Actinomycetes - University of Babylon



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 gram-negative 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 catalase-positive. 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

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