Mycobacterium: Clinical and Laboratory Characteristics of Rapidly Growing Mycobacteria

Prior to the advent of modern molecular technology in the clinical laboratory, the rapidly growing mycobacteria (RGM) were routinely identified by phenotypic characteristics, including batteries of biochemical tests, growth rates, and colonial morphology. The impact of the molecular revolution in the clinical laboratory has brought more rapid, efficient, and accurate identification than was possible with most of the techniques based solely on phenotypic characteristics. Moreover, these technologies have introduced a large number of new species, including some pathogens and some environmental species with no current claim to pathogenicity. Molecular methods such as DNA gene sequence analysis are now recognized as the gold standard for the identification of nontuberculous mycobacteria (NTM). Newer techniques, such as matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) may, however, soon supplant DNA (sequencing) technology. The current disadvantage of all of the newer systems, including sequencing technology, is that complete databases may be lacking. The Clinical and Laboratory Standards Institute (CLSI) has approved guidelines not only for molecular identification methods but also for antimicrobial susceptibility testing of the RGM, thus allowing clinical laboratories to better standardize testing of the RGM. This chapter details the clinical significance of the RGM species and describes methods of laboratory identification, susceptibility testing, and reporting of these RGM species, with an emphasis on taxonomy and antimicrobial susceptibility changes that have occurred within the last 5 years.