Final answer:
Xylene extraction and Ehrlich's reagent are necessary when testing for indole, a breakdown product of tryptophan by bacteria. This method, specific to microbiology, identifies whether bacteria can produce indole, a critical step in differentiating microbial species.
Step-by-step explanation:
The question pertains to the detection of indole production in microbial identification tests. Using xylene extraction and Ehrlich's reagent is necessary when testing for indole, a compound produced from the degradation of the amino acid tryptophan by certain bacteria. This method is preferred over Kovac's reagent for certain applications. The conversion of lipids to volatile compounds for analysis by gas chromatography is not relevant to indole testing but is a method used to analyze lipid composition of microbes.
Ehrlich's reagent reacts with the indole to produce a pinkish-red compound, which can be observed visually. This is a common confirmatory test in microbiology laboratories for bacteria that are capable of breaking down tryptophan to indole, such as Escherichia coli. In contrast, Kovac's reagent is also used to test for indole production but uses a different extraction and detection protocol.