Lack of usefulness of carbon utilization tests for identification of Mycobacterium mucogenicum

Carbon utilization tests have proven to be useful for the identification of some species of rapidly growing mycobacteria and have been described as one of the few tests useful for the differentiation of Mycobacterium mucogenicum from other rapid growers. We have found the carbon utilization tests to be unreliable for the identification of patient isolates of this species. In this study, using 28 isolates of rapidly growing mycobacteria, we examined several variables which might have an effect on results of citrate, inositol, and mannitol utilization: inoculum concentration, incubation temperature, and medium manufacturer. None of these variables affected results obtained for most species of rapid growers or for ATCC strains of M. mucogenicum. Results for patient isolates of M. mucogenicum were found to be inconsistent regardless of the methodology employed and resulted in an ambiguous identification of these isolates or an incorrect identification as Mycobacterium chelonae. Molecular or cell wall analysis may be the best technique to employ for accurate identification of M. mucogenicum.