Fluorescent in situ hybridization (Fish) is a method employing a fluorescent dye covalently bonded to a specific nucleic acid probe identifying or tracking organisms in the environment. Fluorescent in situ hybridization (fish) can be used to determine; how many salmonella typhimurium cells are present in a sample of unpasteurized apple juice, the phylogenetic diversity of an environmental sample and also whether a specific piece of mRNA is being produced.