Description

Actinomyces are branching Gram-positive bacilli. They are facultative anaerobes, but often fail to grow aerobically on primary culture. They grow best under anaerobic or micro-aerophilic conditions with the addition of 5–10% carbon dioxide. Almost all species are commensals of the mouth and have a narrow temperature range of growth of around 35–37°C. They are responsible for the disease known as actinomycosis. Three-quarters of human cases are caused by Actinomyces israelii. Less common causes include A. gerencseriae, A. naeslundii, A. odontolyticus, A. viscosus, A. meyeri, Arachnia propionica and members of the genus Bifidobacterium.

Concomitant bacteria, notably a small Gram-negative rod, Actinobacillus actinomycetemcomitans, but also Haemophilus species, fusiforms and anaerobic streptococci, are sometimes found in actinomycotic lesions, although their contribution to the pathogenesis of the disease, if any, is unknown. Act. actinomycetemcomitans is a rare cause of endocarditis.

Pathogenesis

Actinomycosis is a chronic disease characterized by multiple abscesses and granulomata, tissue destruction, extensive fibrosis and the formation of sinuses. Within diseased tissues the actinomycetes form large masses of mycelia embedded in an amorphous protein–polysaccharide matrix and surrounded by a zone of Gram-negative, weakly acid-fast, club-like structures. These clubs were once thought to consist, at least in part, of material derived from host tissue, but it now appears that they are formed entirely from the bacteria. The mycelial masses may be visible to the naked eye and, as they are often light yellow in colour, they are called sulphur granules. In older lesions the sulphur granules may be dark brown and very hard because of the deposition of calcium phosphate in the matrix. Various species of actinomyces may colonize diseased tissue, such as lung cancer, but sulphur granules are not seen.

The principal forms of human actinomycosis are:

The lymphatics are not usually involved, but haematogenous spread to the liver, brain and other internal organs occurs occasionally. Involvement of bone is uncommon in human actinomycosis and is usually the result of direct extension of adjacent soft tissue lesions.

Laboratory diagnosis

Specimens should be obtained directly from lesions by open biopsy, needle aspiration or, in the case of pulmonary lesions, by fibreoptic bronchoscopy. Examination of sputum is of no value as it frequently contains oral actinomycetes. Material from suspected cases is shaken with sterile water in a tube. Sulphur granules settle to the bottom and may be removed with a Pasteur pipette. Granules crushed between two glass slides are stained by the Gram and Ziehl–Neelsen (modified by using 1% sulphuric acid for decolorization) methods, which reveal the Gram-positive mycelia and the zone of radiating acid-fast clubs. Sulphur granules and mycelia in tissue sections are identifiable by use of fluorescein-conjugated specific antisera. In-situ PCR has been used to detect A. israelii in tissue biopsies.

For culture, suitable media, such as blood or brain–heart infusion agar, glucose broth and enriched thioglycollate broth, are inoculated with washed and crushed granules. Contamination is reduced by the incorporation of metronidazole and nalidixic acid or cadmium sulphate in the media. Cultures are incubated aerobically and anaerobically for up to 14 days. After several days on agar medium, A. israelii may form so-called spider colonies that resemble molar teeth. The identity may be confirmed by biochemical tests, by staining with specific fluorescent antisera or by gas chromatography of metabolic products of carbohydrate fermentation.

Treatment

Actinomyces are sensitive to many antibiotics, but the penetration of drugs into the densely fibrotic diseased tissue is poor. Thus, large doses are required for prolonged periods, and recurrence of disease is not uncommon. Surgical debridement reduces scarring and deformity, hastens healing and lowers the incidence of recurrences. Prolonged penicillin-based regimens are increasingly being replaced by shorter regimens based on amoxicillin with clavulanic acid (the clavulanic acid is required because lesions are often concomitantly infected with β-lactamase-producing bacteria) or cephalosporins, especially ceftriaxone. Alternative agents include tetracyclines, macrolides, fluoroquinolones and imipenem but in-vitro sensitivity testing is unreliable. Additional drugs, including aminoglycosides and metronidazole, may be required when concomitant organisms are present.

Description

The nocardiae are branched, strictly aerobic, Gram-positive bacteria that are closely related to the rapidly growing mycobacteria. Like the latter, but unlike actinomyces, they are environmental saprophytes with a broad temperature range of growth. The properties of nocardiae and actinomycetes are compared in Table 20.1. Most isolates are acid-fast when decolorized with 1% sulphuric acid.

Table 20.1 Differences between the genera Actinomyces and Nocardia

Many species of nocardia are found in the environment, notably in soil, and a range of species cause human opportunist disease, notably Nocardia asteroides, so named because of its star-shaped colonies, N, abscessus, N. farcinica, N. brasiliensis, N. brevicatena, N. otitidiscaviarum, N. nova and N. transvalensis. A wider range of species is encountered in profoundly immunosuppressed patients.

A related group of non-acid-fast species are assigned to the genus Actinomadura, which includes the species Actinomadura madurae and A. pelletieri, common causes of Madura foot (see below).

Pathogenesis

Nocardiae, principally N. asteroides, are uncommon causes of opportunist pulmonary disease, which usually, but not always, occurs in immunocompromised individuals, including those receiving post-transplant immunosuppressive therapy or chemotherapy for cancer and those with acquired immune deficiency syndrome (AIDS). Corticosteroid therapy is a strong risk factor. As a result, the frequency and diversity of clinical manifestations of nocardial disease has increased over the past few decades. Pre-existing lung disease, notably alveolar proteinosis, also predisposes to nocardial disease. The infection is exogenous, resulting from inhalation of the bacilli. The clinical and radiological features are very variable and non-specific, and diagnosis is not easy. In most cases there are multiple confluent abscesses with little or no surrounding fibrous reaction, and local spread may result in pleural effusions, empyema and invasion of bones. In some cases the disease is chronic, whereas in others it spreads rapidly through the lungs. Secondary abscesses in the brain and, less frequently, in other organs occur in about one-third of patients with pulmonary nocardiasis. Acute dissemination with involvement of many organs occurs in profoundly immunosuppressed persons, notably those with AIDS. Recurrence is common in immunosuppressed patients and mortality is high.

Nocardiae also cause primary post-traumatic, postoperative or post-inoculation cutaneous infections (primary cuteneous nocardiasis). The most frequent cause is N. brasiliensis but some cases are caused by N. asteroides or other species. In the USA and the southern hemisphere, but rarely in Europe, cutaneous infections may result in fungating tumour-like masses termed mycetomas.

Madura foot is a chronic granulomatous infection of the bones and soft tissues of the foot resulting in mycetoma formation and gross deformity. It occurs in Sudan, North Africa and the west coast of India, principally among those who walk barefoot and are therefore prone to contamination of foot injuries by soil-derived organisms. A common causative organism is Actinomadura madurae, but Madura foot is also caused by other actinomycetes including Streptomyces somaliensis and by fungi (see Ch. 61).

Laboratory diagnosis

A presumptive diagnosis of pulmonary nocardiasis may be made by a microscopical examination of sputum. In many cases the sputum contains numerous lymphocytes and macrophages, some of which contain pleomorphic Gram-positive and weakly acid-fast bacilli, and occasional extracellular branching filaments. Nocardiae are not so easily seen in tissue biopsies stained by the Gram or modified Ziehl–Neelsen methods, but may be seen in preparations stained by the Gram–Weigert or Gomori methenamine silver methods.

Nocardiae grow on blood agar, although growth is better on enriched media including Löwenstein–Jensen medium, brain–heart infusion agar and Sabouraud’s dextrose agar containing chloramphenicol as a selective agent. Growth is visible after incubation for between 2 days and 1 month; selective growth is favoured by incubation at 45°C. Colonies are cream, orange or pink coloured; their surfaces may develop a dry, chalky appearance, and they adhere firmly to the medium.

Identification of species is usually undertaken in reference laboratories, with the most common technique being analysis of 16S rRNA gene sequences, a technique that has delineated over 30 species.

Treatment

A widely used regimen is sulfamethoxazole with trimethoprim (co-trimoxazole) for 3–6 months, although this prolonged course often causes adverse drug reactions. In addition, some strains, especially of N. farcinica, are resistant to sulphonamides. An alternative regimen, particularly in severe disease, is high-dose imipenem with amikacin for 4–6 weeks. Minocycline, third generation cephalosporins, amoxicillin–clavulanate combinations and linezolid, an oxazolidinone, are also effective. Drug susceptibility testing is subject to several variables and no standardized methods have been proposed. Mycetomata due to nocardiae are much easier to treat than those due to fungi. Even long-standing cases with extensive mycetoma formation respond well to chemotherapy. Despite therapy, mortality of pulmonary and disseminated nocardiasis is high.