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Denitrifying Bacteria can be Enumerated in Nitrite Broth¹

ABSTRACT

Denitrifying- and NO₃^- reducing bacteria in air dry, moist, and organic matter amended Yalesville fsl (Typic Dystrochrept), lake water, and in both effluent and drainage field soil from a septic tank system were enumerated using Difco nutrient broth, which contained either or both NO₃^- and NO₂^- and was incubated anaerobically. Most probable numbers (MPN) of denitrifiers (1.3 x 10³ to 1.1 x 10⁷/g) were always less than those of NO₃^- reducers (1.5 x 10⁶ to 8.1 x 10⁷/g) in the same sample, and constituted from 0.02% to 70% of bacteria capable of aerobic growth on yeast extract agar. Further, for a given sample, denitrifier populations as estimated in nutrient broth + NO₂^- (NO₂^- broth) were always statistically equivalent to or larger than those determined using nutrient broth + NO₃^- (NO₃^- broth). Therefore, NO₂^- broth may be preferable to NO₃^- broth for estimating denitrifier populations in soil and water samples. Nitrite broth retains the nutritional advantages of nutrient broth but also represses competitive growth effects of NO₃^- reducing bacteria. Further, in order to determine when in vitro denitrification has occurred, one needs only to assess colorimetrically the presence of NO₂^- following culture tube incubation.

¹ Contribution from the Connecticut Agric. Exp. Stn., New Haven, CT 06504.

² Assistant Plant Physiologist.

Received for publication June 25, 1976. Accepted for publication December 22, 1976.