a and Species to Be Considered
Nocardia, Streptomyces, Rhodococcus, ciapter Q1 and Similar Organisms a and Species to Be Considered
• Nocardia nova
9 Thermophilic actinomycetes t:The actinomycetes are a large and diverse group of gram-positive bacilli. For the most part, cells of all actinomycetes elongate to form branching, filamentous forms. The rate and extent of filament (hyphae) elonga-[ 'Hon with lateral branching is dependent on the strain of actinomycetes, the growth medium, and the temperaire of incubation.3 Some organisms form filaments, or hyphae, on the agar surface or into the agar, whereas others extend into the air.
These organisms are aerobic, facultatively anaero-or obligately anaerobic; only the aerobic actinomycetes are discussed in this chapter. In the previous ■edition of this text, Oerskovia and Rothia were also <iiscussed; however, because of additional phylogenetic information, these genera are now reviewed in Chapter Aerobic actinomycetes belong to the order Acti-nomycetales. There are more than 40 genera but only the clinically relevant aerobic actinomycetes genera are considered here (Table 21 -1). In this chapter, only those aerobic actinomycetes that can exhibit branching and/or partial acid-fastness are addressed. Although both belong to the order Actinomycetales, Corynebacterium spp. do not usually exhibit branching filaments or partial acid-fastness and Mycobacterium spp. do not exhibit branching and are strongly (acid-alcohol) acid-fast; for these reasons, the Corynebacteriaceae and Mycobacteriaceae are addressed in Chapters 19 and 45, respectively. Finally, another clinically significant aerobic actinomycete is Tropheryma whippier, because this organism has not been cultured on artificial media, it is reviewed in Chapter 46. For purposes of discussion, the remaining genera of aerobic actinomycetes are divided into the following two large groups: those whose cell walls contain mycolic acid and are therefore partially acid-fast and those whose cell envelopes do not contain mycolic acid and therefore are non-acid-fast.
In general, the aerobic actinomycetes are not frequently isolated in the clinical laboratory; nevertheless, these organisms are causes of serious human disease. Not only are infections caused by these organisms difficult to recognize on clinical grounds, but they are also difficult to isolate. Further complicating matters from the laboratory's perspective is the difficulty in classifying, identifying, and performing antibiotic susceptibilities on aerobic actinomycetes isolated from clinical specimens. At the time of this writing, the taxonomy of the aerobic actinomycetes is complex and continues to evolve. New and reliable methods that can identify cell wall amino acids and sugars and characterize mycolic acid, menaquinones, and phospholipids in conjunction with nucleic add phylogenetic studies are proving extremely useful for resolving the taxonomy of the actinomycetes.
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