Acid-alcohol is a solution commonly used in histology and microbiology for the fixation and staining of tissue samples. It typically consists of a mixture of ethanol and acetic acid, which helps preserve cellular structures while allowing for better penetration of dyes during staining processes. This method is particularly useful for preparing samples for microscopic examination. In the context of Gram staining, acid-alcohol serves as a decolorizing agent. After applying the initial stain, the acid-alcohol removes excess dye from certain cells, allowing for differentiation between Gram-positive and Gram-negative bacteria. This step is crucial for accurate identification and analysis of microbial samples.