Identification of a contaminating Mycobacterium tuberculosis strain with atransposition of an IS6110 insertion element resulting in an altered spoligotype

Molecular fingerprinting with the IS6110 insertion sequence is useful for t racking transmission of Mycobacterium tuberculosis within a population or c onfirming specimen contamination in the laboratory or through instrumentati on, Secondary typing with other molecular methods yields additional informa tion as to the relatedness of strains with similar IS6110 fingerprints. Iso lated, relatively rare, random events within the M. tuberculosis genome alt er molecular fingerprinting patterns with any of the methods; therefore, st rains which are different by two or more typing methods are usually not con sidered to be closely related. In this report, we describe two strains of M . tuberculosis, obtained from the same bronchoscope 2 days apart, that demo nstrated unique molecular fingerprinting patterns by two different typing m ethods. They were closely linked through the bronchoscope by a traditional epidemiologic investigation. Genetic analysis of the two strains revealed t hat a single event, the transposition of an IS6110 insertion sequence in on e of the strains, accounted for both the differences in the IS6110 pattern and the apparent deletion of a spacer in the spoligotype. This finding show s that a single event can change the molecular fingerprint of a strain in t wo different molecular typing systems, and thus, molecular typing cannot be the only means used to track transmission of this organism through a popul ation. Traditional epidemiologic techniques are a necessary complement to m olecular fingerprinting so that radical changes within the fingerprint patt ern can be identified.