Actinomycetes
The aerobic Actinomycetes are a large, diverse group of gram-positive
bacilli with a tendency to form chains or filaments. They are related to the
corynebacteria and include multiple genera of clinical significance such as
Mycobacteria and saprophytic organisms such as streptomyces. As the
bacilli grow, the cells remain together after division to form elongated
chains of bacteria (1 Mm in width) with occasional branches. The extent of
this process varies in different taxa. It is rudimentary in some
actinomycetes—the chains are short, break apart after formation, and
resemble diphtheroids; others develop extensive substrate or aerial filaments
(or both); and either may produce spores or fragment into coccobacillary
forms. Members of the aerobic Actinomycetes can be categorized on the
basis of the acid fast stain. Mycobacteria are truly positive acid fast
organisms; weakly positive genera include Nocardia,Rhodococcus, and a
few others of clinical significance. Streptomyces and Actinomadura, two
agents that cause actinomycotic mycetomas, are acid fast stain negative.
Actinomycosis
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. The etiologic agents are several closely related members of the
normal flora of the mouth and gastrointestinal tract. Most cases are due to
Actinomyces israelii, Actinomyces naeslundii, and related anaerobic or
facultative bacteria. Based on the site of involvement, the three common
forms are cervicofacial, thoracic, and abdominal actinomycosis. Regardless
of site, infection is initiated by trauma that introduces these endogenous
bacteria into the mucosa.
Often, in addition to the primary agent of actinomycosis, there are
concomitant bacteria present. Some of these are relatively fastidious gramnegative bacilli such as Actinobacillus actinomycetemcomitans,
Haemophilus aphrophilus, Eikenella corrodens, and Capnocytophaga
species. Occasionally, staphylococci, streptococci, or enteric gram-negative
bacilli are found.
Morphology & Identification
Most strains of A israelii and the other agents of actinomycosis are
facultative anaerobes that grow best in an atmosphere with increased carbon
dioxide. On enriched medium, such as brain-heart infusion agar, young
colonies (24–48 hours) produce gram-positive substrate filaments that
fragment into short chains, diphtheroids, and coccobacilli. After a week,
these "spider" colonies develop into white, heaped-up "molar tooth"
colonies. In thioglycolate broth, A israelii grows below the surface in
compact colonies. Species are identified based on cell wall chemotype and
biochemical reactions.
The sulfur granules found in tissue are yellowish in appearance, up to 1 mm
in size, and are composed of macrophages, other tissue cells, fibrin, and the
bacteria. Eosinophilic club-shaped enlargements of the bacterial cells often
project from the periphery of the granule.
Pathogenesis & Pathology
Regardless of the body site, the natural history is similar. The bacteria bridge
the mucosal or epithelial surface of the mouth, respiratory tract, or lower
gastrointestinal tract-associated with dental caries, gingivitis, surgical
complication, or trauma. Aspiration may lead to pulmonary infection. 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. There is often
superinfection with other endogenous bacteria.
Clinical Findings
Cervicofacial disease presents as a swollen, erythematous process in the jaw
area. 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: mild fever, cough, and purulent
sputum. Eventually, lung tissue is destroyed, sinus tracts may erupt 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, including
the kidneys, vertebrae, and liver. Genital actinomycosis is a rare occurrence
in women that results from colonization of an intrauterine device with
subsequent invasion.
Diagnostic Laboratory Tests
Pus from draining sinuses, sputum, or specimens of tissue are examined 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 are cultured in thioglycolate broth and on brain-heart
infusion blood agar plates, which are incubated anaerobically or under
elevated carbon dioxide conditions. Growth is examined for typical
morphology and biochemical reactions. The main agents of actinomycosis
are catalase-negative, whereas most other actinomycetes are catalasepositive. Surface lesions may also contain other bacterial species.
Treatment
Prolonged administration (6–12 months) of a penicillin is effective in many
cases. Clindamycin or erythromycin is effective in penicillin-allergic
patients. However, drugs may penetrate the abscesses poorly, and some of
the tissue destruction may be irreversible. Surgical excision and drainage
may also be required.
Epidemiology
Because A israelii and the related agents of actinomycosis are endogenous
members of the bacterial flora, they cannot be eliminated. Some individuals
with recurrent infections are given prophylactic penicillin, especially prior to
dental procedures