germinate, outgrow and recolonize the host's GI tract causing relapse of CDI. Consequently, the germination and colonization processes can be considered as the earliest and most essential steps for the development as well as relapse of CDI. The aim of this review is to provide an overview on the molecular basis involved in C. difficile spore germination and colonization.

The LMW surface-layer proteins of Clostridium difficile PCR ribotypes 027 and 001 share common immunogenic properties

The aim of this study was to investigate the S-layer proteins (SLPs) of the hypervirulent Clostridium difficile PCR ribotype 027 and compare them with those of PCR ribotype 001 and other PCR ribotypes involved in C. difficile infection and outbreaks, by molecular analysis and immunological assays. It has been demonstrated previously that PCR ribotype 027 SlpA is conserved in C. difficile strains belonging to this PCR ribotype and that it is a new variant, showing 88 % identity with SlpA of PCR ribotype 001. As the low-molecular-weight (LMW) SLPs of C. difficile are immunodominant antigens, attention was focused on this region of the genome. Sequencing of strains of different PCR ribotypes (001, 012, 014, 017, 027 and 078) showed that SlpA was conserved among strains belonging to the same PCR ribotype. Comparison of the LMW SLP region among these strains identified ten regions with sequence identity between PCR ribotypes 027 and 001, and low conservation with the other PCR ribotypes. In particular, two of these regions corresponded to areas predicted to be surface exposed. Three specific peptides, including those of the two surface-exposed regions, were recognized by human sera against PCR ribotypes 027 and 001 and by a rabbit polyclonal serum against the SLPs of PCR ribotype 027. In contrast, these peptides were not recognized by a polyclonal serum against the SLPs of PCR ribotype 012 used as a control. These results confirm the antigenic role of the LMW SLP and suggest that it may have a role in evasion of the host immune response.

Immunomodulatory activities of surface-layer proteins obtained from epidemic and hypervirulent Clostridium difficile strains

Surface-layer proteins (SLPs) have been detected in all Clostridium difficile strains and play a role in adhesion, although an involvement in the inflammatory process may also be supposed, as they cover the bacterial surface and are immunodominant antigens. The aim of this study was to evaluate the immunomodulatory properties of SLPs obtained from hypervirulent and epidemic (H/E) or non-H/E C. difficile strains, to try to determine whether they contribute to hypervirulence. SLPs were purified from H/E PCR ribotype 027 and 001 and non-H/E PCR ribotype 012 C. difficile strains, and the ability to modulate these properties was studied in human ex vivo models of monocytes and monocyte-derived dendritic cells (MDDCs). The results indicated that SLPs were able to induce immunomodulatory cytokines [interleukin (IL)-1β, IL-6 and IL-10] in monocytes. SLPs induced maturation of MDDCs, which acquired enhanced antigen-presenting activity, a crucial function of the mature stage. SLP-primed MDDCs expressed high levels of IL-10, an important regulatory cytokine. No significant differences were found in the activation induced in monocytes and MDDCs by SLP preparations from H/E and non-H/E strains. Overall, these findings show an important role for SLPs in modulation of the immune response to C. difficile. However, SLPs from H/E strains did not show a specific immunomodulatory pattern compared with SLPs from non-H/E strains, suggesting that SLPs are not involved in the increased severity of infection peculiar to H/E strains.