Which test is commonly used to identify gram-negative bacteria?

The oxidase test is commonly used to identify gram-negative bacteria, particularly members of the Enterobacteriaceae family and other non-fermenters. This test detects the presence of cytochrome c oxidase, an enzyme involved in the electron transport chain. A positive reaction is indicated by a color change, typically to blue or purple, upon the addition of a reagent like tetramethyl-p-phenylenediamine.

While the coagulase test is used primarily to differentiate Staphylococcus aureus (a gram-positive cocci) by its ability to coagulate plasma, it is not relevant for identifying gram-negative bacteria. Similarly, the catalase test detects the presence of the enzyme catalase in bacteria, differentiating between catalase-positive and catalase-negative organisms, but it also does not specifically identify gram-negative bacteria.

Thus, recognizing the oxidase test's role in distinguishing certain gram-negative bacteria highlights its importance in clinical microbiology, underlining that it is a more targeted choice than the coagulase or catalase tests for this specific application.