MRCP UK EXAMINATIONS

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ABDOMINAL

GASTROINTESTINAL

Question [CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
In the laboratory examination of the food sample, the results were obtained:Gram negative staining of rods, culture on Mac Conkey media obtained pink colonies, biochemical test of lactose fermentation (-) glucose (+), Citrate (+), SIM motility (+) sulfur ( +), TSIA H2S (+). The most appropriate conclusion from the identification results is ____
A
Salmonella typhi
B
Escherichia coli
C
Shigella dysentery
D
Staphylococcus aureus
Explanation: 

Detailed explanation-1: -Lactose fermenting species will grow pink colonies. Lactose fermentation will produce acidic byproducts that lower the pH, and this turns the pH indicator to pink. Example of Lac positive species: Escherichia coli, Enterobacteria, Klebsiella.

Detailed explanation-2: -Glucose broth with Durham tubes This is a test commonly used when trying to identify Gram-negative enteric bacteria, all of which are glucose fermenters but only some of which produce gas. Like MSA, this medium also contains the pH indicator, phenol red.

Detailed explanation-3: -This medium is both selective and differential. The selective ingredients are the bile salts and the dye, crystal violet which inhibit the growth of Gram-positive bacteria. The differential ingredient is lactose.

Detailed explanation-4: -Streak plate of Escherichia coli and Serratia marcescens on MacConkey agar. Both microorganisms grow on this selective media because they are gram-negative non-fastidious rods. Growth of E. coli, which ferments lactose, appears red/pink on the agar.

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