Acidification of the culture medium may lead to a false-negative result of the oxidase reaction. Hence, if the medium contained a carbon source or another component that is convertible to acid, a negative oxidase reaction should be considered inconclusive. In order to eliminate false-negative reactions, a reagent freshly adjusted to pH 5-7 might be used; this reagent should be checked on known oxidase-negative strains. However, we recommend that the oxidase test be performed with the normal reagent after (sub)culturing on a non-acidogenic medium.