8. Sulfur-Indole-Motility medium uses the same mechanism to detect sulfur reduct
ID: 302594 • Letter: 8
Question
8. Sulfur-Indole-Motility medium uses the same mechanism to detect sulfur reduction as some of our Enteric plates. Which enteric plates use the same mechanism as SIM to detect the presence of hydrogen sulfide? 9. Bile esculin agar (BEA) contains oxgall, which is the bile from an ox. Bile is also found in Hektoen agar, MacConkey agar, and Shigella/Salmonella agar. A. What is the purpose of oxgall in BEA? to inhibit the growth p gram postive organisms other than entervcocci and group D streptocacei B. Staphylococci typically produce BEA negative results. Explain why this is the case.Explanation / Answer
8. Hektoen enteric (HE) agar uses the same mechanism as SIM to detect the presence of hydrogen sulfide. HE agar is a differential and selective medium for isolation of enteric pathogen from clinical sample. It uses thiosulfate and ferric ammonium citrate to detect H2S production by bacteria. SIM uses ferric ammonium sulfate and thiosulfate for the same. A black precipitate of ferrous sulfide is formed due to reaction of ferric ammonium citrate or ferric ammonium sulfate with H2S gas produced by bacteria. Thiosulfate in both medium acts as a source of sulfur.
a) Bile esculin agar is used for isolation of enterococci and group D streptococci which are gram postitive organisms. Bile used in Hektoen agar inhibits the growth of gram positive organisms. Oxgall is added in BEA to inhibit growth of gram positive organism other than enterococci and group D steptococci. It also inhibits growth of non group D streptococci.
b) BEA contains bile which inhibits growth of gram positive organisms but staphylococcus being a gram positive bacteria is able to grow in presence of bile salts. However, Esculin in BEA can only be hydrolyzed by enterococci and group D streptococci not by staphylococcus. Esculin is hydrolysed to dextrose and esculetin. Esculetin reacts with ferric ions in the medium to produce insoluble salt of iron causing blackening of the medium.
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