Isolation of a Moderately Acidophilic Nitrobacter from a Nitrifying Community Supplied with Urea

Nitrite-oxidizing bacteria (NOB), which perform the second step of aerobic nitrification, play an important role in soil. In the present study, we report a novel isolate from agricultural soil affiliated with the genus Nitrobacter and its physiological characteristics. We sampled the surface soil of a vegetable field and obtained mixed culture A31 using the most probable number (MPN) method with inorganic medium containing 0.75‍ ‍mM urea (pH 5.5). The dilution-extinction procedure on culture A31 led to the isolation of a strain that was designated as Nitrobacter sp. A67. The nxrB1 gene sequence of Nitrobacter sp. A67 (302 bp) was classified into Cluster 5, and the highest sequence identity was 96.10% with Nitrobacter sp. BS5/19. The NO₂^- oxidation activity of Nitrobacter sp. A67 was investigated at various pH. The optimum pH for NO₂^- oxidation was 5.8-6.4. This result indicates that Nitrobacter sp. A67 is a moderately acidophilic nitrite-oxidizing bacterium.