Answers

1. Mobiluncus is associated with bacterial vaginosis.

2. Infections with Actinomyces are characteristically chronic, requiring weeks to months to develop. The organisms also grow slowly in culture and respond slowly to antibiotic treatment.

3. Lactobacilli can cause transient bacteremia (typically from a genitourinary tract source), endocarditis, or opportunistic septicemia in an immunocompromised patient. Treatment of serious infections requires using the combination of penicillin with an aminoglycoside.

Anaerobic Gram-Positive Cocci (Table 37-1)

At one time, all clinically significant anaerobic cocci were included in the genus Peptostreptococcus. Unfortunately, it was recognized that these organisms were organized in a single genus based primarily on their Gram-stain morphology and inability to grow aerobically. More sophisticated methods have since been used to reclassify many of these species into six genera. The most common isolates are listed in Table 37-2. Although some anaerobic cocci are more virulent than others, and some are associated with specific diseases, specific identification of the different genera is generally unnecessary, and knowledge that anaerobic cocci are associated with an infection is typically sufficient.

Table 37-1 Important Anaerobic Gram-Positive Bacteria

Table 37-2 New Classification of Selected Anaerobic Cocci Formerly in the Genus Peptostreptococcus

The anaerobic gram-positive cocci normally colonize the oral cavity, gastrointestinal tract, genitourinary tract, and skin. They produce infections when they spread from these sites to normally sterile sites. For example, bacteria colonizing the upper airways can cause sinusitis and pleuropulmonary infections; bacteria in the intestines can cause intraabdominal infections; bacteria in the genitourinary tract can cause endometritis, pelvic abscesses, and salpingitis; bacteria on the skin can cause cellulitis and soft-tissue infections; and bacteria that invade the blood can produce infections in bones and solid organs (Figure 37-1).

Figure 37-1 Diseases associated with anaerobic cocci and Actinomyces, Propionibacterium, and Mobiluncus, the latter three being anaerobic, non–spore-forming, gram-positive rods.

Laboratory confirmation of infections with anaerobic cocci is complicated by the following three factors: (1) care must be taken to prevent contamination of the clinical specimen with the anaerobic cocci that normally colonize the skin and mucosal surfaces; (2) the collected specimen must be transported in an oxygen-free container to prevent loss of the organisms; and (3) specimens should be cultured on nutritionally enriched media for a prolonged period (i.e., 5 to 7 days). In addition, some species of staphylococci and streptococci grow initially in an anaerobic atmosphere only and may be mistaken for anaerobic cocci. However, these organisms eventually grow well in air supplemented with 10% carbon dioxide (CO₂), so they cannot be classified as anaerobes.

Anaerobic cocci are usually susceptible to penicillins and carbapenems (e.g., imipenem, meropenem); have intermediate susceptibility to broad-spectrum cephalosporins, clindamycin, erythromycin, and the tetracyclines; and are resistant to the aminoglycosides (as are all anaerobes). Specific therapy is generally indicated in monomicrobic infections; however, because most infections with these organisms are polymicrobic, broad-spectrum therapy against aerobic and anaerobic bacteria is usually selected.

Anaerobic, Non–Spore-Forming, Gram-Positive Rods (See Table 37-1)

The non–spore-forming, gram-positive rods are a diverse collection of facultatively anaerobic or strictly anaerobic bacteria that colonize the skin and mucosal surfaces (Table 37-3). Actinomyces, Mobiluncus, Lactobacillus, and Propionibacterium are well-recognized opportunistic pathogens, whereas other genera, such as Bifidobacterium and Eubacterium, can be isolated in clinical specimens but rarely cause human disease.

Table 37-3 Anaerobic, Non–Spore-Forming, Gram-Positive Rods

Actinomyces

Physiology and Structure

Actinomyces organisms are facultatively anaerobic or strictly anaerobic gram-positive rods. They are not acid-fast (in contrast to the morphologically similar Nocardia species), they grow slowly in culture, and they tend to produce chronic, slowly developing infections. They typically develop delicate filamentous forms or hyphae (resembling fungi) in clinical specimens or when isolated in culture (Figure 37-2). However, these organisms are true bacteria in that they lack mitochondria and a nuclear membrane, reproduce by fission, and are inhibited by penicillin but not antifungal antibiotics. Numerous species have been described; Actinomyces israelii, Actinomyces naeslundii, Actinomyces radingae, and Actinomyces turicensis are responsible for most human infections.

Figure 37-2 Macroscopic colony (left) and Gram stain (right) of Actinomyces.

Pathogenesis and Immunity

Actinomyces organisms colonize the upper respiratory, gastrointestinal, and female genital tracts. These bacteria are not normally present on the skin surface. The organisms have a low virulence potential and cause disease only when the normal mucosal barriers are disrupted by trauma, surgery, or infection.

Classic disease caused by actinomyces is termed actinomycosis (in keeping with the original idea that these organisms were fungi or “mycoses”). Actinomycosis is characterized by the development of chronic granulomatous lesions that become suppurative and form abscesses connected by sinus tracts. Macroscopic colonies of organisms resembling grains of sand can frequently be seen in the abscesses and sinus tracts. These colonies, called sulfur granules because they appear yellow or orange, are masses of filamentous organisms bound together by calcium phosphate (Figure 37-3). The areas of suppuration are surrounded by fibrosing granulation tissue, which gives the surface overlying the involved tissues a hard or woody consistency. Actinomycosis is now relatively uncommon. Currently, most infections involving actinomyces are polymicrobic, oral infections such as endodontic infections, odontogenic abscesses, and dental implant–associated infections.

Figure 37-3 Sulfur granule collected from the sinus tract in a patient with actinomycosis. Delicate filamentous rods (arrow) are seen at the periphery of the crushed granule.

Epidemiology

Infections caused by actinomyces are endogenous, with no evidence of person-to-person spread or disease originating from an external source, such as soil or water. Cervicofacial infections are seen in patients who have poor oral hygiene or have undergone an invasive dental procedure or oral trauma. In these patients, the actinomyces that are present in the mouth invade into the diseased tissue and initiate the infectious process.

Patients with thoracic infections generally have a history of aspiration, with the disease becoming established in the lungs and then spreading to adjoining tissues. Abdominal infections most commonly occur in patients who have undergone gastrointestinal surgery or have suffered trauma to the bowel. Pelvic infection can be a secondary manifestation of abdominal actinomycosis or may be a primary infection in a woman with an intrauterine device (Figure 37-4). Central nervous system infections usually represent hematogenous spread from another infected tissue, such as the lungs.

Figure 37-4 Actinomyces species can colonize the surface of foreign bodies, such as this intrauterine device, leading to the development of pelvic actinomycosis.

(From Smith E. In Lambert H, Farrar W, editors: Infectious diseases illustrated, London, 1982, Gower.)

Clinical Diseases (Clinical Case 37-1)

Most actinomyces infections are the cervicofacial type (Figure 37-5). The disease may occur as an acute, pyogenic infection or as a slowly evolving, relatively painless process. The finding of tissue swelling with fibrosis and scarring, as well as draining sinus tracts along the angle of the jaw and neck, should alert the physician to the possibility of actinomycosis. Symptoms of thoracic actinomycosis are nonspecific. Abscesses may form in the lung tissue early in the disease and then spread into adjoining tissues as the disease progresses. Abdominal actinomycosis can spread throughout the abdomen, potentially involving virtually every organ system. Pelvic actinomycosis can occur as a relatively benign form of vaginitis or, more commonly, there can be extensive tissue destruction, including the development of tuboovarian abscesses or ureteral obstruction. The most common manifestation of central nervous system actinomycosis is a solitary brain abscess, but meningitis, subdural empyema, and epidural abscess are also seen. Actinomycosis in patients with chronic granulomatous disease, presenting as a nonspecific febrile illness, has recently been described.

Clinical Case 37-1

Pelvic Actinomycosis

Quercia and associates (Med Mal Infect 36:393–395, 2006) described a classic presentation of pelvic actinomycosis associated with an intrauterine contraceptive device (IUD). The patient is a 41-year-old woman who presented with a 5-month history of abdominal and pelvic pain, weight loss, malaise, and a yellow vaginal discharge. Since 1994 she had used an IUD, which was removed in June 2004. Her symptoms began soon after removal of the IUD. A computed tomography scan revealed a large pelvic mass involving the fallopian tubes as well as numerous hepatic abscesses. A surgical biopsy was performed, and Actinomyces was recovered in culture. She underwent surgical debridement and received oral therapy with a penicillin antibiotic for 1 year. The medical team thought the woman’s pelvis was infected with Actinomyces at the time the IUD was removed. This episode illustrates the chronic nature of actinomycosis and the need for surgical drainage and long-term antibiotic therapy.

Figure 37-5 This patient is suffering from cervicofacial actinomycosis. Note the draining sinus tract (arrow).

Laboratory Diagnosis

Laboratory confirmation of actinomycosis is often difficult. Care must be used during collection of clinical specimens that they not become contaminated with Actinomyces that are part of the normal bacterial population on mucosal surfaces. The significance of Actinomyces isolated from contaminated specimens cannot be determined. Because the organisms are concentrated in sulfur granules and are sparse in involved tissues, a large amount of tissue or pus should be collected. If sulfur granules are detected in a sinus tract or in tissue, the granule should be crushed between two glass slides, stained, and examined microscopically. Thin, gram-positive, branching rods can be seen along the periphery of the granules.

Actinomyces are fastidious and grow slowly under anaerobic conditions; it can take 2 weeks or more for the organisms to be isolated. Colonies appear white and have a domed surface that can become irregular after incubation for a week or more, resembling the top of a molar (Figure 37-6). The individual species of Actinomyces can be differentiated by biochemical tests; however, this process can be time consuming. In general, it is necessary to determine only that the isolate is a member of the genus Actinomyces.

Figure 37-6 Molar tooth appearance of Actinomyces israelii after incubation for 1 week. This colonial morphology serves as a reminder that the bacteria are normally found in the mouth.

Recovery of Actinomyces in blood cultures should be evaluated carefully. Most isolates represent transient, insignificant bacteremia from the oropharynx or gastrointestinal tract. If the isolate is clinically significant, evidence of tissue pathology should be obtained.

Propionibacterium

Propionibacteria are small gram-positive rods often arranged in short chains or clumps (Figure 37-7). They are commonly found on the skin (in contrast with the Actinomyces), conjunctiva, external ear, and in the oropharynx and female genital tract. The organisms are anaerobic or aerotolerant, nonmotile, catalase positive, and capable of fermenting carbohydrates, producing propionic acid as their major byproduct (hence the name). The two most commonly isolated species are Propionibacterium acnes and Propionibacterium propionicum.

Figure 37-7 Gram stain of Propionibacterium in a blood culture.

P. acnes is responsible for two types of infections: (1) acne vulgaris (as the name implies) in teenagers and young adults and (2) opportunistic infections (Clinical Case 37-2) in patients with prosthetic devices (e.g., artificial heart valves or joints) or intravascular lines (e.g., catheters, cerebrospinal fluid shunts). Propionibacteria are also commonly isolated in blood cultures, but this finding usually represents contamination with bacteria on the skin at the phlebotomy site.

The central role of P. acnes in acne is to stimulate an inflammatory response. Production of a low-molecular-weight peptide by the bacteria residing in sebaceous follicles attracts leukocytes. The bacteria are phagocytized and, after release of bacterial hydrolytic enzymes (lipases, proteases, neuraminidase, and hyaluronidase), stimulate a localized inflammatory response. P. propionicum is associated with endodontic abscesses and lacrimal canaliculitis (inflammation of the tear duct).

Propionibacteria can grow on most common media, although it may take 2 to 5 days for growth to appear. Care must be taken to avoid contamination of the specimen with the organisms normally found on the skin. The significance of the recovery of an isolate must also be interpreted in light of the clinical presentation (e.g., a catheter or other foreign body can serve as a focus for these opportunistic pathogens).

Acne is unrelated to the effectiveness of skin cleansing because the lesion develops within the sebaceous follicles. For this reason, acne is managed primarily through the topical application of benzoyl peroxide and antibiotics. Antibiotics such as erythromycin and clindamycin have proved effective for treatment.

Mobiluncus

Members of the genus Mobiluncus are obligate anaerobic, gram-variable or gram-negative, curved rods with tapered ends. Despite their appearance in Gram-stained specimens (Figure 37-8), they are classified as gram-positive rods because they (1) have a gram-positive cell wall, (2) lack endotoxin, and (3) are susceptible to vancomycin, clindamycin, erythromycin, and ampicillin but resistant to colistin. The organisms are fastidious, growing slowly even on enriched media supplemented with rabbit or horse serum.

Figure 37-8 Gram stain of Mobiluncus. The bacterial cells are curved and have pointed ends.

Of the two species of Mobiluncus, M. curtisii is rarely found in the vaginas of healthy women but is abundant in women with bacterial vaginosis (vaginitis). Their microscopic appearance is a useful marker for this disease, but the precise role of these organisms in the pathogenesis of bacterial vaginosis is unclear.

Lactobacillus

Lactobacillus species are facultatively anaerobic or strictly anaerobic rods. They are found as part of the normal flora of the mouth, stomach, intestines, and genitourinary tract. The organisms are most commonly isolated in urine specimens and blood cultures. Because lactobacilli are the most common organism in the urethra, their recovery in urine cultures usually is a result of contamination of the specimen, even when large numbers of the organisms are present. The reason lactobacilli rarely cause infections of the urinary tract is their inability to grow in urine. Invasion into blood occurs in one of the following three settings: (1) transient bacteremia from a genitourinary source (e.g., after childbirth or a gynecologic procedure), (2) endocarditis (Clinical Case 37-3), and (3) opportunistic septicemia in an immunocompromised patient. Strains of lactobacilli are used as probiotics and have occasionally been associated with human infections, most commonly in immunocompromised patients.

Treatment of endocarditis and opportunistic infections is difficult because lactobacilli are resistant to vancomycin (an antibiotic commonly active against gram-positive bacteria) and are inhibited but not killed by other antibiotics. A combination of penicillin with an aminoglycoside is required for bactericidal activity.

Bifidobacterium and Eubacterium

Bifidobacterium and Eubacterium species are commonly found in the oropharynx, large intestine, and vagina. These bacteria can be isolated in clinical specimens but have a very low virulence potential and usually represent clinically insignificant contaminants. Confirmation of their etiologic role in an infection requires their repeated isolation in large numbers from multiple specimens and the absence of other pathogenic organisms.