Indoxyl is a product of the putrefactive decomposition of tryptophan in the intestines. Bacteria like some Helicobacter, Campylobacter
and related genera can hydrolyze indoxyl acetate. In the presence of oxygen, indoxyl changes to indigo. Disk and tube methods are
available. The disk method is faster and cheaper than the tube method.
Disks preparation: prepare a 10% (wt/vol) solution of indoxyl acetate (Sigma) in
acetone. Add 50 µl of the solution to each blank paper disk (diameter, 0.6 cm; BBL).
Dry the disks at room temperature, then store at 4°C in a brown tube with a desiccant.
Tube method: emulsify a large loopful of plate culture in 0.3 ml distilled water (sterile).
Add a indoxyl disk to the suspension. A positive reaction may appear in 10-45 min.
Disk method: place a large loopful of plate culture on an impregnated disk and add 1
drop of distilled water. A positive reaction may appear in 5-10 minutes.
A dark blue color indicates a positive reaction (hydrolysis of indoxyl acetate). A pale
blue color indicates a weak reaction. No color change indicates a negative reaction.
1. Hodge, D. S., A. Borczyk, and L.-L. Wat. 1990. Evaluation of the indoxyl acetate
hydrolysis test for the differentiation of campylobacters. J. Clin. Microbiol. 281482-1483.
2. Tatjana Popovic-Uroic, Charlotte M. Patton, Mabel A. Nicholson, and Julia A. Kiehlbauch. 1990. Eval. of the Indoxyl Acetate Hydrolysis
Test for Rapid Diff. of Campylobacter, Helicobacter, and Wolinella Species. J. Clin. Microiol. 28:2335-2339.
(c) Costin Stoica