Evaluation of a fluorescence in situ hybridization assay for differentiation between tuberculous and nontuberculous Mycobacterium species in smears of Lowenstein-Jensen and Mycobacteria Growth Indicator Tube cultures using peptide nucleic acid probes

A new fluorescence in situ hybridization assay based on peptide nucleic acid probes (MTB and NTM probes targeting tuberculous and nontuberculous species, respectively) for the identification of Mycobacterium tuberculosis complex and differentiation between tuberculous and nontuberculous mycobacteria (NTM) was evaluated using Lowenstein-Jensen (LJ) solid cultures front 100 consecutive sputum samples and 50 acid-fast bacillus (AFB)-positive sputum samples as well as Mycobacteria Growth Indicator Tube (MGIT) liquid cultures from 80 AFB-positive sputum samples. Mycobacterium species could be identified from a total of 53 LJ cultures and 77 MGIT cultures. The diagnostic specificities of the MTB and NTM probes were 100% for both cultures. The diagnostic sensitivities of the MTB probe for the LJ and MGIT cultures were 98 and 99%, respectively, whereas the sensitivities of the NTM probe were 57 and 100%, respectively. The relatively low sensitivity of the NTM probe was due to a high proportion of M. fortuitum, which is not identified by the probe.