Chapter 21: Infections Caused by Anaerobic Bacteria

Medically important infections caused by anaerobic bacteria are common. The infections are often polymicrobial—that is, the anaerobic bacteria are found in mixed infections with other anaerobes, facultative anaerobes, and aerobes (see the glossary of definitions). Anaerobic bacteria are found throughout the human body—on the skin, on mucosal surfaces, and in high concentrations in the mouth and gastrointestinal tract—as part of the normal microbiota (see Chapter 10). Infection results when anaerobes and other bacteria of the normal microbiota contaminate normally sterile body sites.

Several important diseases are caused by anaerobic Clostridium species from the environment or from normal flora: botulism, tetanus, gas gangrene, food poisoning, and pseudomembranous colitis. These diseases are discussed in Chapters 9 and 11.

Anaerobic bacteria do not grow in the presence of oxygen and are killed by oxygen or toxic oxygen radicals (see later discussion); pH and oxidation-reduction potential (E_h) are also important in establishing conditions that favor growth of anaerobes. Anaerobes grow at a low or negative E_h.

Aerobes and facultative anaerobes often have the metabolic systems listed below, but anaerobic bacteria frequently do not.

1. Cytochrome systems for the metabolism of O₂

2. Superoxide dismutase (SOD), which catalyzes the following reaction:

   

3. Catalase, which catalyzes the following reaction:

   

Anaerobic bacteria do not have cytochrome systems for oxygen metabolism. Less fastidious anaerobes may have low levels of SOD and may or may not have catalase. Most bacteria of the Bacteroides fragilis group have small amounts of both catalase and SOD. There appear to be multiple mechanisms for oxygen toxicity. Presumably, when anaerobes have SOD or catalase (or both), they are able to negate the toxic effects of oxygen radicals and hydrogen peroxide and thus tolerate oxygen. Obligate anaerobes usually lack SOD and catalase and are susceptible to the lethal effects of oxygen; such strict obligate anaerobes are infrequently isolated from human infections, and most anaerobic infections of humans are caused by “moderately obligate anaerobes.”

The ability of anaerobes to tolerate oxygen or grow in its presence varies from species to species. Similarly, there is strain-to-strain variation within a given species (eg, one strain of Prevotella melaninogenica can grow at an O₂ concentration of 0.1% but not of 1%; another can grow at a concentration of 2% but not of 4%). Also, in the absence of oxygen, some anaerobic bacteria grow at a more positive E_h.

Facultative anaerobes grow as well or better under anaerobic conditions than they do under aerobic conditions. Bacteria that are facultative anaerobes are often termed aerobes. When a facultative anaerobe such as E coli is present at the site of an infection (eg, abdominal abscess), it can rapidly consume all available oxygen and change to anaerobic metabolism, producing an anaerobic environment and low E_h and thus allow the anaerobic bacteria that are present to grow and produce disease.

ANAEROBIC BACTERIA FOUND IN HUMAN INFECTIONS

Since the 1990s, the taxonomic classification of the anaerobic bacteria has changed significantly due to the application of molecular sequencing and DNA–DNA hybridization technologies. The nomenclature used in this chapter refers to genera of anaerobes frequently found in human infections and to certain species recognized as important pathogens of humans. Anaerobes commonly found in human infections are listed in Table 21-1.

Gram-Negative Anaerobes

A. Gram-Negative Bacilli

1. Bacteroides—The Bacteroides species are very important anaerobes that cause human infection. They are a large group of bile-resistant, non–spore-forming, slender gram-negative rods that may appear as coccobacilli. Many species previously included in the genus Bacteroides have been reclassified into the genus Prevotella or the genus Porphyromonas. Those species retained in the Bacteroides genus are members of the B fragilis group (~20 species).

Bacteroides species are normal inhabitants of the bowel and other sites. Normal stools contain 10¹¹ B fragilis organisms per gram (compared with 10⁸/g for facultative anaerobes). Other commonly isolated members of the B fragilis group include Bacteroides ovatus, Bacteroides distasonis, Bacteroides vulgatus, and Bacteroides thetaiotaomicron. Bacteroides species are most often implicated in intra-abdominal infections, usually under circumstances of disruption of the intestinal wall as occurs in perforations related to surgery or trauma, acute appendicitis, and diverticulitis. These infections are often polymicrobial. Both B fragilis and B thetaiotaomicron are implicated in serious intrapelvic infections such as pelvic inflammatory disease and ovarian abscesses. B fragilis group species are the most common species recovered in some series of anaerobic bacteremia, and these organisms are associated with a very high mortality rate. As discussed later in the chapter, B fragilis is capable of elaborating numerous virulence factors, which contribute to its pathogenicity and mortality in the host.

2. Prevotella—Prevotella species are gram-negative bacilli and may appear as slender rods or coccobacilli. Most commonly isolated are P melaninogenica, Prevotella bivia, and Prevotella disiens. P melaninogenica and similar species are found in infections associated with the upper respiratory tract. P bivia and P disiens occur in the female genital tract. Prevotella species are found in brain and lung abscesses, in empyema, and in pelvic inflammatory disease and tubo-ovarian abscesses.

In these infections, the prevotellae are often associated with other anaerobic organisms that are part of the normal microbiota—particularly peptostreptococci, anaerobic Gram-positive rods, and Fusobacterium species—as well as Gram-positive and Gram-negative facultative anaerobes that are part of the normal microbiota.

3. Porphyromonas—The Porphyromonas species also are Gram-negative bacilli that are part of the normal oral microbiota and occur at other anatomic sites as well. Porphyromonas species can be cultured from gingival and periapical tooth infections and, more commonly, breast, axillary, perianal, and male genital infections.

4. Fusobacteria—There are approximately 13 Fusobacterium species, but most human infections are caused by Fusobacterium necrophorum and Fusobacterium nucleatum. Both species differ in morphology and habitat as well as the range of associated infections. F necrophorum is a very pleomorphic, long rod with round ends and tends to make bizarre forms. It is not a component of the healthy oral cavity. F necrophorum is quite virulent, causing severe infections of the head and neck that can progress to a complicated infection called Lemierre’s disease. The latter is characterized by acute jugular vein septic thrombophlebitis that progresses to sepsis with metastatic abscesses of the lungs, mediastinum, pleural space, and liver. Lemierre’s disease is most common among older children and young adults and often occurs in association with infectious mononucleosis. F necrophorum is also seen in polymicrobial, intra-abdominal infections. F nucleatum is a thin rod with tapered ends (needle-shaped morphology) and is a significant component of the gingival microbiota as well as the genital, gastrointestinal, and upper respiratory tracts. As such, it is frequently encountered in a variety of clinical infections such as pleuropulmonary infections, obstetric infections, significantly chorioamnionitis, and occasionally brain abscesses complicating periodontal disease. Rarely does it cause bacteremia in neutropenic patients.

Bacterial vaginosis is a common vaginal condition of women of reproductive age. It is associated with premature rupture of membranes and preterm labor and birth. Bacterial vaginosis has a complex microbiology; one organism, Gardnerella vaginalis, has been most specifically associated with the disease process.

GARDNERELLA VAGINALIS

G vaginalis is a serologically distinct organism isolated from the normal female genitourinary tract and also associated with vaginosis, so named because inflammatory cells are not present. In wet smears, this “nonspecific” vaginitis, or bacterial vaginosis, yields “clue cells,” which are vaginal epithelial cells covered with many Gram-variable bacilli, and there is an absence of other common causes of vaginitis such as trichomonads or yeasts. Vaginal discharge often has a distinct “fishy” odor and contains many anaerobes in addition to G vaginalis. The pH of the vaginal secretions is greater than 4.5 (normal pH is <4.5). The vaginosis attributed to this organism is suppressed by metronidazole, suggesting an association with anaerobes. Oral metronidazole is generally curative.

Gram-Positive Anaerobes

A. Gram-Positive Bacilli

1. Actinomyces—The Actinomyces group includes several species that cause actinomycosis, of which Actinomyces israelii and Actinomyces gerencseriae are the ones most commonly encountered. Several new, recently described species that are not associated with actinomycosis have been associated with infections of the groin, urogenital area, breast, and axilla and postoperative infections of the mandible, eye, and head and neck. Some species have also been implicated in cases of endocarditis, particularly among substance abusers. These newly described species are aerotolerant and form small, nondescript colonies that are probably frequently overlooked as contaminants. On Gram stain, they vary considerably in length; they may be short and club shaped or long, thin, beaded filaments. They may be branched or unbranched. Because they often grow slowly, prolonged incubation of the culture may be necessary before laboratory confirmation of the clinical diagnosis of actinomycosis can be made. Some strains produce colonies on agar that resemble molar teeth. Some Actinomyces species are oxygen tolerant (aerotolerant) and grow in the presence of air; these strains may be confused with Corynebacterium species (diphtheroids; see Chapter 12). Actinomycosis is a chronic suppurative and granulomatous infection that produces pyogenic lesions with interconnecting sinus tracts that contain granules composed of microcolonies of the bacteria embedded in tissue elements (Figure 21-1). Infection is initiated by trauma that introduces these endogenous bacteria into the mucosa. The organisms grow in an anaerobic niche, induce a mixed inflammatory response, and spread with the formation of sinuses, which contain the granules and may drain to the surface. The infection causes swelling and may spread to neighboring organs, including the bones.

Based on the site of involvement, the three common forms are cervicofacial, thoracic, and abdominal actinomycosis. Cervicofacial disease presents as a swollen, erythematosus process in the jaw area (known as “lumpy jaw”). With progression, the mass becomes fluctuant, producing draining fistulas. The disease will extend to contiguous tissue, bone, and lymph nodes of the head and neck. The symptoms of thoracic actinomycosis resemble those of a subacute pulmonary infection and include a mild fever, cough, and purulent sputum. Eventually, lung tissue is destroyed, sinus tracts may erupt through to the chest wall, and invasion of the ribs may occur. Abdominal actinomycosis often follows a ruptured appendix or an ulcer. In the peritoneal cavity, the pathology is the same, but any of several organs may be involved. Genital actinomycosis is a rare occurrence in women that results from colonization of an intrauterine device with subsequent invasion.

Diagnosis can be made by examining pus from draining sinuses, sputum, or specimens of tissue for the presence of sulfur granules. The granules are hard, lobulated, and composed of tissue and bacterial filaments, which are club shaped at the periphery. Specimens should be cultured anaerobically on appropriate media. Treatment requires prolonged administration of penicillin (6–12 months). Clindamycin or erythromycin is effective in penicillin-allergic patients. Surgical excision and drainage may be required.

2. Propionibacterium—Propionibacterium species are members of the normal microbiota of the skin, oral cavity, large intestine, conjunctiva, and external ear canal. Their metabolic products include propionic acid, from which the genus name derives. On Gram stain, they are highly pleomorphic, showing curved, clubbed, or pointed ends; long forms with beaded uneven staining; and occasionally coccoid or spherical forms. Propionibacterium acnes, often considered an opportunistic pathogen, causes the disease acne vulgaris and is associated with a variety of inflammatory conditions. It causes acne by producing lipases that split free fatty acids off from skin lipids. These fatty acids can produce tissue inflammation that contributes to acne formation. In addition, P acnes is frequently a cause of postsurgical wound infections, particularly those that involve insertion of devices, such as prosthetic joint infections, particularly of the shoulder, central nervous system shunt infections, osteomyelitis, endocarditis, and endophthalmitis. Because it is part of the normal skin microbiota, P acnes sometimes contaminates blood or cerebrospinal fluid cultures that are obtained by penetrating the skin. It is therefore important (but often difficult) to differentiate a contaminated culture from one that is positive and indicates infection.

3. Clostridium—Clostridia are Gram-positive, spore-forming bacilli (see Chapter 11).

B. Gram-Positive Cocci

The group of anaerobic gram-positive cocci has undergone significant taxonomic expansion. Many species within the genus Peptostreptococcus have been reassigned to new genera such as Anaerococcus, Finegoldia, and Peptoniphilus. The species contained within these genera, as well as Peptococcus niger, are important members or the normal microbiota of the skin, oral cavity, upper respiratory tract, gastrointestinal tract, and female genitourinary system. The members of this group are opportunistic pathogens and are most frequently found in mixed infections particularly from specimens that have not been carefully procured. However, these organisms have been associated with serious infections such as brain abscesses, pleuropulmonary infections, necrotizing fasciitis, and other deep skin and soft tissue infections, intra-abdominal infections, and infections of the female genital tract.

PATHOGENESIS OF ANAEROBIC INFECTIONS

Infections caused by anaerobes commonly are caused by combinations of bacteria that function in synergistic pathogenicity. Although studies of the pathogenesis of anaerobic infections have often focused on a single species, it is important to recognize that the anaerobic infections most often are caused by several species of anaerobes acting together to cause infection.

B fragilis is a very important pathogen among the anaerobes that are part of the normal microbiota. The pathogenesis of anaerobic infection has been most extensively studied with B fragilis using a rat model of intra-abdominal infection, which in many ways mimics human disease. A characteristic sequence occurs after colon contents (including B fragilis and a facultative anaerobe such as E coli) are placed via needle, gelatin capsule, or other means into the abdomens of rats. A high percentage of the study animals die of sepsis caused by the facultative anaerobe. However, if the animals are first treated with gentamicin, a drug effective against the facultative anaerobe but not Bacteroides species, few of the animals die, and after a few days, the surviving animals develop intra-abdominal abscesses from the Bacteroides infection. Treatment of the animals with both gentamicin and clindamycin, a drug effective against Bacteroides species, prevents both the initial sepsis and the later development of abdominal abscesses.

The capsular polysaccharides of Bacteroides are important virulence factors. A unique feature of infections with B fragilis is the ability of the organism to induce abscess formation as the sole infecting organism. When injected into rats’ abdomens, purified capsular polysaccharides from B fragilis cause abscess formation, but those from other bacteria (eg, Streptococcus pneumoniae and E coli) do not. The mechanism by which the B fragilis capsule induces abscess formation is not well understood.

Bacteroides species have lipopolysaccharides (endotoxins; see Chapter 9) but lack the lipopolysaccharide structures with endotoxic activity (including β-hydroxymyristic acid). The lipopolysaccharides of B fragilis are much less toxic than those of other gram-negative bacteria. Thus, infection caused by Bacteroides does not directly produce the clinical signs of sepsis (eg, fever and shock) so important in infections caused by other gram-negative bacteria. When these clinical signs appear in Bacteroides infection, they are a result of the inflammatory immune response to the infection.

B fragilis elaborates a number of enzymes important in disease. In addition to proteases and neuraminidases, production of two cytolysins acts together to cause hemolysis of erythrocytes. An enterotoxin capable of causing diarrhea and whose gene is contained on a pathogenicity island is found in the majority of isolates that are recovered from blood cultures.

B fragilis produces an SOD and can survive in the presence of oxygen for days. When a facultative anaerobe such as E coli is present at the site of infection, it can consume all available oxygen and thereby produce an environment in which Bacteroides species and other anaerobes can grow (see earlier).

F necrophorum likewise possesses important virulence factors that enable it to cause Lemierre’s syndrome and other seriously invasive infections. One of these factors is a leukotoxin likely responsible for the necrosis seen with these infections. Other factors include a hemagglutinin, a hemolysin, and lipopolysaccharide (endotoxin). In addition, F necrophorum is capable of causing platelet aggregation. The exact pathogenic interplay, if any, among these factors in the pathogenesis of human infections remains to be elucidated.

Many anaerobic bacteria produce heparinase, collagenase, and other enzymes that damage or destroy tissue. It is likely that enzymes play a part in the pathogenesis of mixed anaerobic infections, although laboratory experiments have not been able to define specific roles.

THE POLYMICROBIAL NATURE OF ANAEROBIC INFECTIONS

Most anaerobic infections are associated with contamination of tissue by normal microbiota of the mucosa of the mouth, pharynx, gastrointestinal tract, or genital tract. Typically, multiple species (five or six species or more when standard culture conditions are used) are found, including both anaerobes and facultative anaerobes. Oropharyngeal, pleuropulmonary, abdominal, and female pelvic infections associated with contamination by normal mucosal microbiota have a relatively equal distribution of anaerobes and facultative anaerobes as causative agents; about 25% have anaerobes alone, about 25% have facultative anaerobes alone, and about 50% have both anaerobes and facultative anaerobes. Aerobic bacteria may also be present, but obligate aerobes are much less common than anaerobes and facultative anaerobes. Anaerobic bacteria and associated representative infections are listed in Table 21-2.

DIAGNOSIS OF ANAEROBIC INFECTIONS

Clinical signs suggesting possible infection with anaerobes include the following:

1. Foul-smelling discharge (caused by short-chain fatty-acid products of anaerobic metabolism)

2. Infection in proximity to a mucosal surface (anaerobes are part of the normal microbiota)

3. Gas in tissues (production of CO₂ and H₂)

4. Negative aerobic culture results

Diagnosis of anaerobic infection is made by anaerobic culture of properly obtained and transported specimens (see Chapter 47). Anaerobes grow most readily on complex media such as trypticase soy agar base, Schaedler’s blood agar, brucella agar, brain–heart infusion agar, and others—each highly supplemented (eg, with hemin, vitamin K₁, blood). A selective complex medium containing kanamycin is used in parallel. Kanamycin (similar to all aminoglycosides) does not inhibit the growth of obligate anaerobes; thus, it permits them to proliferate without being overshadowed by rapidly growing facultative anaerobes. Cultures are incubated at 35–37°C in an anaerobic atmosphere containing CO₂.

Colony morphology, pigmentation, and fluorescence are helpful in identifying anaerobes. Biochemical activities and production of short-chain fatty acids as measured by gas-liquid chromatography are used for laboratory confirmation.

TREATMENT OF ANAEROBIC INFECTIONS

Treatment of mixed anaerobic infections is by surgical drainage (under most circumstances) plus antimicrobial therapy.

The B fragilis group of organisms found in abdominal and other infections universally produces β-lactamase, as do many of the P bivia and P disiens strains found in genital tract infections in women. Fortunately, these β-lactamases are inhibited by β-lactam–β-lactamase inhibitor combinations such as ampicillin–sulbactam. Therapy with antimicrobials (other than penicillin G) is necessary to treat infections with these organisms. At least two-thirds of the P melaninogenica strains from pulmonary and oropharyngeal infections also produce β-lactamase.

The most active drugs for treatment of anaerobic infections are clindamycin and metronidazole, although clindamycin resistance among the B fragilis group has increased in the past decade. Clindamycin is preferred for infections above the diaphragm. Relatively few anaerobes are resistant to clindamycin (B fragilis group excepted) and few, if any, are resistant to metronidazole. Alternative drugs include cefoxitin, cefotetan, some of the other newer cephalosporins, and piperacillin, but these drugs are not as active as clindamycin and metronidazole. The carbapenem antibiotics, ertapenem, imipenem, meropenem, and doripenem, have good activity against many anaerobes, and resistance is still uncommon. Tigecycline, has good in vitro activity against a variety of anaerobe species, including the B fragilis group. Penicillin G remains the drug of choice for treatment of anaerobic infections that do not involve β-lactamase–producing Bacteroides and Prevotella species.

• Anaerobic bacteria are organisms that do not grow in the presence of oxygen and require special handling to recover them from clinical material.

• Anaerobes constitute a substantial component of the normal human microbiota, yet some produce potent exotoxins that cause serious, life-threatening infections.

• Anaerobes are often implicated in mixed bacterial infections when an important mucosal barrier has been compromised such as in the case of trauma.

• B fragilis is one of the most frequently isolated gram-negative anaerobes from clinical material; it possesses a capsule capable of causing abscess formation.

• Treatment of anaerobic infections requires drainage of abscesses and antibiotics such as penicillin (for non–β-lactamase producers), clindamycin, cefoxitin, metronidazole, and the carbapenems.