Burden of unidentifiable mycobacteria in a reference laboratory

Modern identification techniques at the genomic level have greatly improved the taxonomic knowledge of mycobacteria. In adjunct to nucleic acid sequen ces, mycobacterial identification has been endorsed by investigation of the lipidic patterns of unique mycolic acids in such organisms. In the present investigation, the routine use of high-performance liquid chromatography ( HPLC) of mycolic acids, followed by the sequencing of the 16S rRNA, allowed us to select 72 mycobacterial strains, out of 1,035 screened, that do not belong to any of the officially recognized mycobacterial species. Most stra ins (i.e., 47) were isolated from humans, 13 were from the environment, 3 w ere from animals, and 9 were from unknown sources. The majority of human is olates were grown from the respiratory tract and were therefore most likely not clinically significant. Some, however, were isolated from sterile site s (blood, pleural biopsy, central venous catheter, or pus). Many isolates, including several clusters of two or more strains, mostly slow growers and scotochromogenic, presented unique genetic and lipidic features. We hope th e data reported here, including the results of major conventional identific ation tests, the HPLC profiles of strains isolated several times, and the w hole sequences of the 16S rRNA hypervariable regions of all 72 mycobacteria , may encourage reporting of new cases. The taxonomy of the genus Mycobacte rium is, in our opinion, still far from being fully elucidated, and the rep orting of unusual strains provides the best background for the recognition of new species. Our report also shows the usefulness of the integration of novel technology to routine diagnosis, especially in cases involving slow-g rowing microorganisms such as myrobacteria.