Immunological properties of surface proteins of Clostridium difficile

Identification of a novel zinc metalloprotease through a global analysis of Clostridium difficile extracellular proteins

Clostridium difficile is a major cause of infectious diarrhea worldwide. Although the cell surface proteins are recognized to be important in clostridial pathogenesis, biological functions of only a few are known. Also, apart from the toxins, proteins exported by C. difficile into the extracellular milieu have been poorly studied. In order to identify novel extracellular factors of C. difficile, we analyzed bacterial culture supernatants prepared from clinical isolates, 630 and R20291, using liquid chromatography-tandem mass spectrometry. The majority of the proteins identified were non-canonical extracellular proteins. These could be largely classified into proteins associated to the cell wall (including CWPs and extracellular hydrolases), transporters and flagellar proteins. Seven unknown hypothetical proteins were also identified. One of these proteins, CD630_28300, shared sequence similarity with the anthrax lethal factor, a known zinc metallopeptidase. We demonstrated that CD630_28300 (named Zmp1) binds zinc and is able to cleave fibronectin and fibrinogen in vitro in a zinc-dependent manner. Using site-directed mutagenesis, we identified residues important in zinc binding and enzymatic activity. Furthermore, we demonstrated that Zmp1 destabilizes the fibronectin network produced by human fibroblasts. Thus, by analyzing the exoproteome of C. difficile, we identified a novel extracellular metalloprotease that may be important in key steps of clostridial pathogenesis.

Clostridium difficile infection in the twenty-first century

Clostridium difficile is a spore-forming gram-positive bacillus, and the leading cause of antibiotic-associated nosocomial diarrhea and colitis in the industrialized world. With the emergence of a hypervirulent strain of C. difficile (BI/NAP1/027), the epidemiology of C. difficile infection has rapidly changed in the last decade. C. difficile infection, once thought to be an easy to treat bacterial infection, has evolved into an epidemic that is associated with a high rate of mortality, causing disease in patients thought to be low-risk. In this review, we discuss the changing face of C .difficile infection and the novel treatment and prevention strategies needed to halt this ever growing epidemic.

Recombinational switching of the Clostridium difficile S-layer and a novel glycosylation gene cluster revealed by large-scale whole-genome sequencing

BACKGROUND

Clostridium difficile is a major cause of nosocomial diarrhea, with 30-day mortality reaching 30%. The cell surface comprises a paracrystalline proteinaceous S-layer encoded by the slpA gene within the cell wall protein (cwp) gene cluster. Our purpose was to understand the diversity and evolution of slpA and nearby genes also encoding immunodominant cell surface antigens.

METHODS

Whole-genome sequences were determined for 57 C. difficile isolates representative of the population structure and different clinical phenotypes. Phylogenetic analyses were performed on their genomic region (>63 kb) spanning the cwp cluster.

RESULTS

Genetic diversity across the cwp cluster peaked within slpA, cwp66 (adhesin), and secA2 (secretory translocase). These genes formed a 10-kb cassette, of which 12 divergent variants were found. Homologous recombination involving this cassette caused it to associate randomly with genotype. One cassette contained a novel insertion (length, approximately 24 kb) that resembled S-layer glycosylation gene clusters.

CONCLUSIONS

Genetic exchange of S-layer cassettes parallels polysaccharide capsular switching in other species. Both cause major antigenic shifts, while the remainder of the genome is unchanged. C. difficile genotype is therefore not predictive of antigenic type. S-layer switching and immune escape could help explain temporal and geographic variation in C. difficile epidemiology and may inform genotyping and vaccination strategies.

Immune responses to Clostridium difficile infection

Clostridium difficile is the causal agent of antibiotic-associated diarrhea and is a leading cause of hospital-acquired infections in the US. C. difficile has been known to cause severe diarrhea and colitis for more than 30 years, but the emergence of a newer, hypervirulent strain of C. difficile (BI/NAP1) has further compounded the problem, and recently both the number of cases and mortality associated with C. difficile-associated diarrhea have been increasing. One of the major drivers of disease pathogenesis is believed to be an excessive host inflammatory response. A better understanding of the host inflammation and immune mechanisms that modulate the course of disease and control host susceptibility to C. difficile could lead to novel (host-targeted) strategies for combating the challenges posed by this deadly infection. This review summarizes our current knowledge of the host inflammatory response during C. difficile infection.

Induction of cytokines in a macrophage cell line by proteins of Clostridium difficile

Clostridium difficile is a major cause of nosocomial diarrhoea. The toxins produced by C. difficile are responsible for the characteristic pathology observed in C. difficile disease, but several surface-associated proteins of C. difficile are also recognized by the immune system and could modulate the immune response in infection. The aim of this study was to assess the induction of cytokines in a macrophage cell line in response to different antigens prepared from five C. difficile strains: the hypervirulent ribotype 027, ribotypes 001 and 106 and reference strains VPI 10463 and 630 (ribotype 012). PMA-activated THP-1 cells were challenged with surface-layer proteins, flagella, heat-shock proteins induced at 42 and 60 °C and culture supernatants of the five C. difficile strains. The production of the pro-inflammatory cytokines such as TNF-α, IL-1β, IL-6, IL-8 and IL-12p70 was observed in response to the surface-associated proteins, and high levels of TNF-α, IL-1β and IL-8 were detected in response to challenge with culture supernatants. The immune response triggered by the surface-associated proteins was independent of the strain from which the antigens were derived, suggesting that these proteins might not be related to the varying virulence of the hypervirulent ribotype 027 or ribotypes 001 and 106. There was no interstrain difference observed in response to the culture supernatants of the tested C. difficile strains, but this was perhaps due to toxicity induced in the macrophages by large amounts of toxin A and toxin B.

Cyclic diguanylate inversely regulates motility and aggregation in Clostridium difficile

Clostridium difficile-associated disease is increasing in incidence and is costly to treat. Our understanding of how this organism senses its entry into the host and adapts for growth in the large bowel is limited. The small-molecule second messenger cyclic diguanylate (c-di-GMP) has been extensively studied in gram-negative bacteria and has been shown to modulate motility, biofilm formation, and other processes in response to environmental signals, yet little is known about the functions of this signaling molecule in gram-positive bacteria or in C. difficile specifically. In the current study, we investigated the function of the second messenger c-di-GMP in C. difficile. To determine the role of c-di-GMP in C. difficile, we ectopically expressed genes encoding a diguanylate cyclase enzyme, which synthesizes c-di-GMP, or a phosphodiesterase enzyme, which degrades c-di-GMP. This strategy allowed us to artificially elevate or deplete intracellular c-di-GMP, respectively, and determine that c-di-GMP represses motility in C. difficile, consistent with previous studies in gram-negative bacteria, in which c-di-GMP has a negative effect on myriad modes of bacterial motility. Elevated c-di-GMP levels also induced clumping of C. difficile cells, which may signify that C. difficile is capable of forming biofilms in the host. In addition, we directly quantified, for the first time, c-di-GMP production in a gram-positive bacterium. This work demonstrates the effect of c-di-GMP on the motility of a gram-positive bacterium and on aggregation of C. difficile, which may be relevant to the function of this signaling molecule during infection.

The host immune response to Clostridium difficile

Clostridium difficile is the most common cause of nosocomial bacterial diarrhoea in the Western world. Diarrhoea and colitis are caused by the actions of toxins A and B released by pathogenic strains of C. difficile. Adaptive immune responses to these toxins influence the outcomes of C. difficile infection (CDI). Symptomless carriers of toxinogenic C. difficile and those with a single episode of CDI without recurrence show more robust antitoxin immune responses than those with symptomatic and recurrent disease. Immune-based approaches to CDI therapy and prevention have been developed using active vaccination or passive immunotherapy targeting C. difficile toxins. Innate immune responses to C. difficile and its toxins are also central to the pathophysiology of CDI. An acute intestinal inflammatory response with prominent neutrophil infiltration and associated tissue injury is characteristic of CDI. Furthermore, inhibiting this acute inflammatory response can protect against the intestinal injury that results from exposure to C. difficile toxins in animal models. Studies examining host risk factors for CDI have led to validated clinical prediction tools for risk of primary and of recurrent disease. Risk factors associated with severe CDI with poor clinical outcomes have also been identified and include marked elevation of the peripheral white blood cell count and elevated creatinine. However, further work is needed in this area to guide the clinical application of new approaches to disease prevention and treatment including new antimicrobials as well as passive and active immunization.

Mutagenic analysis of the Clostridium difficile flagellar proteins, FliC and FliD, and their contribution to virulence in hamsters

Although toxins A and B are known to be important contributors to the acute phase of Clostridium difficile infection, the role of colonization and adherence to host tissues in the overall pathogenesis of these organisms remains unclear. Consequently, we used the recently introduced intron-based ClosTron gene interruption system to eliminate the expression of two reported C. difficile colonization factors, the major flagellar structural subunit (FliC) and the flagellar cap protein (FliD), to gain greater insight into how flagella and motility contribute to C. difficile's pathogenic strategy. The results demonstrate that interrupting either the fliC or the fliD gene results in a complete loss of flagella, as well as motility, in C. difficile. However, both the fliC and fliD mutant strains adhered better than the wild-type 630Δerm strain to human intestine-derived Caco-2 cells, suggesting that flagella and motility do not contribute to, or may even interfere with, C. difficile adherence to epithelial cell surfaces in vitro. Moreover, we found that the mutant strains were more virulent in hamsters, indicating either that flagella are unnecessary for virulence or that repression of motility may be a pathogenic strategy employed by C. difficile in hamsters.

Roles of cysteine proteases Cwp84 and Cwp13 in biogenesis of the cell wall of Clostridium difficile

Clostridium difficile expresses a number of cell wall proteins, including the abundant high-molecular-weight and low-molecular-weight S-layer proteins (SLPs). These proteins are generated by posttranslational cleavage of the precursor SlpA by the cysteine protease Cwp84. We compared the phenotypes of C. difficile strains containing insertional mutations in either cwp84 or its paralog cwp13 and complemented with plasmids expressing wild-type or mutant forms of their genes. We show that the presence of uncleaved SlpA in the cell wall of the cwp84 mutant results in aberrant retention of other cell wall proteins at the cell surface, as demonstrated by secretion of the proteins Cwp66 and Cwp2 into the growth medium. These phenotypes are restored by complementation with a plasmid expressing wild-type Cwp84 enzyme but not with one encoding a Cys116Ala substitution in the active site. The cwp13 mutant cleaved the SlpA precursor normally and had a wild-type-like colony phenotype. Both Cwp84 and Cwp13 are produced as proenzymes which are processed by cleavage to produce mature enzymes. In the case of Cwp84, this cleavage does not appear to be autocatalytic, whereas in Cwp13 autocatalysis was demonstrated as a Cys109Ala mutant did not undergo processing. Cwp13 appears to have a role in processing of Cwp84 but is not essential for Cwp84 activity. Cwp13 cleaves SlpA in the HMW SLP domain, which we suggest may reflect a role in cleavage and degradation of misfolded proteins at the cell surface.

Clostridium difficile has two parallel and essential Sec secretion systems

Protein translocation across the cytoplasmic membrane is an essential process in all bacteria. The Sec system, comprising at its core an ATPase, SecA, and a membrane channel, SecYEG, is responsible for the majority of this protein transport. Recently, a second parallel Sec system has been described in a number of gram-positive species. This accessory Sec system is characterized by the presence of a second copy of the energizing ATPase, SecA2; where it has been studied, SecA2 is responsible for the translocation of a subset of Sec substrates. In common with many pathogenic gram-positive species, Clostridium difficile possesses two copies of SecA. Here, we describe the first characterization of the C. difficile accessory Sec system and the identification of its major substrates. Using inducible antisense RNA expression and dominant-negative alleles of secA1 and secA2, we demonstrate that export of the S-layer proteins (SLPs) and an additional cell wall protein (CwpV) is dependent on SecA2. Accumulation of the cytoplasmic precursor of the SLPs SlpA and other cell wall proteins was observed in cells expressing dominant-negative secA1 or secA2 alleles, concomitant with a decrease in the levels of mature SLPs in the cell wall. Furthermore, expression of either dominant-negative allele or antisense RNA knockdown of SecA1 or SecA2 dramatically impaired growth, indicating that both Sec systems are essential in C. difficile.

Manganese binds to Clostridium difficile Fbp68 and is essential for fibronectin binding

Clostridium difficile is an etiological agent of pseudomembranous colitis and antibiotic-associated diarrhea. Adhesion is the crucial first step in bacterial infection. Thus, in addition to toxins, the importance of colonization factors in C. difficile-associated disease is recognized. In this study, we identified Fbp68, one of the colonization factors that bind to fibronectin (Fn), as a manganese-binding protein (K(D) = 52.70 ± 1.97 nM). Furthermore, the conformation of Fbp68 changed dramatically upon manganese binding. Manganese binding can also stabilize the structure of Fbp68 as evidenced by the increased T(m) measured by thermodenatured circular dichroism and differential scanning calorimetry (CD, T(m) = 58-65 °C; differential scanning calorimetry, T(m) = 59-66 °C). In addition, enhanced tolerance to protease K also suggests greatly improved stability of Fbp68 through manganese binding. Fn binding activity was found to be dependent on manganese due to the lack of binding by manganese-free Fbp68 to Fn. The C-terminal 194 amino acid residues of Fbp68 (Fbp68C) were discovered to bind to the N-terminal domain of Fn (Fbp68C-NTD, K(D) = 233 ± 10 nM, obtained from isothermal titration calorimetry). Moreover, adhesion of C. difficile to Caco-2 cells can be partially blocked if cells are pretreated with Fbp68C, and the binding of Fbp68C on Fn siRNA-transfected cells was significantly reduced. These results raise the possibility that Fbp68 plays a key role in C. difficile adherence on host cells to initiate infection.

A real-time quantitative PCR assay for evaluating Clostridium difficile adherence to differentiated intestinal Caco-2 cells

Herein we describe a real-time quantitative PCR assay for evaluating the adherence of Clostridium difficile to differentiated human intestinal Caco-2 cells. Our investigations demonstrated that the method, employing the C. difficile-specific triose-phosphate isomerase gene, is as reliable but less time-consuming than counting c.f.u. We conclude that the method will be useful for evaluating the role of host cell adherence in the pathogenesis of C. difficile infection.

Effects of subinhibitory concentrations of antibiotics on colonization factor expression by moxifloxacin-susceptible and moxifloxacin-resistant Clostridium difficile strains

Recent outbreaks of Clostridium difficile infection have been related to the emergence of the NAP1/027 epidemic strain. This strain demonstrates increased virulence and resistance to the C-8-methoxyfluoroquinolones gatifloxacin and moxifloxacin. These antibiotics have been implicated as major C. difficile infection-inducing agents. We investigated by real-time reverse transcription-PCR the impact of subinhibitory concentrations of ampicillin, clindamycin, ofloxacin, and moxifloxacin on the expression of genes encoding three colonization factors, the protease Cwp84, the high-molecular-weight S-layer protein, and the fibronectin-binding protein Fbp68. We have previously shown in six non-NAP1/027 moxifloxacin-susceptible strains that the presence of ampicillin or clindamycin induced an upregulation of these genes, whereas the presence of fluoroquinolones did not. The objective of this study was to analyze the expression of these genes under the same conditions in four NAP1/027 strains, one moxifloxacin susceptible and three moxifloxacin resistant. Two in vitro-selected moxifloxacin-resistant mutants were also analyzed. Moxifloxacin resistance was associated with the Thr82-->Ile substitution in GyrA in all but one of the moxifloxacin-resistant strains. The expression of cwp84 and slpA was strongly increased after culture with ampicillin or clindamycin in NAP1/027 strains. Interestingly, after culture with fluoroquinolones, the expression of cwp84 and slpA was only increased in four moxifloxacin-resistant strains, including the NAP1/027 strains and one of the in vitro-selected mutants. The overexpression of cwp84 was correlated with increased production of the protease Cwp84. The historical NAP1/027 moxifloxacin-susceptible strain and its mutant appear to be differently regulated by fluoroquinolones. Overall, fluoroquinolones appear to favor the expression of some colonization factor-encoding genes in resistant C. difficile strains. The fluoroquinolone resistance of the NAP1/027 epidemic strains could be considered an ecological advantage. This could also increase their colonization fitness and promote the infection.

Antibiotics involved in Clostridium difficile-associated disease increase colonization factor gene expression

Clostridium difficile is the most common cause of antibiotic-associated diarrhoea. Antibiotics are presumed to disturb the normal intestinal microbiota, leading to depletion of the barrier effect and colonization by pathogenic bacteria. This first step of infection includes adherence to epithelial cells. We investigated the impact of various environmental conditions in vitro on the expression of genes encoding known, or putative, colonization factors: three adhesins, P47 (one of the two S-layer proteins), Cwp66 and Fbp68, and a protease, Cwp84. The conditions studied included hyperosmolarity, iron depletion and exposure to several antibiotics (ampicillin, clindamycin, ofloxacin, moxifloxacin and kanamycin). The analysis was performed on three toxigenic and three non-toxigenic C. difficile isolates using real-time PCR. To complete this work, the impact of ampicillin and clindamycin on the adherence of C. difficile to Caco-2/TC7 cells was analysed. Overall, for the six strains of C. difficile studied, exposure to subinhibitory concentrations (1/2 MIC) of clindamycin and ampicillin led to the increased expression of genes encoding colonization factors. This was correlated with the increased adherence of C. difficile to cultured cells under the same conditions. The levels of gene regulation observed among the six strains studied were highly variable, cwp84 being the most upregulated. In contrast, the expression of these genes was weakly, or not significantly, modified in the presence of ofloxacin, moxifloxacin or kanamycin. These results suggest that, in addition to the disruption of the normal intestinal microbiota and its barrier effect, the high propensity of antibiotics such as ampicillin and clindamycin to induce C. difficile infection could also be explained by their direct role in enhancing colonization by C. difficile.

Immunoreactive cell wall proteins of Clostridium difficile identified by human sera

Clostridium difficile is a leading cause of nosocomial infection in the developed world, causing antibiotic-associated disease in susceptible populations. The identity of immunogenic proteins is important in understanding the pathogenesis of disease and in the design of vaccines. This study analysed the sera of six patients infected during a hospital outbreak of a C. difficile ribotype 017 strain. Using a proteomics-based approach, cell wall proteins were separated by two-dimensional PAGE, and immunoreactive proteins were revealed by reaction with patient sera. The identity of immunoreactive proteins was established by MS. Forty-two different proteins were identified in total. All patient sera reacted with at least one component of the surface-layer protein (SLP), four reacted with both components (high- and low-molecular-mass SLPs), and five reacted with one other cell wall protein, suggesting that these are immunodominant antigens. The role of these proteins as potential vaccine candidates and their roles in pathogenesis deserve further study.

Diminished intestinal colonization by Clostridium difficile and immune response in mice after mucosal immunization with surface proteins of Clostridium difficile

Clostridium difficile pathogenesis is mainly due to toxins A and B. However, the first step of pathogenesis is the colonization process. We evaluated C. difficile surface proteins as vaccine antigens to diminish intestinal colonization in a human flora-associated mouse model. First, we used the flagellar cap protein FliD of C. difficile, in order to test several immunization routes: intranasal, rectal, and intragastric. The rectal route, which is the most efficient, was used to vaccine groups of mice with different antigen combinations. After immunizations, the mice were challenged with the toxigenic C. difficile and a significant statistical difference between the control group and the immunized groups was observed in the colonization levels of C. difficile.

Molecular characterization of Clostridium difficile clinical isolates in a geriatric hospital

The discriminatory potential of a combination of various typing methods was evaluated on a set of 21 Clostridium difficile isolates obtained from symptomatic patients hospitalized in a geriatric unit and 7 non-toxigenic isolates from the same hospital. Isolates were firstly serotyped and toxinotyped. Of the 28 isolates, 19 belonged to serogroup A. PCR-ribotyping and PCR-RFLP on the fliC and slpA genes were then applied to these 19 isolates. The results suggest that the combination of PCR-ribotyping with PCR-RFLP analysis of slpA could be more discriminatory and suitable for studying C. difficile epidemiology.

Variability of Clostridium difficile surface proteins and specific serum antibody response in patients with Clostridium difficile-associated disease

Pathogen attachment is a crucial early step in mucosal infections. This step is mediated by important virulence factors, such as surface proteins. Clostridium difficile surface proteins have been identified as (i) adhesins (the flagellar cap protein FliD; the flagellin FliC; and the cell wall protein Cwp 66 with a two domain-structure [Cw 66 N-terminal and Cwp 66 C-terminal domains]) and (ii) protease (the Cwp 84 protein). To address the roles of these proteins in the pathogenesis of Clostridium difficile and to identify vaccine antigen candidates, we analyzed the variability of the proteins and their immunogenicities in 17 patients with C. difficile-associated disease. PCR-restriction fragment length polymorphism analysis of amplified gene products revealed interstrain homogeneity with fliC and fliD, in contrast to cwp 66 genes. Immunoblot analysis showed that FliC and FliD were detected in the majority of isolates. The N-terminal domain of Cwp 66 and Cwp 84 were present in all strains tested, in contrast to the Cwp 66 C-terminal domain, the expression of which was heterogeneous. The 17 sera from the corresponding patients were analyzed by enzyme-linked immunosorbent assay to detect antibodies directed against these proteins. Many patients developed antibodies to FliC, FliD, Cwp 84, and the Cwp 66 C-terminal domain, but not to the Cwp 66 N-terminal domain. In conclusion, this study confirms the expression of these surface proteins of C. difficile during the course of the disease. In addition, the FliC, FliD, and Cwp 84 proteins appeared to be good potential vaccine candidates.