FIG. 1. Indole test results with sulfur-indole-motility (SIM) agar.  SIM medium is a bacterial growth medium that is designed to test for multiple characteristics in an effort to identify an organism.  All tubes were incubated for 48 hours at 37°C after inoculation.  Kovács reagent was added after incubation.

Tube 1 contains uninoculated SIM medium.

Tube 2 shows a negative indole result given by Enterobacter aerogenes as indicated by the lack of color change in the top of the tube after the addition of Kovács reagent. 

Tube 3 shows a positive indole result given by Escherichia coli as indicated by the red layer at the top of the tube after the addition of Kovács reagent.

Tube 4 shows a positive indole result given by Proteus vulgaris as indicated by the red layer at the top of the tube after the addition of Kovács reagent. Note the tube is positive for motility as indicated by hazy growth throughout and positive for hydrogen sulfide production as indicated by the

FIG. 2.  Results of the indole test using sulfur-indole-motility medium. The positive indole result given by Escherichia coli (right) is indicated by the red layer at the top of the tube after the addition of Kovács reagent. The negative indole result given by Klebsiella pneumoniae (left) is indicated by the lack of color change in the top of the tube after the addition of Kovács reagent. (Javier Gutierrez-Jimenez, Universidad de Ciencias y Artes de Chiapas, Mexico; Lorena M. Luna-Cazares, Universidad de Ciencias y Artes de Chiapas, Mexico)

FIG. 3. Result of the indole test using sulfur-indole-motility medium and Proteus vulgaris after a 24-hour incubation at 37°C.  A positive indole result is indicated by the red layer at the top of the tube after the addition of Kovács reagent. Note the tube is positive for motility as indicated by hazy growth throughout and positive for hydrogen sulfide production as indicated by the black precipitate in the butt of the tube. (Anne Hanson, University of Maine, Orono, ME)

FIG. 5.  Result of the indole test using sulfur-indole-motility medium and Escherichia coli after a 24-hour incubation at 37°C. A positive indole result is indicated by the red layer at the top of the tube after the addition of Kovács reagent. Note the tube is positive for motility as indicated by hazy growth throughout and negative for hydrogen sulfide production as indicated by the lack of black precipitate in the butt of the tube. (Anne Hanson, University of Maine, Orono, ME)