Learn how catalase, oxidase, urease, and 50+ other biochemical tests work — with expected results, clinical significance, and exam mnemonics.
The organism grew overnight on blood agar. It is Gram-positive and catalase-positive. Now what? The next step is a panel of biochemical tests — each one asking a specific question about the organism's metabolism and together they narrow a field of thousands of possible bacteria down to a single species.
Biochemical tests are the chemical reactions used to identify bacteria based on their enzymatic activity and metabolic products. They are the bridge between "something grew" and "we know what it is."
This section covers every major test in clinical and teaching laboratory use:
Each article follows the same logic a clinical microbiologist uses at the bench: What does this test detect? Why does this organism give this result? How do you remember which organisms are positive?
If you are working through a biochemical identification flowchart for the first time, start with the catalase test and follow the logic forward.
LAP (leucine aminopeptidase) test: principle, procedure, and expected results, and how it works alongside PYR to identify catalase-negative Gram-positive cocci.
Mixed acid fermentation splits glucose into a mixture of strong acids, lactate, acetate, formate, succinate, plus ethanol, in variable proportions. That flood of acid drops the pH...
Bial’s Test: Principle, Procedure, and Application
Biuret Test: Principle, Procedure, and Uses
Lipids are too large for a bacterium to import, so lipase-producing organisms digest them outside the cell, clearing the opaque tributyrin agar into a transparent halo. That cleari...
SIM medium principle, procedure, and how to read sulfide, indole, and motility correct including why it catches weak H2S producers that TSI and KIA miss.
Casein Hydrolysis Test: Principle, Procedure, and Uses
Litmus Milk Test: Principle, Procedure, and Results
Malonate Test: Principle, Procedure, and Results
A vancomycin-resistant Gram-positive rod in a blood culture sounds alarming, until catalase and PYR results point toward a harmless lactic acid bacterium instead. The gas test that...
The acetamide utilization test detects acylamidase, the enzyme that deaminates acetamide to release ammonia. How to read the green-to-blue result and why it helps identify Pseudomo...
API and RAPID ID For Microbial Identification
Benedict’s Test: Principle, Procedure, Uses, and Limitation
H₂S turns a medium black when the gas meets an iron or lead salt. But the same organism can read H₂S-positive on lead acetate paper and negative on TSI, because the methods differ...
Amylase-positive organisms clear a halo in starch agar that only appears when you flood the plate with iodine. This is a test you read by where the color fails to develop. It also...
