Iodine Test
DESCRIPTION
Some bacterial species like some Neisseria produce a starch-like polysaccharide from sucrose. The polysaccharide reacts with
iodine producing a dark blue-purple or black color.
Polysaccharide-producing Neisseriae:
N. denitrificans, N. dentiae, N. flavescens,
N. macacae, N. mucosa, N. perflava, N. polysaccharea,
and N. sicca.
MEDIUM PREPARATION

Tryptic soy agar, 5% sucrose
Tryptic soy agar (Difco) ..................................................... 40.0 g
Sucrose ............................................................................... 50.0 g (or 10g for 1% conc.)
Distilled water ad ................................................................1000 ml
Dissolve ingredients in water by heating and mixing frequently.
Adjust to pH 7.3. Autoclave 15 min at 121 ºC.

Reagent
Lugol's iodine solution (Gram's iodine solution diluted 1:4)

PROCEDURE
Harvest a well isolated colony and inoculate a Tryptic soy agar + 5% sucrose plate. Incubate at 37 ºC, 18-24 hours (or other required
conditions).
After incubation add one drop of Gram's iodine to growth on plate. The colonies that produce polysaccharide will quickly change color.

READING
Reaction is positive if medium around colonies changes color to dark blue-purple or black.
Reaction is negative if medium remains unchanged. A light brown color should be considered negative (this is the color of the iodine
reagent).

QUALITY CONTROLS
Neisseria polysaccharea, ATCC 43768 - positive control;
Neisseria cinerea, CDC 10,256 - negative control

NOTES
False-negative reactions may be obtained if inoculated medium is incubated longer than 48 hours.
A 5% sucrose concentration may be inhibitory for some strains. In this case it is recommended to lower the concentration to 1%.

REFERENCES:
1. Centers for Disease Control and Prevention: Identification of
N. gonorrhoeae and Related Species; at: http://www.cdc.
gov/std/gonorrhea/lab/ident.htm, accessed November 2016.
2. Tone Tonjum, 2005. Order IV. Neisseriales ord. nov. In:  Bergey’s Manual of Systematic Bacteriology, Second edition,Vol two, part C
The Alpha-, Beta-, Delta-, and Epsilonproteobacteria, George M. Garrity (Editor-in-Chief), pp 774-863.
(c) Costin Stoica
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