SIM medium is used to differentiate members of the family Enterobacteriaceae. A
ID: 66887 • Letter: S
Question
SIM medium is used to differentiate members of the family Enterobacteriaceae. A haziness that spreads from the stab line will indicate a positive test for motility. The tubes must be compared to an uninoculated tube to differentiate between faint haziness and motility. A red color deveopment after addition of Kovacs reagent indicates indole production. A black precipitate indicates H2S production. Thus, we would expect in the result to have no haziness, red colouration as well as formation of black ppt.
How would you expect this result to change if you eliminated the sodium thiosulfate? Ferrous ammonium sulfate?
Explanation / Answer
Some bacteria utilize thiosulfate anion as a terminal electron acceptor, reducing it to sulfide. If this occurs, the newly-formed hydrogen sulfide (H2S) reacts with ferrous sulfate in the medium to form ferrous sulfide, which is visible as a black precipitate. The blackening of the medium is almost always observed in the butt (bottom) of the medium.
Result (slant/butt) Symbol Interpretation 1 Red/Yellow K/A Glucose fermentation only, peptone catabolized. 2 Yellow/Yellow A/A Glucose and lactose and/or sucrose fermentation. 3 Red/Red K/K No fermentation, Peptone catabolized. 4 Yellow/Yellow with bubbles A/A,G Glucose and lactose and/or sucrose fermentation, Gas produced. 5 Red/Yellow with bubbles K/A,G Glucose fermentation only, Gas produced. 6 Red/Yellow with bubbles and black precipitate K/A,G,H2S Glucose fermentation only, Gas produced, H2S produced. 7 Yellow/Yellow with bubbles and black precipitate A/A,G,H2S Glucose and lactose and/or sucrose fermentation, Gas produced, H2S produced. 8 Red/Yellow with black precipitate K/A,H2S Glucose fermentation only, H2S produced. 9 Yellow/Yellow with black precipitate A/A,H2S Glucose and lactose and/or sucrose fermentation, H2S produced.Related Questions
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