Reduction of Nitrates and Nitrites by Representatives of the Brucella Group

Abstract

The Brucella group is generally believed to lack the ability to reduce nitrates. Bergey 1 characterizes both Br. abortus and Br. melitensis as being non-reducers. Duncan and Whitby 2 record similar findings. Evans, 3 Topley and Wilson 4 and Lustig and Vernoni 5 report that nitrates are occasionally reduced.

An investigation was made to determine the factors which influence nitrate reduction and the cause of reported irregularities. A medium of the following composition was used: Peptone 2 gm., beef extract 1 gm.; NaCl 3 gm.; agar 2 gm.; H₂O 1000 cc. The reaction was adjusted to pH 6.8. The semisolid consistency favors the growth of the Brucella and accentuates the zone phenomenon, particularly when inoculated with a dilute suspension of cells. The majority of the Brucella grow in limited zones 2 to 6 mm. below the surface. The abortus type 20 is usually the deepest, the suis 20 are nearest the top and the melitensis 20 are intermediate. Upon the addition of 0.2% KNO₃ to the medium, a reversal of positions is observed. Now the suis types multiply throughout the medium in the form of a pseudo-anaerobic growth, the abortus do so to a less extent but the growth zone of the melitensis is confined to a few mm. below the surface. The two former varieties are apparently able to obtain their oxygen requirements from the disintegration of nitrates.

Every strain of a collection of 425 Brucella tested was found to reduce nitrates vigorously. The presence of nitrites, the disappearance of nitrates and the formation of gas were observed as criteria of the reduction. Frequently 0.1% KNO₃ is quantitatively reduced in 2 days. There is very little tendency for nitrites to accumulate because they are simultaneously reduced, which may account for the apparently negative results reported in the literature.