Proteus penneri showing a green colour reaction with Kovacs' indole reagent.

All 33 investigated strains of Proteus penneri gave a green colour reaction with Kovacs' indole reagent after an incubation of about three days at 36 degrees C. Furthermore, 26% of the 51 strains of Proteus mirabilis studied also showed the green colour reaction but somewhat more weakly. The initial compound of this reaction seems to be a product of tryptophan metabolism which is formed only under aerobic conditions.