Comparison of MALDI-TOF MS with HPLC and nucleic acid sequencing for the identification of Mycobacterium species in cultures using solid medium and broth.

Objectives: The genus Mycobacterium contains over 150 species including pathogenic and nonpathogenic strains. Accurate species level identification can aid in differentiating environmental contamination from true infection, and also can aid in selection of antimicrobial therapy. Methods: We evaluated the performance of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for the routine identification of clinical isolates of mycobacteria using 2 commercially available spectral reference libraries, and also assessed the impact of mycobacterial culture using solid medium and broth on MALDI-TOF MS–based identification. Results: All results were compared with those obtained on high-pressure liquid chromatography and nucleic acid sequencing. Optimal results were obtained with a mycobacterium-specific reference library (Mycobacterium Library v1.0). The identification rate was 89.8% (79/88) for isolates cultured on solid medium and 98.8% (85/86) for isolates analyzed directly from broth. Among these, the proportion identified with a high confidence level was 50.0% (44/88) for isolates cultured on solid medium and 80.2% (69/86) for isolates cultured in broth. Conclusions: Agreement with nucleic acid sequencing for species present in Mycobacterium Library v1.0 was 97.6% (81/83) for isolates cultured on solid medium and 97.5% (79/81) for those cultured in broth. Mycobacterial infections, including tuberculosis, affect millions of people worldwide each year. Most of the mycobacteria described belong to species other than Mycobacterium tuberculosis complex and are collectively referred to as nontuberculous mycobacteria (NTM). The NTM are commonly isolated from natural and municipal (treated) water sources and are particularly well suited to colonize household, industrial, and hospital water supply lines. 1-5 Of the more than 150 species of NTM, only a relatively small number of species are strongly associated with human disease. 6,7 The ubiquity of NTM in the environment, including health care facilities, and the variable pathogenicity within the genus can pose a problem for clinicians when differentiating among asymptomatic colonization, environmental contamination, and true infection. 8-10 Species-level identification of NTM isolated from respiratory sources can aid in patient care by differentiating species with increased pathogenic potential, such as M avium/M intracellulare (MAI) or M abscessus, from other species with low pathogenic potential such as M gordonae. 7,11 Likewise, identifying NTM in postsurgical wound infections or indwelling venous catheters can aid in differentiating NTM associated with invasive infection from those that are environmental or commonly associated with colonization of catheters such as M mucogenicum. 12 In both respiratory and cutaneous specimens, species-level identification of NTM can guide decisions about the necessity of antimicrobial therapy, and in some cases, can assist in selecting appropriate antimicrobial therapy because of the predictable susceptibility profiles in specific NTM species. 6,11 Historically, NTM were identified using the Runyon scheme, based on physical and biochemical properties of