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

It was recently reported that chloramphenicol inhibits existing denitrification enzyme activity in sediments and carbon-starved cultures of ''Pseudomonas denitrificans.'' Therefore, we studied the effect of chloramphenicol on denitrification by Flexibacter canadensis and ''P. denitrificans.'' Production of N2O from nitrate by F. canadensis cells decreased as the concentration of chloramphenicol was increased, and 10.0 mM chloramphenicol completely inhibited N2O production. ''P. denitrificans'' was less sensitive to chloramphenicol, and production of N2O from nitrate was inhibited by only about 50% even in the presence of 10.0 mM chloramphenicol. These results suggested that inhibition of denitrification enzyme activity depended on the concentration of chloramphenicol. Increasing the concentration of chloramphenicol decreased the rate of production of nitrite from nitrate by F. canadensis cells, and the concentration of chloramphenicol which resulted in 50% inhibition of production of nitrite from nitrate was 2.5 mM. In contrast, the rates of production of nitrite from nitrate by intact cells and cell extracts of ''P. denitrificans'' were inhibited by only 58 and 54%, respectively, at a chloramphenicol concentration of 10.0 mM. Chloramphenicol caused accumulation of NO from nitrite but not from nitrate and inhibited NO consumption in F. canadensis; however, it had neither effect in ''P. denitrificans.'' Chloramphenicol did not affect N2O consumption by these organisms. We concluded that chloramphenicol inhibits denitrification at the level of nitrate reduction and, in F. canndensis, also at the level of NO reduction.