In low-income countries some infections caused by nontuberculous mycobacteria are misdiagnosed as multidrug-resistant tuberculosis. In most of these settings the observation of microscopic cords is the only technique used to identify Mycobacterium tuberculosis in the laboratory. In this article we definitively demonstrate that Mycobacterium abscessus, an emerging pulmonary pathogen, also forms microscopic cords.Mycobacterium tuberculosis complex (MTC) species, when grown in a liquid medium without detergent, form cords in tight bundles, consisting of bacilli aligned in parallel, end to end, and side to side, along the long axis of the cord (see reference 6 for a recent review). M. tuberculosis microscopic cords were first observed by Robert Koch in 1882, but their significance increased in 1947, when studies performed by Middlebrook et al. linked this phenotypic characteristic to the virulence of MTC organisms (16). From these pioneering studies to now, cord morphology has been considered to be a distinctive feature of the MTC, and its detection, in smears from liquid cultures, is a reliable criterion for the rapid presumptive identification of MTC isolates in many laboratories around the world (6,12,14,20,23). The first demonstration that cord formation could occur in other mycobacterial species was in 2008, when microscopic cords in a human clinical isolate of Mycobacterium marinum were described (21).In 2010 we demonstrated that M. marinum produced true cords (bacilli arranged in parallel along the long axis of the cord) and not clumps (aggregates of bacilli in a random orientation), obtaining the first scanning electron microscopy (SEM) images of the mycobacterial cord ultrastructure (11). In the same work we demonstrated that the phenomenon of cording was also present in rough variants of other nontuberculous mycobacteria (NTM) such as Mycobacterium vaccae, Mycobacterium gilvum, Mycobacterium obuense, Mycobacterium chubuense, and Mycobacterium parafortuitum (11).The study of the cording phenomenon by SEM aims to eliminate any doubt regarding the ability of NTM to form cords. This is necessary because the use of cording as a criterion for the identification of MTC isolates from clinical cultures has been adopted as a standard methodology by clinical microbiologists. As a result of this bias, when cordlike morphology is observed for NTM, these structures have been characterized as clumps, pseudocords, or loose aggregates (7,14,20). The resolution of the optical microscope does not allow the accurate visualization of the organization of the bacilli in these aggregates; thus, the idea that NTM cannot form true cords has persisted to the present time.Mycobacterium abscessus is one of the most pathogenic and chemotherapy-resistant rapid-growing mycobacteria (15,17). This mycobacterium has been reported to account for 10% of NTM lung infections in the United States and is the second most common cause of NTM lung disease in South Korea (13). M. abscessus causes lung disease in patients with underlying lung disorder...
Our results suggest that killed but metabolically active bacillus Calmette-Guérin could be considered a safer immunotherapy alternative to treatment with the live bacillus.
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