Indole Microbiology Test

Theoretical Background/Context

•  The Indole microbiology test is one of the four tests of the IMViC series, which tests for evidence of an enteric bacterium. The other three tests include: the methyl red test [M], the Voges–Proskauer test [V], and the citrate test. 
• The indole test principle depends on determining the ability of the organism to convert tryptophan into indole through a biochemical test performed on bacterial species. This division is performed by a chain of a number of different intracellular enzymes, a system generally referred to as "tryptophanase.“
• The indole test principle depends on determining the ability of the organism to convert tryptophan into indole through a biochemical test performed on bacterial species. This division is performed by a chain of a number of different intracellular enzymes, a system generally referred to as "tryptophanase.”
• Indole-Positive Bacteria

Bacteria that test positive for cleaving indole from tryptophan include: Aeromonas hydrophila, Aeromonas punctata, Bacillus alvei, Edwardsiella sp., Escherichia coli, Flavobacterium sp., Haemophilus influenzae, Klebsiella oxytoca, Proteus sp. (not P. mirabilis and P. penneri), Plesiomonas shigelloides, Pasteurella multocida, Pasteurella pneumotropica, Enterococcus faecalis, Vibrio sp., and Lactobacillus reuteri.

• Indole-Negative Bacteria

Bacteria which give negative results for the indole production test include: Actinobacillus spp., Aeromonas salmonicida, Alcaligenes sp., most Bacillus sp., Bordetella sp., Enterobacter sp., most Haemophilus sp., most Klebsiella sp., Neisseria sp., Mannheimia haemolytica, Pasteurella ureae, Proteus mirabilis, P. penneri, Pseudomonas sp., Salmonella sp., Serratia sp., Yersinia sp., and Rhizobium sp.

• The Indole microbiology test is one of the four tests of the IMVC series, which tests for evidence of an enteric bacterium. The other three tests include: the methyl red test [M], the Voges–Proskauer test [V] and the citrate test.
• The cytochrome system is usually only present in aerobic organisms which are capable of utilising oxygen as the final hydrogen receptor. The end product of this metabolism is either water or hydrogen peroxide (broken down by catalase).  

Principle of Work

Some bacteria split amino acid tryptophan into indole and pyruvic acid using the enzyme called tryptophanase. Indole can be detected with Kovac’s reagent or Ehrlich’s reagent. Indole reacts with the aldehyde in the reagent to give a red color which concentrates as a ring at the top.

This test is frequently performed using biochemistry virtual lab environments or virtual lab biochemistry simulations to train students on the indole test protocol without handling live cultures.