A method was developed to detect NO- or N2O-producing bacteria in solid or liquid medium by their ability to oxidize the redox indicator resazurin from its reduced colorless form to its oxidized pink form. The method was sensitive to as little as 35 nM N2O or 0.5 nM NO. Ninety-one percent of the colonies that oxidized resazurin on plates also produced N2O in slant cultures. Forty-four percent of the colonies that did not oxidize resazurin did produce N2O. This percentage was reduced to 15% when colonies in which the coloration was difficult to discern were picked to slants to determine whether they oxidized the slant. The production of N2O preceded the oxidation of resazurin by liquid cultures of Escherichia coli and a sludge isolate. With the denitrifying sewage isolate, the disappearance of N2O was followed by the return of resazurin to its reduced state. Wolinella succinogenes was found to produce small amounts of N2O from NO3−, which resulted in a transient oxidation of resazurin.
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