Denitrifying Bacteria can be Enumerated in Nitrite Broth

Denitrifying‐ and NO 3 ‐ 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 NO 3 ‐ and NO 2 ‐ and was incubated anaerobically. Most probable numbers (MPN) of denitrifiers (1.3 × 10 3 to 1.1 × 10 7 /g) were always less than those of NO 3 ‐ reducers (1.5 × 10 6 to 8.1 × 10 7 /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 + NO 2 ‐ (NO 2 ‐ broth) were always statistically equivalent to or larger than those determined using nutrient broth + NO 3 ‐ (NO 3 ‐ broth). Therefore, NO 2 ‐ broth may be preferable to NO 3 ‐ 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 NO 3 ‐ reducing bacteria. Further, in order to determine when in vitro denitrification has occurred, one needs only to assess colorimetrically the presence of NO 2 ‐ following culture tube incubation.