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.
Posted by Tim Sandle
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