Nocardiosis Review of Clinical and Laboratory Experience

Members of the genus Nocardia are associated with the group of microorganisms known as the aerobic actinomycetes and belong specifically to the family Mycobacteriaceae. The nocardiae contain tuberculostearic acids but differ from the mycobacteria by possession of shorter-chained (40- to 60-carbon) mycolic acids. They have a type IV cell wall, characterized by a peptidoglycan made up of meso-diaminopimelic acid, arabinose, and galactose (15). The systematics of this group of organisms originated from intuitive principles based on microscopic morphology and phenotypic characterization. The nocardiae are gram-positive, bacillary, branching bacteria whose hyphae often fragment to coccobacillary forms. Recent application of modern taxonomic procedures, inclusive of more extensive phenotypic evaluation, molecular characterization, and numerical taxonomic methods, has expanded our knowledge of their phylogenetic relatedness and taxonomic status (9, 13, 15). The taxonomy within the genus Nocardia is changing rapidly as the recognition and description of new species continue. As expected, there are differences of opinion as to the number of validly described species within the genus at this time, with recent publications citing from 22 to 30 such valid species (3, 14). Although a large number of species have been characterized both phenotypically and genotypically within the genus, the genotype remains greatly heterogeneous and will continue to evolve (3, 14). Sixteen species have been implicated in human infections (Table ​(Table1);1); but the geographic prevalence of each may change dramatically throughout the world, and some are uncommon. The species found most frequently in Arizona (Table ​(Table2)2) may vary substantially from those isolated in other parts of the United States. TABLE 1. Species of the genus Nocardia associated with infections in humansa TABLE 2. Isolates of Nocardia spp. recovered from individual patients in the Phoenix cosmopolitan area between 1998 and 2002