Phenotypic Characterization

For many years, the results of the Gram stain, modified acid-fast stain (MAS), colonial growth patterns, a few biochemicals, and hydrolysis of specific substrates (eg, casein, xanthine, tyrosine) were all the tests that were needed to identify Nocardia and Streptomyces.^23,24 However, with the advent of genotypic identification methods and the many resulting taxonomy reclassifications, phenotypic characterization is no longer as useful to clinical microbiologists, as it takes time and special bacteriology resources and training. At best, the stains, colonial morphology, biochemicals, and substrate testing assist in pointing the microbiologist to a possible genus (in order to assist the clinician with initial treatment); however, definitive identification must be performed at the genotypic level. Routine clinical laboratories must usually package and ship suspected isolates to their nearest public health laboratory (local or state, where isolates may be sent on to the CDC), or to a large reference (specialty) laboratory for identification and susceptibility workup.