Question

how would the results be confounded if both Glucose and Citrate were included as fermentable carbohydrates in the Simmons Citrate test? (your answer should address how your ability to interpret the test results would be impacted)

Answer 1

In the Simmons citrate agar test, the ability of organisms to utilize citrate as a carbon source is testes. In common the Simmons citrate agar contains sodium citrate as the sole source of carbon and ammonium dihydrogen phosphate as the sole source of nitrogen, other nutrients. The pH indicator used is bromthymol blue. This test used as a part of the IMViC tests and to differentiate the Enterobacteriaceae.

Adding glucose for fermentation along with citrate in Simmons test keeps the pH lower. The colour will turn from green to yellow.

In the regular test, bacteria that survive in the medium and utilize the citrate also convert the ammonium phosphate to ammonia and ammonium hydroxide. Both reactions increase the pH turning bromothymol blue from green to blue. By adding the glucose the pH will be lower and the bromothymol blue can be converted to green or even yellow.

In general or Simmons citrate results: positive – the colour turns blue or even a small amount of blue while citrate is utilized, the pH is around 7.6.

Negative Simmons citrate results: there will be no colour change, no growth, citrate is not utilized.