[Personal View] Fluorescent acid-fast stains for diagnosing mycobacteria and beyond: back to the future?

Acid-fast stains (AFS) remain indispensable in modern diagnostic microbiology; they are used for detecting mycobacteria (including Mycobacterium tuberculosis and Mycobacterium leprae), acid-fast parasites, and some acid-variable bacteria as well as in histopathology. Fluorescent AFS surpass brightfield AFS (Ziehl–Neelsen) in sensitivity, particularly when pathogen loads are low. However, latest evidence suggests that these stains target nucleic acids, whereas lipid-rich, intact cell walls merely prevent decolourisation; this evidence corrects the long-held assumption that AFS bind to mycolic acids.