Bacteria utilize glucose and other carbohydrates through various metabolic pathways. Some pathways are oxidative, while others involve fermentation reactions.
The Oxidation-Fermentation (O/F) test, also known as the Ox-Ferm test, is used to determine which metabolic pathway is utilized by a microorganism.
This test is commonly used to differentiate between bacterial species, particularly Gram-negative rods, and to distinguish between the genera Staphylococcus and Micrococcus.
The Oxidation-Fermentation (O/F) test is used to determine whether bacteria metabolize carbohydrates oxidatively, fermentatively, or are non-saccharolytic and therefore unable to utilize the carbohydrate present in the medium.
Oxidative organisms metabolize glucose or other carbohydrates only under aerobic conditions, where oxygen acts as the final electron (hydrogen) acceptor.
Other organisms ferment glucose, and the final electron acceptor is an organic substance rather than oxygen.
This fermentative process is independent of oxygen, and fermentative organisms may grow under aerobic or anaerobic conditions.
The end product of carbohydrate metabolism is typically acid, which lowers the pH of the medium.
The method, commonly known as the Hugh and Leifson O/F test, uses a semi-solid medium containing the carbohydrate being tested (usually glucose) and a pH indicator.
Two tubes are inoculated with the test organism. One tube is left open to allow aerobic conditions, while the other is sealed immediately (usually with mineral oil) to create anaerobic conditions.
Members of the Enterobacteriaceae family ferment glucose and produce acid throughout the medium in both tubes, resulting in a color change of the pH indicator.
Organisms that cannot utilize the carbohydrate either aerobically or anaerobically, such as Alcaligenes faecalis, produce an alkaline reaction in the open tube and show no change in the covered tube.
Hugh and Leifson’s medium can also be used to observe gas production and motility.
Staphylococci and micrococci are commonly tested using the Baird–Parker modification of the medium.