Tuberculosis (TB) is caused by the acid-fast bacterium Mycobacterium tuberculosis. Although most TB cases affect the lungs (pulmonary TB), some infections occur in other body sites (extrapulmonary TB). Among these cases, lymph node tuberculosis (LNTB) is the most common. Diagnosis of LNTB by conventional methods (staining, biochemical tests) has limitations—for example, formaldehyde preservation of tissue biopsies interferes with the acid-fast stain, leading to underdiagnosis or misdiagnosis. Now, fluorescent in situ hybridization (FISH) is being used to diagnose extrapulmonary TB.
When detecting microorganisms, a probe that hybridizes to sequences on ribosomal RNA (rRNA) is generally used. This is because:
RNA is a single-stranded molecule, but DNA is a double helix and must be denatured.
rRNA is in the cytoplasm, so it is easy to detect, but DNA is in the nucleus, so it is harder to detect.
Cells that are dividing can have thousands of copies of rRNA, increasing the technique's sensitivity.
These are all reasons why rRNA is targeted.