Diseases Caused by Clostridium

The genus Clostridium includes over 200 described species. Members of this genus participate in a variety of invasive and toxigenic infections. They can cause disease that is strictly toxin mediated, such as antibiotic-associated colitis (AAC) and foodborne botulism, or contribute to invasive infections, including bacteremia, clostridial myonecrosis (gas gangrene), and other suppurative infections driven by histotoxins and enzymes that destroy soft tissue. Historically, clostridial infections were recognized as discrete clinical syndromes well before the germ theory of disease was proposed. The clinical features of tetanus were well described by some of the earliest medical writers, such as Hippocrates, and the toxic nature of this species was noted as early as the 1870s. Clostridia are often isolated as part of a mixed microbiota during suppurative infections that occur as a result of fecal or soil contamination of otherwise sterile tissues. Prior to 1977, the most commonly reported clostridial infections and intoxications were those caused by Clostridium perfringens. Other species, most notably Clostridium tetani in nonimmunized individuals and Clostridium botulinum, also generated considerable interest due to the severity and often fatal nature of the intoxication they caused.

With the discovery of the etiology of AAC first in an animal model and subsequently in humans, it soon became clear that in the antibiotic era Clostridioides difficile (formerly Clostridium difficile) was the most common clostridial species associated with the human disease formerly called C. difficile –associated diarrhea and currently called C. difficile infection (CDI). Within the hospital setting, CDI has become a significant worldwide nosocomial infection problem resulting in both toxin-mediated diarrheal disease and more fulminant presentations such as pseudomembranous enterocolitis and toxic megacolon. Recent recognition that more virulent strains of C. difficile occur in health care settings has provoked an increased awareness of this nosocomial infection and has prompted a demand for both rapid methods for diagnosis and more aggressive treatment of CDI and AAC. While the well-recognized pathogenic members of the genus Clostridium continue to participate in a broad array of infectious processes, it is also important to note the important role that previously obscure species, such as C. difficile, play in human disease.

Characteristics of Clostridium Species

Microbiology

Members of this genus are phenotypically characterized as anaerobic, gram-positive rods capable of forming endospores. Clostridium spp. are ubiquitous in nature, found in soils and sediments throughout the world and as members of the intestinal microbiome of humans and most other animals. Over 70% of humans are colonized with clostridia at concentrations of 10 ⁸ to10 organisms per gram of feces. Clostridia can also be isolated as part of the vaginal microbiome of healthy women, although they tend to be transient members of this microbiome, occurring in low numbers as a result of contamination by intestinal microbiota, rather than as part of the autochthonous community. Most members of this genus are obligate anaerobes, while strains of a few Clostridium species, such as C. tertium, C. histolyticum, C. innocuum, and C. perfringens, are aerotolerant and can be confused with members of the genus Bacillus during laboratory diagnosis. Based on 16S ribosomal DNA (rDNA) sequence data, members of the genus Clostridium are part of the phylum Firmicutes, a diverse group of gram-positive organisms including both spore-forming and non–spore-forming genera. Based on 16S rDNA sequence analysis, the clostridia can be divided into 11 homology groups, with most of the clinically significant species belonging to homology group 1. Traditional phenotypic classification methods for the clostridia rely on carbohydrate fermentation profiles, detection of short-chain fatty acid end products of fermentation, Gram stain morphology, colony morphology on agar media, and detection of specific toxins. More recently, proteomic analysis using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry has been employed for identification of certain species. Although many different species have been isolated from human clinical material, only a small number of species are regularly associated with human disease ( Table 246.1 ).

TABLE 246.1

Clostridial Species Commonly Associated With Human Disease

SPECIES	SPORE LOCATION	LECITHINASE PRODUCED	LIPASE	ENTEROTOXINS PRODUCED	HISTOTOXINS, HEMOLYSINS, PROTEASES	NEUROTOXINS PRODUCED
Tissue Infections
C. perfringens	ST, C	+	−	Yes	Yes	No
C. ramosum	T	−	−	No	Yes	No
C. septicum	ST	−	−	No	Yes	No
C. sordellii	ST	+	−	No	Yes	No
C. bifermentans	ST	+	−	No	Yes	No
C. tertium	T	−	−	No	Yes	No
C. sphenoides	ST	−	−	No	Yes	No
C. baratii	ST	−	−	No	Yes	No
C. novyi	ST	+	+	No	Yes	No
C. histolyticum	ST	−	−	No	Yes	No
Intoxications
C. difficile	ST	−	−	Yes	Yes	No
C. botulinum	ST, T	−	+	No	Yes	Yes
C. tetani	T	−	−	No	Yes	Yes

C, Centrally; ST, subterminally; T, terminally.

Microscopically, the vegetative cells of Clostridium species are rod-shaped, often pleomorphic, and found as short chains, as clusters, or in pairs. The cells of most species have rounded ends. This may vary, with some species showing more pointed ends ( Clostridium ramosum ). Some species form long chains ( Clostridium spiroforme ), which may be tightly packed to form coils. Clostridia usually stain gram positively in young cultures, with some species losing this staining characteristic in older cultures. Species such as Clostridium clostridioforme and C. ramosum rarely show the typical gram-positive appearance and present as gram-variable or gram-negative rods ( Fig. 246.1 ). When spores are present, they tend to be ovoid or spherical, with the spore often distending the vegetative cell to produce a “club-shaped” appearance. Spores may be located centrally, subterminally, or as terminal structures, depending on the species. Spore location is used as part of the phenotypic identification process. Most clostridia are motile by virtue of peritrichous flagellae, with the notable exception of the common clinical isolates C. perfringens and C. ramosum,

FIG. 246.1, Gram stain characteristics of Clostridium species.

Clostridium spp. have diverse metabolic pathways and can be saccharolytic, proteolytic, both, or neither. Clostridium spp. are not known to reduce sulfate. The end products of fermentative metabolism are mixtures of short-chain fatty acids and alcohols, a characteristic that can be used for identification purposes in the clinical laboratory. Aerotolerant strains of clostridia do not form spores in the presence of oxygen, are catalase negative, and grow more abundantly under anaerobic conditions. Clostridia do not have a complete cytochrome system and are therefore oxidase negative. Most strains are catalase and superoxide dismutase negative, although trace amounts of activity have been reported for some species. Clostridia produce a variety of biologically active proteins, including hemolysins, proteolytic enzymes, and other toxins. It is the protein toxins produced by clostridia that account for their importance in human disease. Clostridia produce a greater diversity of toxins than any other genera of bacteria. These include neurotoxins, enterotoxins, collagenases, proteases, necrotoxins, lecithinases, lipases, DNases, and neuraminidases. The potency of some of these toxins, such as botulinum neurotoxin (BoNT) and tetanus neurotoxin (TeNT), render them among the most lethal substances yet described; less than 0.2 ng of purified TeNT is fatal in mice.

Pathogenesis

Invasive infections caused by clostridia are invariably due to organisms that are either part of the normal intestinal and vaginal microbiome or acquired by a traumatic injury breaching the skin that becomes contaminated with soil, unsanitary water, or fecal material. Intoxications can occur either in response to endogenous toxin production, such as that associated with CDI, or by ingestion of preformed toxins contaminating food, as is the case for noninfant botulism. Apart from environmental spread of C. difficile within a susceptible population, such as hospitalized patients on broad-spectrum antibiotic therapy and residents in nursing homes, clostridia rarely cause infection through person-to-person contact.

The spores of clostridia account for their persistence in hostile environments and their exogenous acquisition by humans. In addition to their long-term survival in soil or food, clostridial spores may spread via aerosol transmission as part of naturally occurring dust clouds. C. difficile is of concern because this species may be part of the intestinal microbiome of an individual or may be acquired through contact with individuals or contaminated surfaces and equipment within the hospital environment harboring spores. The vegetative cells of clostridia are generally susceptible to routinely used disinfectants; however, spores can survive hostile environments, including heat, desiccation, and exposure to many commonly employed disinfectants. This allows pathogenic clostridia to persist in the environment, even following routine disinfection procedures. Methods for eliminating clostridial spores from some environments, such as C. difficile in the hospital setting, include finding methods to promote germination of the spores so that the vegetative cells can be destroyed.

Clostridium spp., particularly members of clusters IV and XIVa derived from the gut microbiota, have recently been shown to have beneficial effects on the immune system. One can speculate that this may be why almost all children have C. difficile present in their intestine during the first year of life. Specifically, a consortia of Clostridium species promoted accumulation of colonic regulatory T-cell development in mice. Inoculation of these Clostridium spp. into mice increased their resistance to chemically induced colitis and blunted their allergic responses. This finding casts this genus in a new light, suggesting its members are not only fearsome pathogens but also may hold therapeutic potential for autoimmune and allergic conditions.

Major Infections and Intoxications

Clostridioides difficile Infection

Historical Perspective

Until it was identified as the primary cause of AAC in 1977, C. difficile was not regarded as a particularly common or important pathogen; however, the association of C. difficile with CDI and AAC has brought this organism to prominence as the most common clostridial species associated with human disease. Hall and O'Toole published the first description of C. difficile in 1935 and suggested that it might be involved in intestinal disease in children. Interestingly, the clinical description of pseudomembranous colitis dates to the 1890s. An animal model for antibiotic-associated intestinal disease was first reported in the 1940s, with several additional observations on the induction of bowel inflammation by antibiotics in hamsters, guinea pigs, and rabbits made in the 1950s. The occurrence of pseudomembranous colitis in patients receiving broad-spectrum antibiotics prior to the 1970s was not uncommon. Based on laboratory analysis, it was often attributed to Staphylococcus aureus, one of the major nosocomial pathogens of the antibiotic era. Cultures of stool from these patients often yielded high levels of S. aureus; however, obligately anaerobic organisms were not evaluated in these early studies. Given the common isolation of S. aureus from stool samples obtained from healthy individuals, these earlier observations were something of a self-fulfilling prophecy. While certain strains of S. aureus produce potent enterotoxins that may be responsible for some cases of AAC, there is little evidence to suggest that this species is a common cause of pseudomembranous colitis.

In 1974, investigators in St. Louis noted that about 20% of patients receiving the lincosamide antibiotic clindamycin developed diarrhea, and half of these patients had pseudomembranous colitis when examined endoscopically. Publication of these observations set the stage for more detailed examination of the role of antibiotics in the occurrence of pseudomembranous colitis and led to the search for an etiologic agent. The breakthrough that ultimately led to identification of C. difficile as the causative agent of CDI involved a hamster model of AAC demonstrating that vancomycin prevented the occurrence of AAC induced by clindamycin, suggesting that a gram-positive organism was involved in the hamster disease. Armed with this information and the knowledge that the disease appeared to be toxin-mediated, based on molecular size filtration studies, these same investigators isolated clostridial species from the ceca of hamsters with AAC and showed that one species, C. difficile, can cause disease in other hamsters as pure cultures or culture filtrates in the absence of prior antibiotic exposure. The link to human disease was made when the same toxin isolated from the hamster model was found in stools of AAC patients using a combination of cytotoxicity assays and an anti-clostridial antibody capable of neutralizing the cytotoxic effect. Vancomycin remains the antibiotic of choice for treating serious CDI in humans (see Chapter 243 for a more extensive discussion of treatment alternatives), and the same cytotoxicity assay used to correlate animal and human disease remains the gold standard for evaluating other diagnostic assays.

Clinical Manifestations

The clinical manifestations of CDI range from a self-limiting diarrheal disease that disappears when antibiotics are discontinued to fulminant presentations with characteristic pseudomembranes within the large intestine and progression to toxic megacolon, often with fatal complications. Pseudomembranes, while present in 97% of CDI, are not pathognomonic for CDI and may also occur in antibiotic-associated diarrhea not caused by C. difficile. Symptoms may occur while patients are receiving antibiotics, usually after 5 to 10 days of therapy, or can occur 2 to 10 weeks after antibiotic therapy has been completed. All classes of antibiotics have been associated with CDI, including the penicillins, cephalosporins, macrolides, lincosamides, and aminoglycosides. Diarrhea accompanied by fever occurs in most patients and resembles the symptoms caused by many other intestinal pathogens. In severe cases, bloody diarrhea may be present. Diarrhea is often accompanied by bloating and cramping. Leukocytosis is not uncommon. CDI is more common in the elderly and in hospitalized patients receiving broad-spectrum antibiotic therapy. When surgical intervention is required for severe disease, colostomy carries a substantial risk of mortality regardless of the age of the patient.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here