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Chapter 23 : General Approaches to the Identification of Aerobic Gram-Positive Rods
This chapter emphasizes that the Gram stain (performed on 24- to 48-h-old colonies from rich media) and macroscopic morphologies are initial key features for the differentiation of aerobic gram-positive rods. All strains of aerobic gram-positive rods (except the non-rapidly growing mycobacteria) are initially grown on blood agar plates. Catalase activity should be tested with colonies grown on media lacking heme groups. Type of metabolism can be evaluated using oxidative-fermentative media or in cystine Trypticase agar (CTA) medium. Genera which contain strictly anaerobic gram-positive rods may also contain species, or strains within a species, which grow reasonably well aerotolerantly or aerobically. Some gram-positive rods (e.g., Rhodococcus spp. or Dermabacter might be initially misidentified as gram-positive cocci because of their initial gram stain appearance. Molecular approaches to characterize or subtype pathogens are described in the chapter.