The Genus Nocardioides
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The Genus Nocardioides JUNG-HOON YOON AND YONG-HA PARK
The genus Nocardioides was proposed by Prauser (1976) for Gram-positive, non-acid-fast, aerobic, and mesophilic nocardioform actinomycetes, developing a mycelium that fragments into irregular rod- to coccus-like elements. Originally, the genus contained two species, i.e., Nocardioides albus and Nocardioides luteus (Prauser, 1984), both forming welldeveloped mycelia. The primary mycelium shows abundantly branching hyphae growing on the surface and penetrating into agar media. The hyphae of the aerial mycelium either do not branch or are sparsely and irregularly branched, resembling the structure of the aerial mycelium of nocardiae. Both the primary and the aerial mycelia can break up into short rod-like fragments, which give rise to new mycelia, by extruding one or two hyphae. The inclusion of additional species to the genus Nocardioides resulted in the morphologic heterogeneity of the genus. Other species subsequently transferred to Nocardioides were Arthrobacter simplex (O’Donnell et al., 1982) and Pimelobacter jensenii (Suzuki and Komagata, 1983). The genus Pimelobacter was originally proposed by Suzuki and Komagata (1983) for Arthrobacter species with LL-2,6-diaminopimelic acid in the peptidoglycan. Three Pimelobacter species were recognized, namely, P. jensenii (previously regarded as an atypical strain of Arthrobacter simplex), P. tumescens (previously, type strain of Arthrobacter tumescens), and P. simplex (previously regarded as the type strain of Arthrobacter simplex and two strains of “Brevibacterium lipolyticum”). As a consequence of a taxonomic reevaluation (Collins et al., 1989b), Pimelobacter jensenii was transferred to the genus Nocardioides as N. jensenii, and P. tumescens was reclassified in a new genus, Terrabacter, as T. tumescens (Collins et al., 1989b). The reclassified N. simplex and N. jensenii are coryneform actinomycetes that form neither a primary mycelium nor an aerial mycelium. The number of species belonging to the genus Nocardioides has continuously increased. Two more coryneform species, Nocardioides fastidiosa (Collins and Stackebrandt, 1989a) and
Nocardioides plantarum (Collins et al., 1994), were isolated from herbage. Nocardioides fastidiosa has MK-9(H4) as the predominant menaquinone. Consistent with this, the genus Nocardioides was known to have MK-8(H4) or MK-9(H4) as the predominant menaquinone; however, Tamura and Yokota (1994) subsequently transferred N. fastidiosa to the genus Aeromicrobium as A. fastidiosum. Later, three more Nocardioides species, N. pyridinolyticus, N. nitrophenolicus and N. aquaticus, were added to the genus. Strains of these species form neither a primary mycelium nor an aerial mycelium. Currently, there are eight validly described Nocardioides species, namely, N. albus (Prauser, 1976), N. aquaticus (Lawson et al., 2000), N. jensenii (Suzuki and Komagata, 1983; Collins et al., 1989b), N. luteus (Prauser, 1984), N. nitrophenolicus (Yoon et al., 1999), N. plan
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