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.
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.
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.
Actinomyces species. A: Colony of Actinomyces species after 72 hours growth on brain–heart infusion agar, which usually yields colonies about 2 mm in diameter; they are often termed “molar tooth” colonies. (Courtesy of CDC Public Health Image Library, L Georg.) B: Granule of Actinomyces species in tissue with Brown and Breen stain. Original magnification ×400. Filaments of the branching bacilli are visible at the periphery of the granule. Such granules are commonly called “sulfur granules” because of their unstained gross yellow color. (Courtesy of CDC Public Health Image Library.) C: Actinomyces naeslundii in a brain abscess stained with methylamine silver stain. Branching bacilli are visible. Original magnification ×1000. (Courtesy of CDC Public Health Image Library, L Georg.)
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).
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.
TABLE 21-2Anaerobic Bacteria and Associated Representative Infections ||Download (.pdf) TABLE 21-2 Anaerobic Bacteria and Associated Representative Infections
|Brain abscesses |
| Peptostreptococci, Fusobacterium nucleatum, and others |
|Oropharyngeal infections |
| Oropharyngeal anaerobes; Actinomyces, Prevotella melaninogenica, Fusobacterium species |
|Pleuropulmonary infections |
| Peptostreptococci; Fusobacterium species; Prevotella melaninogenica, Bacteroides fragilis in 20–25%; others |
|Intra-abdominal infections |
| Liver abscess: Mixed anaerobes in 40–90%; facultative organisms |
| Abdominal abscesses: Bacteroides fragilis; other gastrointestinal flora |
|Female genital tract infections |
| Vulvar abscesses: Peptostreptococci and others |
| Tubo-ovarian and pelvic abscesses: Prevotella sp., peptostreptococci; others |
|Skin, soft tissue, and bone infections |
| Mixed anaerobic flora; Propionibacterium acnes |
| Bacteroides fragilis; peptostreptococci; propionibacteria; Fusobacteria; Clostridium; others |
| Bacteroides fragilis; Actinomyces |
DIAGNOSIS OF ANAEROBIC INFECTIONS
Clinical signs suggesting possible infection with anaerobes include the following:
Foul-smelling discharge (caused by short-chain fatty-acid products of anaerobic metabolism)
Infection in proximity to a mucosal surface (anaerobes are part of the normal microbiota)
Gas in tissues (production of CO2 and H2)
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.