Question

suppose that when you examined your tubes after incubating them, you noticed that the unsealed control contained slight yellowing at the top. suppose further that pair#1 showed complete yellowing of broth tubes and pairs #2 and #3 showed light yellowing of the unsealed tube. assuming all other tubes were green, what conclusionscould you safely make? which results, if any, are reliable? why?
which results, if any, are not reliable? why not?

Answer 1

The Oxidation-Fermentation Test is used to differentiate bacteria based on their ability to oxidize or ferment specific sugars.

Once microbes are inoculated,
-One tube is sealed with a layer of sterile mineral oil to promote anaerobic growth and fermentation.
-The other tube is left unsealed to allow aerobic growth and oxidation.

Organisms able to ferment the carbohydrate or ferment and oxidize the carbohydrate will turn the sealed and unsealed yellow throughout.

Organisms able only to oxidize the sugar will turn the unsealed yellow medium and leave the sealed medium green or blue.

Slow or weak fermenters will turn both tubes slightly yellow at the top.

Organisms not able to metabolize the sugar will either produce no color change or will turn the medium blue due to alkaline products from amino acids degradation.

Since Pair #1 showed complete yellowing for sealed and unsealed, these Organisms able to ferment the carbohydrate or ferment and oxidize the carbohydrate. So our interpretation will be that the organism has: Oxidation and fermentation OR fermentation only.

For tubes #2 and #3, the sealed tubes were green throughout suggests that they need oxygen for aerobic growth, and the fact that their unsealed tubes showed light yellowing is evidence for oxidation. Sealed - Green and Unseal - Yellow. Our interpretation for these pairs of tubes would be : Oxidation

Tube 1 can be either Oxidation and fermentation OR fermentation only. So reliability of this needs to be confirmed more with additional testing.

Tubes 2 and 3 are most reliable because they can only be oxidation only and no fermentation.