Unreliability of Cord Formation for Presumptive Identification of Mycobacterium tuberculosis Complex
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Abstract
Cord formation is the important characteristic of Mycobacterium tuberculosis complex (MTBC) that is used for identification of MTBC from the past until now. The mechanism and factors for cord formation are still unknown at the present time. In the past, scientists believed that cord formation was associated with glycolipid on cell wall of MTBC that is called cord factor or trehalose 6,6′-dimycolate (TDM), the important virulence factor of MTBC. Almost mycobacterial species, with the exception of M. leprae, have TDM but mycobacteria other than tuberculosis or Non-tuberculous mycobacteria (NTM) rarely form cord so there may be multiple factors that cause the cord as well. Because some species of NTM can form cord and some strains of MTBC cannot form cord, false positive and false negative can be occurred. Therefore, the use of cord formation alone is not appropriate to distinguish MTBC from NTM. It must be used in conjunction with other macroscopic characteristics such as colony morphology and growth rate for separation. All of these are only presumptive identification. It is necessary to use a higher specific test to confirm the identification of MTBC.
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