When conducting this test, some microbiologists inoculate two sets of broth, one

Question

When conducting this test, some microbiologists inoculate two sets of broth, one with carbohydrate and one without (PR Base Broth). Given that an uninoculated control is used to increase the reliability of the test, why would the second set of broths is used?

And assuming you tested organisms using PR Glucose and PR Base broths, which of the combinations of results in the following table would be reliable? Interpret each of the combinations.

PR Glucose

PR Base

Interpretation

Reliable Y/N?

A/G

A/G

Fermentation with acid and gas end products

A/G

-/-

Fermentation with acid and gas end products. No fermentation within PR Base plate.

K

-/-

Degradation of peptone; alkaline end products in PR Gluclose Plate. No fermentation in PR Base.

A/G

A/-

Fermentation with acid and gas end products in Glucose. Fermentation with acid end products; no gas produced in PR Base Plate

PR Glucose

PR Base

Interpretation

Reliable Y/N?

A/G

A/G

Fermentation with acid and gas end products

A/G

-/-

Fermentation with acid and gas end products. No fermentation within PR Base plate.

K

-/-

Degradation of peptone; alkaline end products in PR Gluclose Plate. No fermentation in PR Base.

A/G

A/-

Fermentation with acid and gas end products in Glucose. Fermentation with acid end products; no gas produced in PR Base Plate

Explanation / Answer

The uninoculated control is to rule out the presence of any other substance in the solution which may cause color. The broth tubes without carbohydrate is used to prove that the color change is due to the action of microbes on the carbohydrate added and not on any other ingredient of the medium. ----------------------------------------- The first combination would not be reliable as both the glucose and the base tubes give the same result. The second combination would be reliable as there is no production of acid and gas in the base broth. The third combination is also not reliable as it does not give the standard byproducts of carbohydrate fermentation The fourth combination is also not reliable as acid formation is seen in the base tubes.

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