Oxidase test helps in identification of each bacterium except one:
**Core Concept**
Oxidase test is a biochemical test used to identify bacteria that produce cytochrome c oxidase, an enzyme involved in the electron transport chain of bacterial respiration. This test is particularly useful in distinguishing between different species of gram-negative bacteria, which often have similar morphological and biochemical characteristics.
**Why the Correct Answer is Right**
The oxidase test is based on the principle that bacteria that produce cytochrome c oxidase will turn a chromogenic substrate, such as 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), blue in color. This is because the enzyme catalyzes the oxidation of the substrate, resulting in a color change. The oxidase test is a rapid and simple method for identifying bacteria such as *Pseudomonas* and *Alcaligenes*, which are both known to produce cytochrome c oxidase.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, but it would be incorrect if it was a bacterium that does not produce cytochrome c oxidase.
**Option B:** This option is not provided, but it would be incorrect if it was a bacterium that does produce cytochrome c oxidase, such as *Pseudomonas* or *Alcaligenes*.
**Option C:** This option is not provided, but it would be incorrect if it was a bacterium that does produce cytochrome c oxidase, such as *Pseudomonas* or *Alcaligenes*.
**Option D:** This option is not provided, but it would be incorrect if it was a bacterium that does produce cytochrome c oxidase, such as *Pseudomonas* or *Alcaligenes*.
**Clinical Pearl / High-Yield Fact**
The oxidase test is a useful tool for identifying bacteria that are resistant to cephalosporins and penicillins, as these antibiotics are often ineffective against bacteria that produce cytochrome c oxidase.
**Correct Answer:** D.