ACTINOMYCETES

            CLASSIFICATION

            Domain – Bacteria

                        Phylum – Actinobacteria                                                     

                                    Class – Actinobacteria

                                                Order – Actinomycetales

                                                            Family – Actinomycetaceae/Actinomycetes

            Actinomycetes are traditionally considered to be transitional form between bacteria and fungi. Like fungi they form a mycelia network of branching filaments but, like bacteria they are thin, possess cell walls containing muramic acid, have prokaryotic nuclei and are susceptible to antibacterial antibiotics. They are therefore true bacteria, bearing a superficial resemblance to fungi. Actinomycetes are Gram-positive, non-motile, non-sporing, non-capsulated filaments that break up into bacillary and coccoid elements. Most are free living particularly in soil.

            Actinomycetes include many genera of medical interest such as – anaerobic Actinomyces, Arachnia, Bifidobacterium, Rothia and aerobic Nocardia, Actinomadura, Dermatophilus and Streptomyces.

            The major pathogenic genus Actinomyces is anaerobic or microaerophilic and non-acid fast, while Nocardia species are aerobic and may be acid fast. Some species of Streptomyces may cause disease, but their importance is the major source of antibiotics.

GENUS: ACTINOMYCES

            The name Actinomyces was coined by Harz to refer to the ray-like appearance to the organisms in the granules that characterize the lesions (actinomyces, meaning ray fungus). Wolf and Israel (1891) isolated an anaerobic bacillus from human lesions and produced experimental infections in rabbits and guinea pigs. This was named Actinomyces israelii. It causes human actinomycosis.

            PATHOGENICITY:- Actinomycosis is the disease which is a chronic granulomatous infection occurring in human beings and animals. It is characterized by the development of indurated swellings, mainly in the connective tissue, suppuration and the discharge of sulphur granules. The lesion often points towards the skin, leading to multiple sinuses.

            Actinomycosis in human beings is an endogenous infection. The Actinomyces species are normally present in the mouth, intestine and vagina as commensals. Trauma, foreign bodies or poor oral hygiene may favor tissue invasion. Actinomycosis is usually a cooperative disease, the Actinomyces being accompanied by other associated bacteria which may enhance the pathogenic effect.

            The disease occurs throughout the world but its incidence in the advanced countries has been declining probably as a result of the widespread use of antibiotics. Actinomycosis is more common in rural areas and in agricultural workers. Young male persons (10-30 years of age) are most commonly affected.

            The disease responds to prolonged treatment with penicillin or tetracycline. Treatment will have to be continued for several months and supplemented by surgery, where necessary.

GENUS: NOCARDIA

            Nocardia resembles Actinomycetes morphologically but are aerobic. All species are Gram-positive and some such as N. astroides and N. brasiliensis are acid fast.

            PATHOGENICITY:- Nocardia are frequently found on soil and infection may be exogenous. Infection with Nocardia (Nocardiosis) cause cutaneous, sub-cutaneous or systemic lesions in humans. The species causing disease are – N. asteroids, N. brasiliensis and N. caviae.

            Cutaneous infection may lead to local abscesses, cellulitis or lymphocutaneous lesions. 
        Sub-cutaneous infection is actinomycotic mycetoma which is a localized chronic, granulomatous involvement of the sub-cutaneous and deeper tissues, commonly affecting the foot and less often the hand and other parts, and presenting as a tumour with multiple discharging sinuses.

            Systemic infection usually caused by N. asteroids manifests primarily as pulmonary disease, pneumonia, lung abscess or other lesions resembling tuberculosis. Metastatic manifestations may involve the brain, kidneys and other organs. Systemic infection occurs more often in immunodeficient persons.

            Cotrimoxazole given for several months may be useful in treatment of the diseases. Minocycline, amikacin and cefotaxime are effective.

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