Actinomycetes
The actinomycetes are gram-positive organisms that tend to grow slowly as branching elements. They are prokaryotes but have fungus-like characteristics. Actinomycetes are the most abundant organisms in the soil. Actinomycosis generally arise from endogenous inhabitants of the oral cavity. Whereas norcardiosis results from inhalation of soil organisms. An allergic pneumonitis called farmer’s lung occurs among agricultural workers who have inhaled dust from moldy plant material; it has been traced to at least three actinomycetes.
Several species of Actinomyces have been implicated as the cause of actinomycosis in humans and animals. A. israelli is usually responsible for disease in man. It is part of the normal oral flora; it can be cultured from the majority of human tonsils and is nearly always found in scrapings of gums and teeth. The conditions that lead the organism to become invasive are not definitely known but may involve trauma and dental surgery. In general, actinomycotic infections are accompanied by a mixed flora of gram-negative bacteria (actinobacillus, Eikenella, Fusobacterium, and Bacteroides).
Actinomycosis is distributed worldwide but is relatively rare. Its incidence is higher in men than women and in persons over 20. Actinomycosis is characterised by chronic destructive abscesses of connective tissues; the abdomen (especially the caecum and appendix), the lungs, the chest wall, and the face and neck may be involved. Wherever the lesions occur, they are basically the same. Abscesses expand into contiguous tissues and eventually form burrowing, tortuous sinuses to the skin surface, where they discharge purulent material.
When pus from an abscess or infected sputum is examined carefully, yellow sulphur granules are occasionally seen. These are small colonies of actinomycetes which may be up to several millimetres in diameter. Detection of granules is not required to establish a diagnosis of actinomycosis but their presence facilitates identification of the organism. Penicillin is the drug of choice. The organism is also sensitive to tetracycline.
In contrast to Actinomyces, species of Nocardia are inhabitants of the soil rather than commensals in animals and they are aerobic. Nocardia species are gram-positive and two species are pathogenic for man: N. asteroides, and N. brasiliensis, and are somewhat acid-fast. There are two common modes of infection by Nocardia, pulmonary nocardiosis arises from inhalation of the organisms, whereas chronic subcutaneous absesses (mycetomas) arise from contamination of skin wounds, usually on the feet, and hands of labourers. In pulmonary nocardiosis, the lesions may simulate miliary or pulmonary tuberculosis. N. asteroides lie scattered though the abscesses in the form of tangled, fine, branching filaments. Aggregations into granules do not occur. Different species of Nocardias are associated with mycetomas in different parts of the world, such as N. brasiliensis in Mexico. These abscesses are clinically very similar to those of Streptomyces and to various fungi.
Nocardia are widely distributed throughout temperate and tropical climates. The diseases it causes are seen frequently in association with immunosuppression or underlying chronic diseases such as Hodgkin’s disease. Once nocardiosis becomes clinically evident, it tends to become progressive and fatal, even with aggressive therapy: about 50% of patients succumb. Various antibacterial drugs are used in the treatment of nocardiosis, and sulfonamides are reported to be most effective.
Streptomyces are characterised by the stability of their filaments and by the formation of spores on the aerial mycelia and by the formation of spores on the aerial mycelia that project above the surface of the culture medium. With increasing appreciation of the distinction between norcardiae and streptomycetes, it has been realised that both cause actinomycotic abscesses. Because streptomycetes are ubiquitous in soil, infection is attributed to contamination of scratches and penetrating wounds. Mycetomas caused by streptomycetes are indistinguishable clinically from other actinomycetes.