API for Microorganism Identification | API test for Bacteria | API 20E

The API Biochemical Gallery is a testing system commonly used in microbiology to aid in the identification of bacteria. In this article, we will explore the principle, usage, and interpretation of the API system strip.

◉ Introduction

API test strip (analytical profile index) is a miniaturized and standardized gallery of biochemical tests, usable with complete identification databases, the best known of which is the api 20E (20 characters for Enterobacteriaceae).

According to BIOMERIEUX, 10 API® strips are used every minute worldwide, they are also used to assess the performance of other identification products (reference technique)

Initial identification of bacteria via methods such as Gram staining or selective or differential culture media is essential before choosing the appropriate API gallery.

Similar systems are offered by other companies, such as the Liofilchem EnteroPluri-Test.

Galerie API 20E

◉ API 20E identification gallery

The API 20E includes a plastic strip containing 20 mini test wells. Each well contains dehydrated media with chemically defined compositions to generally detect enzymatic activity, primarily related to carbohydrate fermentation or protein and amino acid catabolism by the inoculated organisms.


The different tests in API 20E

    1- ONPG test : The ONPG test is used to detect the enzyme β-galactosidase

    2- ADH : Decarboxylation of the amino acid arginine by arginine dihydrolase

    3- LDC: Decarboxylation of the amino acid lysine by lysine decarboxylase

    4- ODC: Decarboxylation of the amino acid ornithine by ornithine decarboxylase

    5- CIT : Use of citrate as sole carbon source

    6- H2S : Hydrogen sulfide production

    7- URE : Urease enzyme test

    8- TDA (Tryptophan deaminase) : Detection of the tryptophan deaminase enzyme (detected by adding Ferric chloride).

    9- IND : Indole test : production of indole from tryptophan by the enzyme tryptophanase (Indole is detected by adding Kovac's reagent).

    10- VP : The Voges-Proskauer test for the detection of acetoin (acetylmethylcarbinol) produced by fermentation of glucose by bacteria using the butylene glycol pathway

    11- GEL : Production test of the enzyme gelatinase which liquefies gelatin

    12- GLU : Glucose fermentation (hexose sugar)

    13- MAN : Fermentation of mannose (sugar hexose)

    14- INO : Fermentation of inositol (cyclic polyalcohol)

    15- SOR : Fermentation of sorbitol (sugar alcohol)

    16- RHA : Rhamnose fermentation (methyl pentose sugar)

    17- SAC : Fermentation of sucrose (disaccharide)

    18- MEL : Fermentation of melibiose (disaccharide)

    19- AMY : Fermentation of amygdalin (glycoside)

    20- ARA Fermentation of arabinose (pentose sugar)

A bacterial suspension is used to rehydrate each well, and the strips are then incubated. During this incubation, the metabolic activities of the bacteria cause color changes, which appear spontaneously or are revealed by the addition of reagents.

◉ Preperation of the API gallery

❶ Take a single isolated colony (from a pure culture) and make a bacterial suspension in sterile distilled water (0.5 McFarland).

❷ Combine the bottom and lid of an incubation box and distribute water in the cells to create a humid atmosphere. Then place the gallery in the incubation box under sterile conditions

❸ Take a Pasteur pipette and fill these compartments with the bacterial suspension as follows (photo) :

réaliser API 20E

❹ Mark the tray with the identification number (Patient ID or Organization ID), date and your initials.

❺ Incubate the tray at 37 o C for 18-24 hours.

❻ After incubation add reagents as follows :

Reagents to add to API 20E
Well Reagent
TDA A drop of TDA reagent
IND A drop of James' reagent or kovacs
VP A drop of VP1 then VP2 reagent

◉ Results, reading and interpretation of the API 20E

The reading of these reactions (positive or negative) is done according to the variations of colors


API 20E positive vs negative

  • You can use a free online tool to interpret your API test, such as UPBM (note that the database may not be up to date).
  • You can also enter your results into the Excel spreadsheet Excel spreadsheet
  • Another option is to use apiwebTM biomerieux

◉ Download Protocols from API Galleries

🏾 We have collected the main manuals to carry out your biochemical identification API (API 20e, API NE, API STREPT, API STAPH, API CANDIDA...)

Files (protocols) for carrying out API galleries
Galleries Protocols
API 20E pdf Download
API NE pdf Download
API STREPT pdf Download
API STAPH pdf Download
API LISTERIA pdf Download
API CANDIDA pdf Download
API CH50 pdf Download
API 20C pdf Download


API 20E positive vs negative


API 20NE positive vs negative


API Strep positive vs negative


API NH positive vs negative


API Staph positive vs negative

You can have all the information necessary for the handling and interpretation of API 20e, API NE, API STREPT, API STAPH, API CANDIDA... in the following file Download