General Characteristics of Nocardia, Streptomyces, Rhodococcus, and Similar Organisms
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p296-298
2025-07-02
631
The genera Nocardia, Rhodococcus, Gordonia, and Tsukamurella are partially acid-fast aerobic actinomycetes.
Nocardia and Rhodococcus belong to the family Nocardiaceae, and Gordonia and Tsukamurella are in the Gordoniaceae and Tsukamurellaceae families, respectively. However, the variability associated with the classification of an organism as partially acid-fast depends on the particular strain and culture conditions. Therefore, this characteristic should be interpreted with caution. The genus Actinomadura includes approximately 67 species and subspecies, with significant variation. The cell walls of this group contain the sugar madurose, a characteristic shared with the genus Dermatophilus.
Partially Acid-Fast Aerobic Actinomycetes
Nocardia spp. Organisms belonging to the genus Nocardia are gram positive (often with a beaded appearance), variably acid fast, catalase positive, and strictly aerobic. As they grow, Nocardia spp. form branched filaments that extend along the agar surface (substrate hyphae) and into the air (aerial hyphae). As the organisms age, nocardiae fragment into pleomorphic rods or coccoid elements. Nocardiae also are characterized by the presence of mesodiaminopimelic acid (DAP) and the sugars arabinose and galactose in peptidoglycan in the cell wall.
Currently, the taxonomy in the genus Nocardia is changing rapidly. Recognition and description of new species continue and remain controversial regarding the number of validly described species; recent publications cite 22 to 30 valid species. Of significance, Cloud et al.1 reported that the most commonly identified species was Nocardia cyriacigeorgica, not N. asteroides, as determined by partial 16S rRNA DNA sequencing, followed by N. farcinica, N. nova, N. africana, and N. veterana. The species considered human pathogens or that have been implicated as human pathogens are listed in Box 1. N. asteroides, N. nova, N. farcinica, N. brasiliensis, N. otitidiscaviarum (formerly N. caviae), N. pseudobrasiliensis, and N. transvalensis account for most of the diseases in humans caused by Nocardia spp.
Box1. Nocardia spp. Considered Human Pathogens or Have Been Implicated in Human Disease
Rhodococcus, Gordonia, Tsukamurella spp.
Organisms belonging to the Rhodococcus, Gordonia, and Tsukamurella genera are similar to Nocardia spp. in that they are gram-positive, aerobic, catalase-positive, partially acid-fast, branching, filamentous bacteria that can fragment into rods and cocci. The extent of acid-fastness depends on the amount and complexity of mycolic acids in the organism’s cell envelope and on culture conditions. The differentiation of these three genera, as well as species identification, is difficult. In particular, the genus Rhodococcus consists of a very diverse group of organisms in terms of morphology, biochemical characteristics, and ability to cause disease. As previously mentioned, the taxonomy of these organisms continues to evolve; species included in these three genera, as of this writing, are summarized in Table 1.
Table1. Species Included in the Genera Rhodococcus, Gordonia, and Tsukamurella
Non–Acid-Fast Aerobic Actinomycetes: Streptomyces, Actinomadura, Dermatophilus, Nocardiopsis, and the Thermophilic Actinomycetes
The non–acid-fast aerobic actinomycetes (i.e., Streptomyces, Actinomadura, Dermatophilus, Nocardiopsis, and the thermophilic actinomycetes) are gram-positive, branching filaments that do not contain mycolic acids in their cell envelopes and are therefore non–acid-fast. This group of actinomycetes is heterogeneous and is encountered infrequently in the clinical laboratory. Only the non–acid-fast actinomycetes associated with human disease are addressed (Table 2).
Table2. Non–Acid-Fast Aerobic Actinomycetes Associated with Human Disease
Another group of non–acid-fast actinomycetes, the thermophilic actinomycetes, are associated with infections in humans and include the medically relevant genera Thermoactinomyces, Saccharomonospora, and Saccharopolyspora.
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