Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Clostridioides difficile (Formerly Clostridium difficile ) Infection
Reviewed for currency November 8, 2020
The administration of antibiotics can be complicated by a number of unintended consequences, among which gastrointestinal side effects are quite common. Gastrointestinal symptoms occur in up to 25% to 50% of patients depending on the specific antimicrobial agent, patient population, and epidemiology. While the majority of these side effects are mild, consisting of minor antibiotic-associated diarrhea (AAD) without systemic signs of illness, some patients can develop frank colitis and severe clinical manifestations, including toxic megacolon, intestinal perforation, sepsis, and death. Infection with Clostridioides difficile (formerly Clostridium difficile ) is thought to be responsible for about 25% of all cases of AAD and is the underlying etiology in nearly all cases of severe disease and pseudomembranous colitis (PMC).
While Koch's postulates were fulfilled for C. difficile 40 years ago, we still have an incomplete understanding of how disruption of the normal indigenous gut microbiota leads to C. difficile colonization and frank disease. Additionally, while adequate antimicrobial therapy for C. difficile infection was described shortly after the etiologic role for the pathogen was established, a recent increase in the apparent severity, prevalence, and recurrence of C. difficile infection (CDI) has prompted additional interest in understanding the pathogenesis of this infection.
Historical Overview
Prior to 1978 the disease pseudomembranous colitis and the organism Clostridioides difficile had never been linked. PMC was described in humans by Finney in 1893, and in animals in 1943 by Hambre and colleagues, following administration of penicillin that resulted in rapid animal death. PMC occurred in patients prior to the antibiotic era, perhaps consistent with the lack of prior antibiotic use in a small subset of current CDI patients. However, it was the antibiotic era that was accompanied by an increase in PMC, particularly in association with use of lincomycins. It was the publication by Tedesco and coworkers in 1974 that started the search for a cause for PMC linked to clindamycin use, at the time called clindamycin-associated colitis. Of 200 clindamycin-treated patients, 21% were found to have diarrhea and nearly half of those (10%) were found to have PMC, including 38% of patients who inexplicably had onset of diarrhea after clindamycin had been stopped.
After 4 years of intensive research, in 1977 and 1978 a succession of papers leading to the identification of C. difficile and its toxins as the cause of clindamycin-associated colitis or PMC and of vancomycin as an effective treatment were published. The laboratories of Larson in the United Kingdom, and Bartlett, Fekety, and Finegold in the United States were largely responsible for these discoveries, using the hamster as an essential animal model for CDI.
In retrospect, the rapidity and thoroughness of the CDI research discoveries of the 1970s may have engendered an early perception that there was little else to learn. This may have led to a clinical perception of CDI as a nuisance disease that was not even monitored as a health care–associated infection in many hospitals. This perception of CDI changed markedly in the early 21st century when CDI epidemics in North America and Europe with high mortality occurred that were caused by a previously little-known C. difficile strain that has been variously described as North American pulsed-field gel electrophoresis type 1 (NAP1), restriction endonuclease analysis (REA) type BI, and PCR ribotype 027, collectively referred to as NAP1/BI/027.
It should be noted that prior to the realization that C. difficile was the etiologic agent for the majority of cases of PMC, other infectious agents had been implicated as causative agents of PMC and AAD (reviewed by Gorkiewicz ). Staphylococcus aureus was one of the first bacteria associated with pseudomembranous enteritis following the administration of antibiotics. Experimental evidence was provided by studies that demonstrated that antibiotic-treated chinchillas would develop enterocolitis after infection with S. aureus. Although C. difficile is responsible for most cases of severe antibiotic-associated colitis, enterotoxigenic S. aureus strains can still be isolated from C. difficile –negative cases. Enterotoxigenic Clostridium perfringens strains have also been isolated from patients with AAD. Another form of severe antibiotic-associated colitis (associated with hemorrhage, but not pseudomembrane formation) appears to be due to infection with Klebsiella oxytoca. These findings indicate that a proportion of cases of antibiotic-associated colitis and AAD that are due to infection are not secondary to C. difficile, but this is only in the minority of cases. Additionally, it needs to be restated that infection (including infection with C. difficile ) accounts only for about a quarter of cases of AAD. In noninfectious cases of AAD, disruption of the normal composition and function of the indigenous gut microbiota alone, as detailed in the following section, is thought to be the underlying cause.
Pathogenesis
The Indigenous Gut Microbiota and Antibiotics
The pathogenesis of antibiotic-associated colitis and AAD, whether or not they are associated with C. difficile infection, is thought to center on the disruption of the normal, indigenous gut microbiota by antimicrobial drug administration. It is important to note that the concept of the indigenous gut microbiota playing a central role in human health has existed for at least 100 years. Recent scientific and technologic advances have permitted the study of the role that the gut microbiome plays in human health and disease, including the development of antibiotic-associated colitis and AAD.
The indigenous gut microbiota are thought to play a number of critical roles in host homeostasis and for the purposes of CDI, the function of importance has been called “colonization resistance.” Colonization resistance refers to the ability of the normal gut microbial community to resist the ingrowth of pathogenic microbes. A number of different mechanisms are speculated to be important for maintaining colonization resistance, including competition for nutrients, occupancy of ecologic and physical niches, production of antimicrobial products, and signaling through the host immune system ( Fig. 243.1 ). Regardless of the specific mechanisms, therapeutic antibiotic administration can profoundly disrupt the indigenous gut microbiota community and therefore disrupt colonization resistance. Studies in human and animal systems indicate that antibiotics can have long-lasting effects on the gut microbiome structure, and thus alter gastrointestinal function. With regard to C. difficile the altered gut microbiome structure and function can directly influence the biology of the pathogen in terms of growth and pathogenesis. In the following sections, as specific topics in the pathogenesis and virulence of C. difficile are discussed, potential influences of the indigenous microbiota are highlighted.
Sporulation and Germination in Clostridioides difficile
As with many anaerobic bacteria, during periods of environmental stress vegetative C. difficile bacteria initiate the sporulation program, which results in the production of the spore form of the organism. This physically robust form is stable to oxygen stress, temperature extremes, and desiccation. Within the hospital environment, this includes resistance to the effects of alcohol-based hand sanitizers. The spore form thus represents a reservoir for the organism, which can persist in the environment.
Following ingestion of spores, germination is apparently triggered by the environment that exists within the gastrointestinal tract. It has been shown that primary bile acids, including taurocholic acid, can serve as potent germinants in vitro, with glycine functioning as a cogerminant. Interestingly, members of the indigenous microbiota efficiently metabolize primary bile acids through the process of deconjugation and dehydroxylation to produce secondary bile acids, some of which have been shown to be inhibitory to C. difficile. Therefore one mechanism by which the disruption of the indigenous microbiome by antibiotics can lead to CDI is through alteration of in vivo bile acid metabolism. This leads to a situation that favors germination of ingested spores and the subsequent vegetative outgrowth of the pathogen. The key role of spore germination in the pathogenesis of C. difficile may serve as an attractive therapeutic target, as has been recently demonstrated with the use of a synthetic bile salt analogue that inhibits germination and has a protective effect in animal studies. Similarly, restoration of bile salt metabolism by administration of a bacterium with 7-α-dehydroxylase activity could restore colonization resistance against C. difficile.
Toxin Production
The histopathologic hallmark of C. difficile infection is damage to the mucosal epithelium with generation of an acute, neutrophil-predominant inflammatory response with the formation of a pseudomembrane consisting of sloughed epithelial cells, inflammatory cells, and a fibrinous exudate ( Fig. 243.2 ). Damage to the epithelium is caused by the most well-characterized C. difficile virulence factor, the large glucosyltransferase toxins TcdA and TcdB. Members of the large clostridial toxin family, TcdA and TcdB are produced within the gastrointestinal lumen during the stationary phase of vegetative growth of C. difficile. Following binding to a number of potential cell surface receptors, these toxins are taken up by the cells of the mucosal epithelium through the process of receptor-mediated endocytosis. Within the endosome the toxin undergoes autocleavage to release the catalytic subunit, which is transferred to the host cell cytoplasm. Subsequently, the active subunit glycosylates host cell guanosine triphosphatases belonging to the Rho family of cytoskeletal regulatory proteins. Intracellular activation of Rho guanosine triphosphatases leads to disruption of the actin cytoskeleton, ultimately leading to apoptotic and necrotic cell death. In cultured epithelial cells, cytoskeletal disruption results in cell rounding and cytotoxicity. In vivo these effects are manifested by loss of cell-cell tight junctions with subsequent increase in epithelial permeability. TcdA and TcdB also induce the secretion of cytokines in host cells, including interleukin-8, which likely leads to the acute neutrophilic inflammatory infiltrate that characterizes C. difficile infection. Recent data suggest that these two related toxins cause cell death and tissue damage via distinct pathways.
Other Virulence Factors
Some strains of C. difficile, including the NAP1/BI/027 strains that are responsible for the recent global outbreaks of CDI, secrete a toxin that is a member of the clostridial binary toxins exemplified by the iota toxin from Clostridium perfringens. C. difficile binary toxin (CDT) is a two-part toxin that is an ADP-ribosyltransferase specific for actin monomers, thus disrupting the actin cytoskeleton. CDT is produced by only a small fraction of C. difficile strains, and thus its role in pathogenesis is unclear. Although some reports suggest that infection with a CDT-producing C. difficile strain results in more severe clinical disease, this is not a universal finding and the actual significance of infection with CDT-producing strains is not clearly defined.
In addition to the three toxins, multiple other potential virulence factors for C. difficile have been studied, including surface layer proteins, surface polysaccharides, flagella, and various adhesins. To date, the specific roles of any of these factors in C. difficile virulence have not been delineated, but ongoing work on these factors may lead to new strategies to prevent and treat CDI.
Host Response to Clostridioides difficile Infection
Patients infected with C. difficile can mount adaptive immune responses to the toxins TcdA and TcdB. The level of antibody response to the C. difficile toxins is inversely correlated with the relative risk of developing recurrent disease. This finding served as the basis for the development of toxin-specific monoclonal antibodies for the prevention of CDI recurrence. Additionally, because the pathogenesis of C. difficile is tightly linked to pathogen expression of TcdA and TcdB, an alternative approach to immunotherapy in the form of toxin-based vaccines is being explored.
Additional work has been done examining the role of innate immune responses in the pathogenesis of CDI. In animal models, mice that are deficient in Toll-like receptor signaling, including MyD88 knockout mice, were shown to be more susceptible to experimental CDI. Similarly, animals deficient in signaling through the intracellular pattern-recognition pathway involving Nod1 are more susceptible to C. difficile infection. It appears that signaling through the innate immune system early during infection is important in host defense against C. difficile. Increased interleukin-23 signaling is encountered in humans with acute CDI, and this finding is replicated in murine models of infection. Protection against acute disease apparently reflects the balance of several innate immune responses to C. difficile infection. Innate lymphoid cells (ILCs) of the ILC-1 type and to a lesser extent of the ILC-3 type are required for recovery from acute disease in a murine model. Different studies demonstrated that eosinophils, stimulated by the microbiota, also provide protection in murine models of acute CDI. Interestingly, these same investigators demonstrated that expression of binary toxin may increase virulence of C. difficile by suppressing this eosinophilic response. These studies illustrate the complex system of interactions among the microbiota, host responses, and pathogen in the pathogenesis of CDI. Importantly, this may lead to novel treatment strategies for CDI, particularly severe acute disease.
Pathogenesis of Recurrent Clostridioides difficile Infection
While the majority of patients with symptomatic CDI respond to antibiotic therapy directed against the pathogen, up to 25% of patients will have recurrent symptoms following the discontinuation of C. difficile therapy for an initial episode of disease. The risk of recurrence increases with each episode, resulting in significant morbidity and mortality in this population of patients. A number of factors have been investigated with regard to the pathogenesis of recurrent infection. As noted previously, the adaptive host response may play a role in determining the risk of recurrent disease.
Several studies have indicated that pathogen characteristics may contribute to the risk of recurrence. Infection with the epidemic NAP1/BI/027 C. difficile strain is associated with an increased risk of recurrence. The reasons for this are unclear, but may have to do with the intrinsic antibiotic resistance, the dynamics of spore biology of the strain, or the presence of an additional toxin, binary toxin.
Because alteration of the indigenous gut microbiota by antibiotics is a prerequisite for the majority of cases of CDI, the role of the gut microbiome in recurrence has been examined. It has been shown that patients with recurrent CDI exhibit decreased microbial diversity of their indigenous gut microbes. Presumably, if the gut microbiota is unable to return to its baseline state, the patient is at increased risk of reinfection or regrowth of residual C. difficile following therapy. This has led to the exploration of restoring microbiome diversity through the administration of probiotic bacteria or the transplantation of feces from normal donors (see “ Treatment ” later).
Altered Virulence in Specific Clostridioides difficile Lineages
The increase in the incidence and apparent severity of CDI over the past 15 years has been associated with the widespread appearance of the NAP1/BI/027 strain of C. difficile. This has prompted a number of investigators to determine if this particular strain of C. difficile is actually associated with worse outcomes in infected patients and whether or not this strain has specific virulence determinants not widespread in other strains of the pathogen.
Several factors are characteristic of NAP1/BI/027 strains. These strains typically are resistant to newer fluoroquinolone antibiotics. Furthermore they possess the binary toxin CDT, and also harbor a characteristic mutation in the anti-sigma factor tcdC, which is theorized to play a role in the regulation of toxins TcdA and TcdB. This mutation in tcdC introduces a frameshift that results in a truncated, nonfunctional protein. Because studies have demonstrated that these epidemic strains can produce increased levels of toxin in vitro, it has been proposed that this characteristic mutation is responsible for this phenotype, although this has been questioned by reconstitution of tcdC that showed no effect on toxin production. There is in vitro evidence that the TcdC regulator can inhibit in vitro expression of the large C. difficile toxins, but the presence of alterations within this regulatory gene does not directly correlate with increased production of toxins within clinical isolates. One group demonstrated that complementation of the tcdC mutation in trans will decrease the amount of toxin produced by a NAP1/BI/027 clinical isolate and reduce the virulence of this strain in the hamster model of infection. However, a second group that utilized a genetic system that allowed correction of the tcdC mutation directly on the chromosome failed to show an association between tcdC genotype and the level of toxin production.
Additional in vitro studies suggested that NAP1/BI/027 strains had an increased ability to sporulate, which could contribute to the ease with which these strains can spread within the health care environment. However, this observation has also been disputed by measuring sporulation in vitro in multiple C. difficile strains, including multiple NAP1/BI/027 strains that did not reveal increased sporulation of these epidemic strains. Perhaps most important to the enhanced virulence of these strains is the presence of a third toxin, binary toxin CDT, an ADP-ribosylating toxin that has been shown to enhance the lethality of toxin A in the hamster model and to cause a modest degree of hamster mortality in the absence of toxins A and B. It is interesting to note that this constellation of characteristics has also arisen within an unrelated epidemic strain of polymerase chain reaction (PCR) ribotype 078 that was identified in Europe (and that arose in swine) and together with ribotype 027 was found to exhibit increased 14-day mortality in CDI patients.
Despite this evidence, there is still debate over the relative importance of these genetic and phenotypic characteristics of the epidemic C. difficile strains in the pathogenesis of the organism. In spite of these conflicting data, it is likely that our understanding of the basic pathogenesis of CDI will result in the development of more effective strategies for prevention and treatment of this infection.
Epidemiology
C. difficile is a ubiquitous organism, able to survive for long periods in the environment through sporulation. al Saif and Brazier showed the widespread presence of C. difficile in environmental sites such as surface water, drinking water, swimming pools, and soil, and in a wide variety of animals, including dogs, cats, horses, sheep, and pigs. In addition, some foods have been shown to be contaminated at a low level with C. difficile spores that are of the same ribotype that cause clinical disease. This includes beef, pork, turkey, and a variety of vegetables, but there has been no evidence of foodborne transmission to date. It is highly likely that humans ingest C. difficile spores frequently, but remain asymptomatic (and uncolonized) as a result of the colonization resistance of an intact gut microbiota.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here