Microbiology: Citrate Agar — Transcript

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1. 0:00my name is Kevin toh Goff and in this
2. 0:01video we're gonna discuss the theory
3. 0:03behind citrate agar test all right so
4. 0:06when we use citrate agar this is
5. 0:08basically Agra that contains citrate
6. 0:10okay it turns out that some bacteria are
7. 0:14actually able to use citrate as a carbon
8. 0:16source this is kind of a general scheme
9. 0:19showing what they might be able to do
10. 0:20citrate can be converted to acetate
11. 0:23which can be used as a carbon source it
12. 0:25can be converted to acetyl co a also
13. 0:27oxaloacetate which is a krebs cycle
14. 0:29intermediate can be converted to pyruvic
15. 0:32acid and carbon dioxide this carbon
16. 0:34dioxide can ultimately react and be
17. 0:37transformed into carbonate which will
18. 0:39form a compound called sodium carbonate
19. 0:42it turns out that the sodium carbonate
20. 0:44will actually turn the agar blue what
21. 0:48the citrate agar test is actually
22. 0:50measuring other than a bacteria's
23. 0:52ability to use citrate as a carbon
24. 0:53source is it's actually measuring the pH
25. 0:57we have an a pH indicator that's a
26. 1:00little bit different it's called
27. 1:01bromothymol blue so bromothymol blue
28. 1:04when it turns alkaline turns blue it's
29. 1:07actually normally green at neutral pH so
30. 1:09actually we'd have a citrate agar that
31. 1:11would actually start out more like the
32. 1:13color on the left side here it would
33. 1:15start out green and if the bacteria
34. 1:18that's inoculated onto here is able to
35. 1:20use citrate as a carbon source we will
36. 1:23form sodium carbonate and it will
37. 1:25actually turn the agar blue because of
38. 1:29the the pH indicator bromothymol blue
39. 1:32okay so we see blue here on the citrate
40. 1:37agar that's indicated of bacteria that
41. 1:39are able to use citrate as a carbon
42. 1:41source bacteria that cannot use citrate
43. 1:45as a carbon source will not grow on this
44. 1:47so we have growth we should see that
45. 1:51blue color transformation okay here's
46. 1:54some more results for this in some cases
47. 1:56you can do this on a plate and in some
48. 1:58cases you can do it on a tube okay but
49. 2:01blue color changes are positive for
50. 2:04citrate meaning the bacteria can use
51. 2:06citrate as a carbon source so in this
52. 2:08picture on the Left we would say this
53. 2:10result right here on my mouse is on the
54. 2:12left side
55. 2:13is a positive citrate result same thing
56. 2:16over here on the right side of the right
57. 2:18figure this is a positive result because
58. 2:20of the blue okay in some cases the blue
59. 2:23may not be distributed all the way to
60. 2:25the bottom of the tube but you should
61. 2:27see the blue if it's a negative result
62. 2:29you will see absolutely no blue this is
63. 2:31all green right here as you can see same
64. 2:33thing as this negative result for
65. 2:35citrate right here in the figure on the
66. 2:37left okay so pretty much the only thing
67. 2:40you have to do here is distinguish
68. 2:41between blue and green a blue result
69. 2:44means citrate can be used as a carbon
70. 2:47source green result means it cannot and
71. 2:50it won't even grow in the first place
72. 2:51okay so make sure to watch the
73. 2:55demonstration video on citrate agar test
74. 2:57and in the next video we're actually
75. 2:59going to discuss the oxidase test