Clostridium difficile

Clostridium difficile is a Gram-positive, toxin-producing anaerobic bacterium belonging to the family Clostridiaceae of the Clostridiales. Though strictly oxygen-intolerant, C. difficile is able to produce aerotolerant endospores under unfavourable conditions that are capable of persisting in an open environment for years. C. difficile is a commensalist species typically housed in the colonic fecal flora of a fairly small subset of the child population, with the number of carriers decreasing as children age (Kelly and LaMont, 1998). When it exists in small numbers, this organism remains non-pathogenic. However, when it does manage to colonize and yield larger populations, its pathogenicity becomes the root cause of a variety of colon
infections.

Figure 1. This micrograph depicts Clostridium difficile bacteria from a stool sample culture.

Infection by C. difficile arises in an opportunistic manner; normal microflora in the gut attributes to the body’s defense against pathogenic domination and so, mass colonization of C. difficile usually occurs in individuals where typical colonic flora has been disrupted as a result of antibiotic use, individuals who are immunosuppressed (AIDS or cancer patients), or people taking so-called intestinal cleansers.

C. difficile inherent pathogenicity lies in its ability to produce toxin. In particular, this pathogen produces two potent cytotoxins, namely, toxin A and toxin B, which ultimately lead to C. difficile-associated infection and disease (Mylonakis et al., 2001).

Toxins A and toxin B are glucosyltransferases that target and inactivate the Rho family of GTPase enzmyes. Toxin A induces actin depolymerization by a mechanism correlated with a decrease in the ADP-ribosylation of the low molecular mass GTP-binding Rho proteins. Another toxin, binary toxin, has also been described, but its role in disease is not yet fully understood. Eventually, this leads to the deterioration of epithelial cell components and inevitably cell death. Additionally, both toxins induce a strong cellular inflammatory reaction and mass cytokine release, as well as activate the enteric nervous system, attracting neutrophils to the site (Jefferson et al., 1999). The overall outcome of toxin A and toxin B production is colonic mucosal injury and inflammation.

C. difficile is transmitted from person to person by the fecal-oral route. Because the organism forms heat-resistant spores, it can remain in the hospital or nursing home environment for long periods of time. It can be cultured from almost any surface in the hospital. Once spores are ingested, they pass through the stomach unscathed because of their acid-resistance. They germinate into vegetative cells in the colon upon exposure to bile acids and multiply.

C. difficile-associated infection is typically restricted to the lower
abdominal regions of the gastrointestinal tract; clinical manifestations of C. difficile infection vary in terms of severity, from diarrhea to acute inflammation of the colon leading to death. Pseudomembranous colitis (PMC) is the one condition most commonly associated with C. difficile, as it has been linked directly to being almost exclusively caused by none other than C. difficile in every case. PMC is characterized as a progression of diarrhea to which the colonic mucosa becomes severely inflamed. Along with symptoms such as diarrhea, fever, and abdominal distention and pain, pseudomembrane formation occurs. This pseudomembrane is composed of a variety of cellular components, including fibrin, mucous, dead cell debris, and leukocytes. Pseudomembranes appear in various regions of the colon walls and, in fatal cases, form a sheath over the entire colonic mucosa (Mylonakis et al., 2001).

Pseudomembranous colitis caused by C. difficile is treated with specific antibiotics, such as vancomycin or metronidazole. To reduce complications, physicians often begin treatment based on clinical presentation before definitive results are available. Knowledge of the local epidemiology of intestinal flora of a particular institution can guide therapy. In addition, oral rehydration therapy is useful in retaining fluids during the duration of diarrhea. Interestingly, several disinfectants commonly used in hospitals may be ineffective against C. difficile spores, and may actually promote spore formation. However, disinfectants containing bleach are effective in killing the organisms and should constantly be used as a sanitizer.

Laboratory culturing and Gram staining procedures are considered too unspecific to identify C. difficile in clinical situations due to its morphological similarity to other Clostridia species that make up the normal microflora of the colon. Typical diagnostic tests select for the presence of toxin A or toxin B production (Mylonakis et al., 2001). Tissue culture assay for specificity of toxin B cytotoxicity remains one of the most sensitive and exact tests used for diagnosis. Enzyme-linked immunosorbent assays (ELISAs) are also able to identify toxin A and/or toxin B in stool, which are rapid, specific, and most frequently used for clinical diagnosis of presumed C. difficile infections.

References:

Jefferson, K.K., Smith, M.F. Jr., & Bodak, D.A. (1999). Roles of intracellular calcium and NF-kappa B in the Clostridium difficile toxin A-induced up-regulation and secretion of IL-8 from human monocytes. Journal of Immunology, 163: 5183-5191.

Kelly, C.P. & LaMont, T. (1998). Clostridium difficile infection. Annual Review of Medicine, 49: 375-390.

Mylonakis, E., Ryan, E.T., & Calderwood, S.B. (2001). Clostridium difficile—Associated Diarrhea. Archives of Internal Medicine, 161: 525-533.