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Question 1

Describe each of the 3 things a SIM test tests for

Answer 1

1) The media has ferrous iron and thiosulfate added to it; when a culture uses thiosulfate a black precipitate forms from H2S reacting with the iron.
2) If the culture is able to break down the amino acid tryptophan, indole will be produced. Indole reacts with Kovach’s reagent and turn red. If not, it stays yellow.
3) The less-dense agar allows for interpretation of motility; if it’s fuzzy, culture is motile

Question 2

1) Is a SIM test selective, differential, or both? What is it used for?
2) What specific bacteria does it test for? Which is positive and which is negative?

Answer 2

1) Only differential (used for gram negative rod ID)
2) E. coli (+ indole production/ red) and Enterobacter aerogenes

Question 3

1) Is SS media selective, differential, or both? What type of bacteria is it best for?
2) What specific bacteria is this test best for and what color are they?
3) What does this media contain? What part of this media is inhibitory?
4) What is this media’s indicator? What color does it turn and when?

Answer 3

1) Both selective and differential; best for gram negative identification
2) Salmonella and Shigella are transparent with dark centers (H2S) (Shig. son may have red or pink colonies).
3) Contains lactose sugar, bile salts, NaCl, and sodium thiosulfate. Bile salts inhibit G+ and coliforms.
4) Neutral red is the indicator; turns medium red when pH is below 6 (when cultures ferment lactose)

Question 4

1) Is EMB selective, differential, or both? When is it best used?
2) What does the medium contain? What ingredient(s) are inhibitory?
3) What makes EMB differential? What color is EMB media?

Answer 4

1) Both selective and differential; best used for G(-) rod ID
2) Lactose and sucrose sugars; eosin Y and methylene blue inhibit G+
3) Colony appearance. EMB is red.

Question 5

EMB:
1) What species have dark centered colonies with clear edges?
2) Which have a metallic sheen and are dark?
3) Which has larger colonies with pinkish brown centers?
4) Which has colorless colonies?

Answer 5

1) Dark centered with clear edges: E. coli and E. aero (coliforms)
2) Dark with metallic sheen: E. coli and K. pneuomonias
3) Larger with pink/brown centers: E. aero
4) Other non-lactose fermenting bacteria that are G(-)

Question 6

1) Is MAC selective or differential? When is it best used?
2) What does this media contain? What ingredient(s) are inhibitory?
3) What is the indicator in MAC? What color does it turn and when?
4) What species grow well on this media but do not turn red? What species doesn’t grow well but turns bright red?

Answer 6

1) Both selective and differential; best used to ID G(-) rods
2) Sugar lactose, NaCl, neutral red; crystal violet and bile salts inhibit G+ and reduce coliforms
3) Neutral red; turns red as acids are produced by lactose fermentation.
4) Salmonella and Shigella grow well and don’t ferment; E. coli is inhibited but turns media bright red

Question 7

1) Is MSA selective or differential? When is it best used?
2) What does this media contain? What ingredient(s) are inhibitory?
3) What is this media’s indicator? What color does it turn and when? Give an example of a positive and negative bacteria.

Answer 7

1) Both selective and differential; best used for G+ cocci but can provide info on G+ rods
2) Contains mannitol sugar and high NaCl (7.5%); NaCl inhibits G- cultures.
3) Phenol red turns yellow when mannitol fermentation occurs and acids are produced (S. aureus) and stays red when it doesn’t (S. epi)