Citrate Test Lab Report

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Citrate Test: Some bacteria can use citrate as a source of carbon. To test if the unknown bacteria uses citrate as a source of carbon, Simmon’s citrate agar was used as the medium on which the bacteria was grown. The Simmon’s citrate agar consists of sodium citrate as the source of carbon, ammonium dihydrogen phosphate as the source of nitrogen along with pH indicator such as bromothymol blue. Procedure: The Citratase activity was detected by inoculating the unknown bacteria on the slant surface of Simmon’s citrate agar. Followed by overnight incubation at 37°C. Day after the slant was observed for a change in color. Results: Two results can be acquired in the citrate test, positive and negative. The positive result indicate growth with the…show more content…
We then incubated our tube of broth at 37 oC overnight. The following day we came back to interpret what we observed. Results: A positive test, a deep-pink or purplish-red color, would indicate that the bacteria produced urease meaning ammonia was formed; ammonia makes the broth more alkaline which causes the pH indicator of phenol red to change color. However, a negative test would result in the color of the broth to be an orange-red color. The following day we observed our tube for a color change but noted an orange-red color, a negative test. Based off of the result of this test we were able to determine that we had Escherichia coli instead of Proteus vulgaris because we interpreted the results to be negative. The negative results indicate that Escherichia coli is present due to the fact that urease was not produced, whereas Proteus vulgaris would have a positive test result meaning that ammonia was formed due to the urease that was produced. To confirm that we indeed did have Escherichia coli we decided that our next test that we would perform was the thioglycollate; this again would differentiate between Escherichia coli and Proteus…show more content…
Before leaving lab for the day, we noted the depth of the colored area by marking on the tube with a marker. Results: The next day when we checked for the results of our test, we observed that our bacteria are facultative anaerobes because there was growth throughout the medium but the most growth was seen nearest the surface. We also noted that our bacteria grew out from the initial insertion site which indicated that we had a positive result for motility. Based off of this test result we were able to once again confirm that we did have Escherichia coli and not Proteus vulgaris due to the positive motility result. The next confirmatory test we decided to perform was the MacConkey agar because both Escherichia coli and Proteus vulgaris have positive results; however, to distinguish between the two different bacteria, it can be observed that Escherichia coli has a pink hue around the bacteria streak. Proteus vulgaris, on the contrary, does not have a pink hue around the bacteria streaked portion of the plate. MacConkey agar
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