|Fermentation of Carbohydrates and Polyalcohols
Test consists in cultivating a bacteria strain in a medium containing a single fermentable nutrient (a carbohydrate or a polyalcohol)
0.5-1% and a pH indicator for acid production. Nutritive non-fermentable substances are also added to support growth. Various pH
indicators may be used: Phenol red, Bromothymol blue, Bromcresol purple.
Simplest medium for fermentation testing is Peptone water (proteose peptone 2g,
NaCl 0.5 g, H2O ad 100 ml) supplemented with a carbohydrate or polyalcohol 1%.
Next step is to add 1.2 ml of bromthymol blue solution (bromthymol blue 1 g, NaOH
n/10 25 ml, distilled water 475 ml) in 100 ml peptone water. If using phenol red then
add 1 ml phenol red solution 2% in 100 ml peptone water.
A demonstration of phenol red preparation is available in video section.
Inoculate the medium and incubate it at 37 °C, 24 hours.
A yellow color after 24 hours show a positive result. Reaction is negative if medium
color is red-orange (phenol red) or blue-green (bromthymol blue).
Fermentation with gas production may also be tested introducing a Durham tube in
the carbohydrate broth. A demonstration of gas production by glucose fermenting
bacteria is available in video section.
This method can be used for testing fermentation of glucose, lactose, saccharose,
maltose, trehalose, raffinose, xylose, arabinose, rhamnose, sorbose, starch, dextrin,
inulin, glycogen, glycerol, erythritol, adonitol, dulcitol, mannitol, sorbitol, inositol,
salicin, esculin and others.
- Cystine is an essential growth factor for most Neisseriae. In this case, sugars
fermentation should be tested in cysteine trypticase agar containing sugars (glucose,
maltose, sucrose etc.) at a final concentration of 1–2%.
- Because the Neisseriae also produce ammonia from peptones (which may
neutralize acid produced from carbohydrates), acid production must be determined in
a medium with a low protein/carbohydrate ratio and a sensitive indicator such as
- Horse serum contains a maltase which breaks down maltose to glucose. Adding
horse serum in medim when testing maltose fermentation may lead to a false-
positive result if the strain ferments glucose.
|Glucose medium with Phenol red and
Durham tubes for fermentation gas capture
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2. Margaret Barnett, 1992. Microbiology Laboratory Exercises. Wm. C. Brown Publishers.
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Washington, DC, 2003.
4. 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.
5. D. G. Hollis, R. E. Weaver, and P. S. Riley: Emended description of Kingella denitrificans (Snell and Lapage 1976): correction of the
maltose reaction. J. Clin. Microbiol. November 1983 18:5 1174-1176.
(c) Costin Stoica