Toluidine Blue O indicator solution is widely used in biological staining and histology. It acts as a metachromatic dye, meaning it changes color depending on the chemical composition of the tissue it binds to. It is particularly effective in staining acidic polysaccharides such as glycosaminoglycans in cartilage and mast cell granules, appearing blue or red-violet based on the tissue type. In microbiology, Toluidine Blue O is used to detect bacterial presence in samples, especially in rapid diagnostic tests. It serves as a redox indicator—color change from blue to colorless indicates reduction, which can signal microbial metabolic activity. It is also applied in cytology for identifying abnormal cells, such as in oral cancer screening, where it selectively stains dysplastic or malignant tissues, helping clinicians locate suspicious areas for biopsy. Additionally, it is used in ophthalmology during corneal staining to highlight damaged epithelial cells. Due to its ability to bind nucleic acids, Toluidine Blue O is employed in chromosome staining and RNA detection in cells, making it valuable in both research and diagnostic laboratories.