Mannitol Salt Agar (MSA) | Results & Theory — Transcript

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1. 0:04[Music]
2. 0:08welcome back to microbiology lab my name
3. 0:11is Kevin toh goth please make sure to
4. 0:13like this video and subscribe to my
5. 0:15channel for future videos and
6. 0:16notifications in this video we're going
7. 0:19to discuss a test called mannitol salt
8. 0:21agar or MSA so what is this agar
9. 0:24well first of all it has two things that
10. 0:26are very important embedded in the agar
11. 0:28itself one of those is the sugar alcohol
12. 0:31called mannitol and one of the things
13. 0:34that MSA will tell us is whether or not
14. 0:36bacteria are able to ferment mannitol
15. 0:39the second thing that it has is a very
16. 0:42high concentration of salt in fact the
17. 0:44salt concentration in mannitol salt agar
18. 0:47is 7.5 percent now in this same week
19. 0:51that we do MSA we also do six and a half
20. 0:54percent salt broad now I mentioned in
21. 0:57this video that six and a half percent
22. 0:58salt broth or just the 6.5 percent salt
23. 1:02is still a very high salt concentration
24. 1:04and enough salt to inhibit the growth of
25. 1:06most bacteria well for a mannitol salt
26. 1:09agar it's one percent higher than that
27. 1:11it's actually going to be 7.5 percent
28. 1:14salt and this high salt concentration is
29. 1:17going to be able to inhibit the growth
30. 1:18of many species of bacteria more so than
31. 1:22would the six and a half percent salt
32. 1:24broth that we discussed in a separate
33. 1:25video ok so those are the two things
34. 1:28sugar and salt now as we mentioned the
35. 1:30salt inhibits growth so the first thing
36. 1:33that we always check for in a mannitol
37. 1:35salt plate is for growth and we see
38. 1:38three examples right here three examples
39. 1:40but what we see in the left - so
40. 1:42Staphylococcus aureus and Staphylococcus
41. 1:45epidermidis is that these organisms are
42. 1:47growth positive and all we do when we're
43. 1:50classifying whether or not there's
44. 1:51growth which is we just look for
45. 1:52colonies on the plate obviously okay and
46. 1:55in both of these there's gonna be
47. 1:57colonies on the plate here's another
48. 1:58view up here you can actually see in
49. 2:00this case they did a zigzag streak on
50. 2:02each half of the plate so there's
51. 2:04organisms on the right side here
52. 2:06organisms on the left and these are both
53. 2:08going to be growth positive in contrast
54. 2:11over here we have micro
55. 2:13coccus luteus this does not grow on
56. 2:16mannitol salt because Micrococcus luteus
57. 2:20cannot survive high salt concentrations
58. 2:22in fact other organisms like
59. 2:24streptococcus will also not be able to
60. 2:26survive at such a high salt
61. 2:27concentration and it's also worth
62. 2:30mentioning that Staphylococcus organisms
63. 2:32are very good at growing on this medium
64. 2:35so one always check for growth first if
65. 2:38there's growth then we can ask whether
66. 2:41or not the bacteria ferment mannitol and
67. 2:43here's the thing we do colonies that
68. 2:46confer mint mannitol will appear yellow
69. 2:48and so we would classify those as
70. 2:50mannitol positive so in this one over
71. 2:53here on the left down here which is
72. 2:54Staphylococcus aureus we obviously see
73. 2:57that there's growth this plate was done
74. 2:59using the streaking for isolation method
75. 3:01but the agar turned yellow and that has
76. 3:04to do with the fact that mannitol is
77. 3:06fermented to acid end products the pH
78. 3:09indicators phenol red and so when acid
79. 3:12end products are produced from mannitol
80. 3:14it lowers the pH of the medium and turns
81. 3:16phenol red yellow and so if we have a
82. 3:18yellow plate around the colonies then we
83. 3:21have a mannitol positive organism which
84. 3:23just means that they ferment mannitol
85. 3:25and so for our MSA our full answer here
86. 3:29if we were classifying the results would
87. 3:30be growth positive mannitol positive all
88. 3:34right but what happens if we have growth
89. 3:38and it turns this sort of rosy pink
90. 3:40color well it's still growth positive
91. 3:42but in this case it's a mannitol
92. 3:44negative reaction for Staphylococcus
93. 3:46epidermidis so colonies that cannot
94. 3:49ferment mannitol will appear pink sort
95. 3:52of a rosy pink like you see in the
96. 3:53middle here and again we would classify
97. 3:55those as growth positive mannitol
98. 3:58negative now one other thing I want to
99. 4:01mention and this is very similar to the
100. 4:03kaki agar if you have negative growth so
101. 4:06there's no growth on the plate you
102. 4:08cannot say that the mannitol reaction is
103. 4:10negative okay if there's no growth in
104. 4:13the first place your mannitol reaction
105. 4:15usually put one of three things either
106. 4:17n/a for not applicable unknown or
107. 4:20inconclusive and the reason is because
108. 4:22this organism Micrococcus luteus either
109. 4:25does permit man
110. 4:27toric does not but we cannot tell based
111. 4:30on this test because there's no growth
112. 4:32in the first place if it managed to grow
113. 4:35we would be able to tell but it can't
114. 4:37survive the salt so we can't actually
115. 4:38determine the mannitol reaction you
116. 4:41would have to do another test to
117. 4:42determine that and most likely the best
118. 4:44one would be a carbohydrate fermentation
119. 4:46with a durum - that would be the best
120. 4:48test to determine whether or not an
121. 4:50organism that's growth negative here
122. 4:51could actually ferment mannitol but just
123. 4:54from this test we have to say mannitol
124. 4:56unknown or inconclusive or n/a all right
125. 4:59and let's conclude this just by looking
126. 5:02at a summary flow chart here that can
127. 5:04actually help you determine the results
128. 5:06of your mannitol salt agar the first
129. 5:08question you always ask is is there
130. 5:10growth going here if there's no growth
131. 5:13like we see on this plate then its
132. 5:15growth negative and for the mannitol
133. 5:17reaction we're forced to say
134. 5:19inconclusive unknown or not applicable
135. 5:21but if there is growth so we have a
136. 5:24growth positive organism we then have to
137. 5:26ask if there's mannitol fermentation if
138. 5:29the area around the colonies is pinkish
139. 5:32red or that rosy pink color that our
140. 5:35answer is no we have a mannitol negative
141. 5:37organism so over here if we saw this on
142. 5:40mannitol salt agar this would be growth
143. 5:42positive to the colonies that are
144. 5:44zigzagging but mannitol negative due to
145. 5:46the pink but if there's growth and the
146. 5:49color is yellow around the colonies then
147. 5:52our answer is yes we do for a mint
148. 5:54minutes also we'd be mannitol positive
149. 5:56so these organisms over here on the
150. 5:58right half of this plate would be growth
151. 6:00positive mannitol positive right so this
152. 6:04is our summary of the mannitol salt agar
153. 6:06hopefully this test makes sense to you
154. 6:09please make sure to LIKE this video and
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157. 6:15much