Eosin

Eosin is a family of synthetic red-orange dyes derived from fluorescein, with eosin Y being the most widely used form. It is an anionic, water-soluble dye that belongs to the xanthene dye class and is commonly employed as a counterstain in light microscopy.

In histology, eosin is used with hematoxylin in standard H&E staining. Hematoxylin stains nuclei blue, while eosin stains the cytoplasm, muscle fibers, collagen, and other eosinophilic structures pink to red. The staining is pH dependent, typically performed in slightly acidic conditions to optimize contrast between tissues. Eosin’s affinity for basic, or eosinophilic, components provides a broad counterstain that helps differentiate tissue architecture.

Outside of histology, eosin is used in microbiology as part of eosin methylene blue (EMB) agar, where it functions as an indicator dye. Along with methylene blue, eosin Y helps differentiate lactose-fermenting Gram-negative bacteria, which produce dark, sometimes metallic colonies, from non-fermenters that remain pale. Eosin Y also exhibits fluorescence under ultraviolet light, emitting an orange-red glow, which can aid certain fluorescence-based techniques.

Forms and handling notes: eosin is available as disodium or other salt forms, soluble in water or aqueous alcohols for staining applications. Solutions are used at low concentrations, and staining procedures typically involve brief exposure times. Eosin can fade with prolonged light exposure, so light protection is often advised. Safety considerations include irritation upon skin or eye contact; handling should follow standard laboratory guidelines, including gloves, proper storage, and regulatory disposal of dye-containing waste.