MacConkey Agar — Transcript

Full transcript

1. 0:04[Music]
2. 0:08welcome back to microbiology lab my name
3. 0:11is Kevin towpath please make sure to
4. 0:12like this video and subscribe to my
5. 0:14channel in this video we're going to
6. 0:16discuss a little bit of the theory
7. 0:17behind in the konkey agar and then also
8. 0:20how to interpret the results on the next
9. 0:22slide alright so first of all what is
10. 0:24macconkey agar so this is first of all
11. 0:27both a selective and differential medium
12. 0:30so it's selective in the sense that it
13. 0:32selects for the growth of gram negative
14. 0:35organisms so if you have growth such as
15. 0:38on either side of this macconkey agar
16. 0:40plate right here anytime you have growth
17. 0:42on macconkey agar that's gonna indicate
18. 0:44that you have a gram negative organisms
19. 0:46so these are gram negative organisms
20. 0:48macconkey agar inhibits the growth of
21. 0:51grande positive organisms they don't
22. 0:52grow on this and the reason behind this
23. 0:55you'll see this in your lab manual there
24. 0:57are substances embedded in the agar
25. 0:59itself called bile salts along with
26. 1:02crystal violet and both of these
27. 1:04substances collectively act to inhibit
28. 1:07the growth of gram positive organisms so
29. 1:09if you plated a grand positive organism
30. 1:12on here you wouldn't see any of these
31. 1:13colonies growing there'd be nothing okay
32. 1:16you might see the little you know a
33. 1:18little bit of tear from your loop and
34. 1:20when you scrape it over the agar itself
35. 1:22but you won't see any growth if it's
36. 1:23gram positive and that's how this agar
37. 1:26is selective but one of the major things
38. 1:28that we also care about is that it's
39. 1:30also differential now the differential
40. 1:32nature of macconkey agar has to do with
41. 1:35also embedded in here is the
42. 1:37disaccharide a carbohydrate called
43. 1:39lactose now not really getting into all
44. 1:41of this it suffice us to say that some
45. 1:44bacteria are going to be able to
46. 1:47metabolize lactose and ultimately
47. 1:50ferment that lactose into acid end
48. 1:54products the major one of which would be
49. 1:56lactic acid but the whole point is some
50. 1:58of these bacteria will be able to
51. 2:00ferment lactose into acid end products
52. 2:02such as lactic acid and so the
53. 2:05differential nature of this is going to
54. 2:07be that some bacteria will be able to
55. 2:09ferment lactose those would be
56. 2:10considered lactose positive and then
57. 2:13some bacteria
58. 2:13will not be able to ferment lactose
59. 2:15those would be lactose negative alright
60. 2:18so let's now get into a detailed
61. 2:20discussion about how we actually
62. 2:21interpret the results alright so
63. 2:24basically the first thing we're going to
64. 2:26need to determine is whether or not
65. 2:27there's growth now this is probably the
66. 2:29easiest thing if you have no growth
67. 2:32meaning the plate is actually growth
68. 2:35negative that means that your organism
69. 2:38is automatically gram positive okay and
70. 2:42so you would not see any of these
71. 2:43colonies growing on the plate you may
72. 2:46see the a little bit of a you know a
73. 2:48line or a tear if you played it a little
74. 2:51too hard where your loop went over the
75. 2:53agar but no colony should be growing on
76. 2:55that and if you have no growth your
77. 2:57organism is gram positive now in the
78. 3:00case where your organism is gram
79. 3:02positive you have no growth in the first
80. 3:04place so you would not be able to tell
81. 3:06what the lactose reaction is okay
82. 3:09whether or not it's lactose positive or
83. 3:10negative so there's two things you
84. 3:12always report you report the gram
85. 3:14reaction if it's gram positive okay and
86. 3:17then if it's gram positive for lactose
87. 3:20you would say lactose unknown or lactose
88. 3:22inconclusive or something that indicates
89. 3:25that you cannot tell the lactose
90. 3:27reaction because there's no growth in
91. 3:29the first place okay now the interesting
92. 3:32part is when you do have growth and on
93. 3:35this plate right here will actually look
94. 3:36at something very similar on the next
95. 3:38slide on the left side of the plate we
96. 3:40actually have one kind of organism this
97. 3:42will actually turn out to be lactose
98. 3:43positive on the left on the right here
99. 3:46this will actually turn out to be
100. 3:47lactose negative and depending on
101. 3:49whether it's lactose positive or
102. 3:50negative it actually changes the color
103. 3:52so in general on both sides of the plate
104. 3:55here we have growth meaning these
105. 3:57bacteria would first of all be growth
106. 3:58positive but that indicates that they're
107. 4:00gram-negative and remember that's
108. 4:03because macconkey agar selects only for
109. 4:05organisms that are gram-negative and it
110. 4:08inhibits the growth of grande positive
111. 4:09and so now that you've determined your
112. 4:11bacteria is gram-negative because
113. 4:13there's growth you now have to determine
114. 4:15the lactose reaction whether it's
115. 4:16lactose positive or lactose negative so
116. 4:19look at right here so I kind of like
117. 4:21this this picture a little better over
118. 4:23here on the right so this is the same
119. 4:25picture from the previous slide
120. 4:27on the Left we have colonies that are at
121. 4:30least around where the bacteria are it's
122. 4:32turned pink and over here on the right
123. 4:35it's mainly a tannish yellow color I
124. 4:37would call it more tan now if you've
125. 4:40determined this growth it's
126. 4:41gram-negative so then you look at the
127. 4:44color that's the second thing you look
128. 4:46at so if the area around the smear is
129. 4:49pink now the whole plate does not have
130. 4:51to be pink we look at the bacteria over
131. 4:53here we have two pink ones right here
132. 4:56this one turned pink only very close to
133. 4:59the smear this one it kind of spread out
134. 5:01a little bit from the smear but if
135. 5:02there's any pink around the smear itself
136. 5:05pink is lactose positive okay so clearly
137. 5:09over here on the left side where my
138. 5:11mouse is okay these bacteria there was
139. 5:14growth so it's gram-negative they're
140. 5:16opposites growth positive gram negative
141. 5:18and then it's pink so it's lactose
142. 5:20positive so if you wanted to fully
143. 5:23report the results here on the Left you
144. 5:26would have to say gram negative lactose
145. 5:28positive and that's just because there's
146. 5:29clearly growth and it's pink around the
147. 5:31smear all right but now let's look over
148. 5:34here on the right side here it's more of
149. 5:37a tan color we can see here these are
150. 5:39non lactose fermenting colonies these do
151. 5:42not ferment lactose and that's because
152. 5:43they're tan but there's clearly growth
153. 5:46here so there's still gram-negative okay
154. 5:48so gram negative but because it's tan
155. 5:50around the smears tan around the
156. 5:52colonies we would consider these
157. 5:54bacteria to be lactose negative okay and
158. 5:58remember back from the beginning at
159. 6:01least the first slide here lactose is
160. 6:03being or potentially is being fermented
161. 6:05into lactic acid which is an acid also I
162. 6:09didn't mention this earlier but embedded
163. 6:11in the agar itself we have a pH
164. 6:13indicator that pH indicator actually
165. 6:15happens to bees usually neutral red and
166. 6:18these kinds of pH indicators when the
167. 6:20agar becomes more acidic meaning the pH
168. 6:23becomes lowered then it turns a reddish
169. 6:25pink color okay
170. 6:27that's what neutral red does so the
171. 6:29reason why if you have a lactose
172. 6:31positive organism it turns pinkish red
173. 6:33is because for lactose positive
174. 6:35organisms they're able to ferment
175. 6:37lactose meaning they're able to convert
176. 6:39lactose ultimately through
177. 6:41like Hollis into lactic acid which is an
178. 6:44acid therefore it lowers the pH it turns
179. 6:47the ph indicator red in the case of over
180. 6:50here on the right these do not permit
181. 6:52lactose so the agar does not turn red it
182. 6:55instead is at least relatively speaking
183. 6:57more alkaline and so it maintains this
184. 7:00tan color okay so in the end let's kind
185. 7:04of regroup here and talk about what we
186. 7:06have to do with macconkey agar the first
187. 7:08thing we do is determine if there's
188. 7:09growth or not which indicates the gram
189. 7:11reaction okay if there's no growth no
190. 7:14colonies its gram positive okay so
191. 7:18growth negative gram positive and in the
192. 7:21case of gram positive it's gonna be
193. 7:23lactose inconclusive okay or lactose
194. 7:26unknown we can't tell the lactose
195. 7:27reaction in that case but if there is
196. 7:30growth growth positive then those
197. 7:32organisms are gram negative and then we
198. 7:34have to do the second thing if it's gram
199. 7:36negative where we look at the color of
200. 7:38the agar to determine the lactose
201. 7:41reaction positive or negative so if it's
202. 7:43gram negative and only if it's gram
203. 7:44negative we say is it pink around the
204. 7:47colonies or tan if it's pink it's gonna
205. 7:50be lactose positive so gram negative
206. 7:52lactose positive but if it's tan around
207. 7:55the colonies then it's gonna still be
208. 7:56gram negative but lactose negative okay
209. 7:59and that's pretty much all there is to
210. 8:01macconkey agar test it's really kind of
211. 8:03a two-step process interpreting is their
212. 8:05growth if no it's gram positive if yes
213. 8:09gram negative and then you look at the
214. 8:11color to determine the lactose reaction
215. 8:13alright so hopefully this video made
216. 8:16sense to you please comment please like
217. 8:19and subscribe to my channel thank you
218. 8:20very much in the next video we're gonna
219. 8:22discuss the carbohydrate fermentation
220. 8:24tests in the Durham tube
221. 8:25thank you