Histology often makes use of some unusual products you wouldn’t normally think of as laboratory materials. One such item is peanut oil, which is commonly used in the Fite Stain. This stain is used for diagnosis of leprosy, as well as other rare bacterial infections such as Nocardiosis. The bacteria causing leprosy, Mycobacterium Leprae, has cell walls that contain mycolic acids. Bacteria that have a high lipid content of mycolic acids are considered “acid fast” because even after decolorization with acid-alcohol, they hold the carbol-fuchsin dye. The problem with the Mycobacterium Leprae is that it tends to lose its acid-fastness during the exposure of the bacteria’s cell walls to xylene. There are two ways that early researchers of the bacteria found to solve this problem; the principle of restoring the acid-fastness, and the principle of protecting the acid-fastness.
The earliest publication in 1938 by Fite first proposes heating the sample with vegetable oil, chicken fat, or some other type of lubricating oil before staining with fuchsin. This method is worked on in subsequent years to become the Fite-Faraco method, but another researcher several years later, Wade, contests his method, the Wade-Fite method of protecting the acid-fastness is superior. Whoever originally developed it, the procedure we use today in what we refer to as the Fite Stain utilizes this method of protection using peanut oil to protect the acid-fastness. The Fite Stain uses peanut oil mixed with xylene in the deparaffinization. The peanut oil coats the bacteria protecting the waxy coat of the cell walls.
So why is peanut oil used as opposed to another type of oil? First of all, it was available at the time the stain was developed. Faraco’s original research into the staining of acid-fast bacilli is published in 1938 with additional publications and alterations in the 1940s. For reference, Canola oil doesn’t come on the scene until the 1970’s so at the time of the research into these methods, peanut oil is the oil commonly used. On top of that, it has highest viscosity at room temperature and is thick enough to stick to the bacteria wall which is necessary for protecting the acid-fastness.
Additional adaptations and research into the Fite method have substituted mineral oil for peanut oil. Mineral oil is made from refined petroleum and is commonly used in lotions and skincare products and is also used as a binding agent in food production. These modifications of the Fite stain have been used in the identification of mycobacterial species in AIDS patients.