PAS reaction stains phagocytosed atypical mycobacteria in paraffin sections

In 4% formaldehyde-fixed, paraffin-embedded tissues, PAS negativity for mycobacteria in the literature has been axiomatic. However, recent observations of disseminated human mycobacteriosis have shown that intracellular (phagocytosed) organisms stained strongly positive with the routine PAS technique. This staining was abolished by a sequential hydrolytic procedure, which suggests that the carbohydrate residues of the mycobacterial peptidoglycolipids are responsible for the reaction. This staining characteristic in tissue sections is of diagnostic importance, since few bacteria of medical relevance are concomitantly acid-fast and PAS positive. The nature of this affinity is for the aqueous form of basic fuchsin.