Carbohydrate Utilization:

Bacteria produce acidic products when they ferment certain carbohydrates. The carbohydrate utilization tests are designed to detect the change in pH which would occur if fermentation of the given carbohydrate occurred. Acids lower the pH of the medium which will cause the pH indicator (phenol red) to turn yellow. If the bacteria do not ferment the carbohydrate then the media remains red. If gas is produced as a by product of fermentation, then the Durham tube will have a bubble in it.

The carbohydrate tests we perform are the:
Glucose (Dextrose) test
Lactose test
Sucrose test

All carbohydrate test media should be inoculated with the transfer loop.



Fermentation results from left to right:
Left tube shows less acid formation than far right tube, but gas is still made
Center shows no carbohydrate utilization to produce acid or gas.
Right tube shows acid was produced as evidenced by the yellow color, and gas was made (look at the bubble in the Durham tube)



Citrate Utilization:

Tests for the ability of bacteria to convert citrate (an intermediate of the Kreb’s cycle) into oxaloacetate (another intermediate of the Kreb’s cycle). In this media, citrate is the only carbon source available to the bacteria. If it can not use citrate then it will not grow. If it can use citrate, then the bacteria will grow and the media will turn a bright blue as a result of an increase in the pH of the media. To inoculate this slant, use the transfer loop.

Left tube is a negat