EFFECT OF CHLORAMPHENICOL ON DENITRIFICATION IN FLEXIBACTER-CANADENSIS AND PSEUDOMONAS-DENITRIFICANS

It was recently reported that chloramphenicol inhibits existing denitr ification enzyme activity in sediments and carbon-starved cultures of ''Pseudomonas denitrificans.'' Therefore, we studied the effect of chl oramphenicol on denitrification by Flexibacter canadensis and ''P. den itrificans.'' Production of N2O from nitrate by F. canadensis cells de creased as the concentration of chloramphenicol was increased, and 10. 0 mM chloramphenicol completely inhibited N2O production. ''P. denitri ficans'' was less sensitive to chloramphenicol, and production of N2O from nitrate was inhibited by only about 50% even in the presence of 1 0.0 mM chloramphenicol. These results suggested that inhibition of den itrification enzyme activity depended on the concentration of chloramp henicol. Increasing the concentration of chloramphenicol decreased the rate of production of nitrite from nitrate by F. canadensis cells, an d the concentration of chloramphenicol which resulted in 50% inhibitio n of production of nitrite from nitrate was 2.5 mM. In contrast, the r ates of production of nitrite from nitrate by intact cells and cell ex tracts of ''P. denitrificans'' were inhibited by only 58 and 54%, resp ectively, at a chloramphenicol concentration of 10.0 mM. Chloramphenic ol caused accumulation of NO from nitrite but not from nitrate and inh ibited NO consumption in F. canadensis; however, it had neither effect in ''P. denitrificans.'' Chloramphenicol did not affect N2O consumpti on by these organisms. We concluded that chloramphenicol inhibits deni trification at the level of nitrate reduction and, in F. canndensis, a lso at the level of NO reduction.