Eiken test is which type of reaction used for E. coli?
**Core Concept**
The Eiken test is a method used to identify and differentiate certain bacteria, including *Escherichia coli* (*E. coli*), based on their biochemical properties. This test is specifically designed to detect the presence of *E. coli* by exploiting its unique enzymatic capabilities. The underlying principle involves the bacterium's ability to ferment certain carbohydrates.
**Why the Correct Answer is Right**
The correct answer involves the type of reaction that *E. coli* exhibits in the Eiken test, which typically involves an **oxidation** or **fermentation** reaction. *E. coli* can ferment lactose, among other carbohydrates, producing acid and causing a color change in the medium, which is indicative of a positive result. This is due to the bacterium's enzymatic machinery, including **lactase** (beta-galactosidase), which breaks down lactose into glucose and galactose.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the biochemical reaction type that *E. coli* exhibits in the Eiken test.
**Option B:** Similarly, this option does not correctly identify the reaction type relevant to the Eiken test for *E. coli*.
**Option C:** This option is also incorrect as it misrepresents the biochemical basis of the Eiken test.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that the Eiken test, among other biochemical tests, helps in the identification and differentiation of *E. coli* from other enteric bacteria based on its metabolic capabilities, particularly its ability to ferment certain carbohydrates.
**Correct Answer:** D. Oxidation-Fermentation test.