Monday, 2 March 2015

Oxidation-fermentation test explained

Bacteria utilise glucose and other carbohydrates through various metabolic pathways. Some are oxidative routes but others involve fermentation reactions. The oxidation-fermentation test, also known as the “oxferm”/ OF test, is used to determine which route is used. The test is used to differentiate between species, particularly Gram negative rods as well as between genera Staphylococcus and Micrococcus.

The oxidative-fermentative test is used to determine if bacteria metabolise carbohydrates oxidatively, by fermentation, or are non-saccharolytic and therefore have no ability to use the carbohydrate in the media.

Oxidative organisms can only metabolise glucose or other carbohydrates under aerobic conditions i.e. oxygen is the ultimate hydrogen acceptor. Other organisms ferment glucose and the hydrogen acceptor is then another substance e.g. sulphur. This fermentative process is independent of oxygen and cultures of organisms may be aerobic or anaerobic. The end product of metabolising a carbohydrate is an acid.

The method described, sometimes referred to as the Hugh and Leifson test employs a semi-solid medium in tubes containing the carbohydrate under test (usually glucose) and a pH indicator. Two tubes are inoculated and one is sealed immediately to produce anaerobic conditions. The Enterobacteriaceae, produce an acid reaction throughout the medium in both tubes. Organisms that cannot break down the carbohydrate aerobically or anaerobically, eg Alcaligenes faecalis, produce an alkaline reaction in the open tube and no change in the covered tube. Hugh and Leifson’s medium can also be used for recording gas production and motility. Staphylococci and micrococci are tested with the Baird-Parker modification of the medium.

In relation to the oxidation-fermentation test, Public Health England has issued a technical report, including safety information. The report can be accessed here.

Posted by Tim Sandle