oxidase negative?

[blondie2006]

What does it mean when they say that enteric bacteria are "oxidase negative" ?

[theprof]

To understand the term "oxidase negative" you need to know about the oxidase test

The oxidase test is a key test to differentiate between Pseudomonadaceae (ox +) and Enterobacteriaceae (ox -) bacteria both of which are present in the gut.

An enzyme known as cytochrome oxidase is involved with the reduction of oxygen at the end of the electron transport chain and the test is used to confirm the presence of cytochrome c and this oxidase. Gram-negative bacteria produce cytochrome c.

To perform the test, one drop of a colourless reagent called Tetramethyl-p-phenylenediamine (Oxidase Reagent) is added to an inoculum of the bacteria under test in a slant tube or a petri dish.

If within 20 seconds, a purple colouration occurs, this is termed an "oxidase positive" reaction. If no reaction occurs within the time period, it is termed an "oxidase negative" reaction.

In practice in the lab, this test can sometimes be difficult to perform as the reagant has to be fresh and in sealed tubes or recently thawed from frozen. The culture also has to be no more than about 24 hours old. The test can be performed on filter paper if the correct equipment is not to hand.

The test is principally used to detect the presence of Neiserria and also to discriminate between aerobic Gram-negative organisms like Pseudomonas aeruginosa and other Enterobacteriaciae.