Nocardia of Actinomycetes Infection
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.