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Chapter 182 Actinomyces
Richard F. Jacobs, Gordon E. Schutze
Actinomyces organisms are slow-growing, gram-positive bacteria that are part of the endogenous oral flora in humans. Their filamentous structure gives them a fungus-like appearance. Infection caused by these bacteria is termed actinomycosis, which is a chronic, granulomatous, suppurative disease characterized by direct extension to contiguous tissue across natural anatomic barriers with the formation of numerous draining fistulas and sinus tracts. These infections usually involve the cervicofacial, thoracic, abdominal, or pelvic regions.
Actinomyces is a member of the order Actinomycetales, which includes gram-positive filamentous bacteria such as Nocardia, Streptomyces, and mycobacteria. Actinomyces israelii is the predominant species causing human actinomycosis. Other implicated species, in order of importance, are Propionibacterium propionicum, Actinomyces turicensis, Actinomyces odontolyticus, Actinomyces meyeri, Actinomyces naeslundii, Actinomyces viscosus, Actinomyces europaeus, and Actinomyces radingae. Arcanobacterium pyogenes (previously in the Actinomyces genus) also causes human actinomycosis.
Actinomyces organisms are non–spore-forming, gram-positive, non–acid-fast, nonmotile, facultative or strictly anaerobic bacilli with variable morphology ranging from diphtheroid to mycelial with short branching forms. Actinomyces organisms are part of the endogenous flora of mucous membranes and are often found in clinical specimens such as sputum, bronchial washes, purulent exudates, and tissues obtained surgically or at necropsy. Staining of crushed tissue specimens rinsed with sterile saline or purulent exudate stained with Gram or acid-fast procedures may reveal organisms within the classic sulfur granules, which are characteristically associated with pulmonary disease caused by A. israelii or A. meyeri. Cultures on brain-heart infusion agar incubated at 37°C anaerobically (95% nitrogen and 5% carbon dioxide) and a separate set incubated aerobically reveal organisms within the lines of streak at 24-48 hr. A. israelii colonies appear as loose masses of delicate, branching filaments with a characteristic spider-like growth. Colonies of A. naeslundii, A. viscosus, and P. propionicum may have similar growth characteristics. Biochemical testing is frequently used for speciation but is limited by the complexity within this group. Newer speciation methods are based on 16S recombinant RNA sequence analysis.
Actinomycosis occurs worldwide among people of all ages, with higher incidence among males, possibly related to increased trauma or poorer dental hygiene. There is no relationship to race, season, or occupation. In a review of 85 cases of actinomycosis, 27% were in persons <20 yr of age, with 7% among children <10 yr of age. The youngest patient in this series was 28 days old. The source of human infection is almost always endogenous flora. The incidence has declined as a result of improved oral hygiene and early antibiotic treatment of oral infections. Risk factors in children include trauma, dental caries, debilitation, and poorly controlled diabetes mellitus. Although actinomycosis is not a common opportunistic infection, disease has been associated with corticosteroid use, leukemia, renal failure, congenital immunodeficiency diseases, and HIV infection. In one study, antecedent disease and surgery predisposed 81 of 181 subjects to infection.
The 3 significant sites of Actinomyces infection are, in order of frequency, cervicofacial, abdominal and pelvic, and pulmonary, although infection may involve any organ in the body. Infection typically follows introduction of the organism into tissues after trauma or surgery. The hallmark of actinomycosis is spread that fails to respect tissue or fascial planes. The use of intrauterine devices (IUDs) may predispose to development of pelvic actinomycosis. Pulmonary actinomycosis occurs after inhalation or aspiration of organisms, introduction of a colonized foreign body, or spread from an existing cervicofacial or abdominal actinomycotic infection.
Infection spreads contiguously and, rarely, hematogenously. Actinomycosis is a chronic, suppurative, scarring inflammatory process. Sites of infection show dense cellular infiltrates and suppuration that form many interconnecting abscesses and sinus tracts. This may be followed by cicatricial healing from which the organism spreads by burrowing along fascial planes, causing deep, communicating scarred sinus tracts. Sulfur granules are characteristic of actinomycosis. On hematoxylin-eosin staining, they appear as an adherent mass of polymorphonuclear neutrophils attached to the radially arranged eosinophilic clubs of the granule, which is the host immune response. They may be microscopic or macroscopic and are typically yellow, accounting for their name, but may be white, gray, or brown.
Actinomycosis, even in closed infections, is usually, if not always, polymicrobial in nature, involving mixed bacteria. In a large study of more than 650 cases, infection with Actinomyces was identified in pure culture in only 1 case and in others was usually identified with other oral flora, most notably members of the HACEK group, which includes Haemophilus aphrophilus, Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae. A. actinomycetemcomitans is a fastidious, gram-negative bacillus that is part of the oral flora and has been implicated as a pathogen in periodontal disease. Other bacterial species frequently isolated concomitantly in human actinomycosis include Fusobacterium, Bacteroides, Capnocytophaga, Staphylococcus, Streptococcus, Enterococcus, and Enterobacteriaceae.
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