Species-specific identification of mycobacterial 16S rRNA PCR amplicons using smart probes

Due to growing problems with new emerging pathogens, cost-effective and manageable methods for their accurate identification in routine diagnostics are urgently required. Of particular importance is the genus Mycobacterium with its more than 100 species. Identification of these species is hampered by their slow growth in the laboratory and by the obligate need for DNA sequence analysis. To provide a fast and reliable diagnostic tool, we developed a novel approach using fluorescently labeled DNA hairpin structures (smart probes) for selective and sensitive detection of mycobacterial 16S rDNA PCR amplicons in homogeneous and heterogeneous assays. Smart probes are singly labeled hairpin-shaped oligonucleotides bearing a fluorescent dye at the 5'-end, which is quenched by guanosine residues in the complementary stem. Upon hybridization to target sequences, a conformational change occurs reflected in an increase in fluorescence intensity. Using optimized parameters for hybridization experiments we established a reliable method for the specific detection of Mycobacterium tuberculosis (M. tuberculosis complex) and Mycobacterium xenopi (member of the atypical mycobacteria) with a detection sensitivity of approximately 2 x 10(-8) M in homogeneous solution. The specificity of the smart probes designed is demonstrated by discrimination of M. tuberculosis and M. xenopi against 15 of the most frequently isolated mycobacterial species in a single assay. In combination with a microsphere-based heterogeneous assay format, the technique opens new avenues for the detection of pathogen-specific DNA sequences with hitherto unsurpassed sensitivity.