Nitrile hydrolysis by Rhodococcus erythropolis BL1, an acetonitrile-tolerant strain isolated from a marine sediment

A number of physiologically different nitrile-hydrolysing bacteria were isolated from coastal marine sediments in Denmark by enrichment culture. One strain, BL1, identified as Rhodococcus erythropolis, grew on acetonitrile as sole carbon and nitrogen source in a defined medium. Growth occurred between 0 and 8% NaCl with an optimum around 2%, thus reflecting the marine origin of the isolate. Intact cells of R. erythropolis BL1 could hydrolyse a large variety of saturated and unsaturated aliphatic nitriles to their corresponding acids. Benzonitrile and benzylcyanide were not hydrolysed, whereas some aromatic compounds containing a -CN group attached to a C₃ or C₄ aliphatic side chain were accepted as substrates. The substrate spectrum of R. erythropolis BL1 was thus markedly different from those of other Grampositive nitrile-hydrolysing bacteria isolated from non-marine environments. Nitrile hydrolysis during growth and in resting cell suspensions usually occurred without intermediate accumulation of amide outside the cells. Detailed studies, however, showed that nitrile hydrolysis by strain BL1 was due to a nitrile hydratase/amidase enzyme system. Nitrile hydratase activity was found to be inducible whereas amidase activity was constitutive. The amidase activity of cells could, however, be enhanced manyfold by growth in media containing acetamide or acetonitrile. In most cases amides were hydrolysed at a much higher rate than the corresponding nitriles, which explained why amides were rarely detected in the surrounding medium during nitrile hydrolysis. R. erythropolis BL1 exhibited the highest tolerance towards acetonitrile ever reported for a nitrile-hydrolysing bacterium, as demonstrated by its ability to grow exponentially in the presence of 900 mM acetonitrile.