Spot indole test: evaluation of four reagents

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Abstract
Kovacs indole reagent, p-dimethylaminobenzaldehyde, Ehrlich indole reagent, and p-dimethylaminocinnamaldehyde were used as spot indole reagents to test 359 strains of gram-negative rods growing on 5% sheep blood agar. Trypticase soy agar and MacConkey agar. The p-dimethylaminocinnamaldehyde reagent was the most sensitive of those tested and provided results that were easiest to interpret. The p-dimethylaminocinnamaldehyde reagent was able to detect Providencia alcalifaciens indole production because of the red-violet color unique to that organism. All reagents tested were accurate in detecting indole produced by members of the Enterobacteriaceae family, with the exception of P. alcalifaciens.

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