Analytical Profile Index
An Analytical Profile Index (API, Analytical Profile Index) is a rapid determination system for the identification ( determination ) of bacteria . The test is based on classic physiological test procedures, which are precisely defined in their combination. They can also be seen as part of a colorful series , but in a miniaturized and standardized form.
Principle of the system
The API rapid determination system combines numerous tests to check the biochemical characteristics of the bacteria, for example the enzymes present and the resulting metabolic properties or the ability to use certain carbohydrates with acid formation. The individual tests are housed in - depending on the system, between 10 and 50 - small reaction vessels that contain the standardized substrates . Meaningful identifications can only be carried out with pure cultures . The reaction vessels are inoculated and incubated with a suspension of the pure culture , usually for 18 to 24 hours. Individual reactions must take place under anoxic conditions; In order to prevent the entry of oxygen into the test tube, the inoculated approach is covered with paraffin oil.
The incubation is followed by an evaluation in which each individual test is assessed with regard to a positive or negative result. Detection reagents need to be added for some tests . The results are noted in the results protocol with + or -, the reactions are divided into groups of three. For a positive result, a number is assigned that depends on the position in the group of three, for example 1, 2 or 4, for a negative reaction a zero is assigned. The added numerical values of each group of three result in a numerical profile that shows the biochemical characteristics of the examined bacterium in abbreviated form as a numerical code . The numerical profile is compared to a database , either in a catalog or by application software . The result is the identification of the bacterium, with further information, for example the probability in% that the identification is correct or information about taxa with a similar result.
Variants and other features
The identification possibilities of these test systems are limited because they were only developed for the rapid identification of clinically relevant bacteria. Accordingly, only these microorganisms can be identified on the basis of their metabolic reactions. If a pure culture is not used, no meaningful result is obtained or, in rare cases, an incorrect result. It is advisable to regularly examine reference strains of bacteria that are stored in a strain collection and whose results and characteristics are known. Despite these limitations, the test system delivers a reliable result if the operating instructions are followed.
With 366 different bacterial strains from the enterobacteria group , a systematic comparison of the results of the rapid determination system was made with results obtained with individually set test tubes or Petri dishes of a colorful series ( e.g. citrate agar according to Simmons or lysine decarboxylase test medium according to Møller ). Only 13 of the 366 strains were incorrectly identified, of which seven were designated as atypical ( biochemical characteristics differing from the type strain ). Overall was accurate (Engl. Accuracy ) of 96.4% determined for each test the values are 90.4 to 100%.
The API 20 E system and the API 20 NE system enable a selection of gram-negative enterobacteria or non-enterobacteria to be identified. In medical microbiology, the latter are usually classified as “ non-fermenting gram-negative rod bacteria”. To distinguish between enterobacteria and non-enterobacteria, the oxidase test is used to test for the presence of cytochrome c oxidase . Enterobacteria are oxidase negative. A systematic comparison of the results of the rapid determination system with those of a conventional colored series was also carried out for the group of non-fermenting gram-negative rod bacteria. Only 10 of the 292 strains were incorrectly identified, so 96.6% of the strains were identified by both methods in agreement. Further rapid identification systems are available for a large number of medically relevant bacterial groups and yeasts , for example API 50 CHB for Bacillus species, API Coryne for Corynebacteria , API Staph for staphylococci , API 20 Strep for streptococci or API Candida for representatives of the yeast genus Candida .
Enterobacteria identification reactions
The following table shows the reactions of the Bunte series in the API 20 E system for the identification of Enterobacteriaceae and other gram-negative, non-demanding rods. It is merely an overview and does not replace the operating instructions, especially since detection reagents have to be added for some tests. Reactions in which the utilization of carbohydrates is checked are highlighted in color ; monosaccharides , disaccharides , sugar alcohols and other glycosides are listed . When carbohydrates are used, the bromothymol blue pH indicator checks whether acids are formed during the breakdown. Further information on the enzymes, substrates or biochemical reactions can be found in the respective articles.
| abbreviation | Reaction, enzyme or substrate | negative result | positive result |
|---|---|---|---|
| ONPG | ONPG test | colorless | yellow |
| ADH | Arginine dihydrolase | yellow | red or orange |
| LDC | Lysine decarboxylase | yellow | red or orange |
| ODC | Ornithine decarboxylase | yellow | red or orange |
| CIT | Citrate utilization | green or yellow | blue-green or blue |
| H 2 S | Hydrogen sulfide formation (see also SIM agar ) | colorless or grayish | black precipitate |
| URE | Urease (see also urea agar ) | yellow | red or orange |
| TDA | Tryptophan deaminase | yellow | red-brown |
| IND | Indole formation | colorless or yellow | pink to cherry red |
| VP | Voges-Proskauer reaction ( formation of acetoin ) | colorless | pink to red |
| GEL | Hydrolysis of gelatin by proteolytic enzymes | colorless | Distribution of the black pigment |
| GLU | Fermentative or oxidative degradation of D - glucose with acid formation | blue or blue-green | yellow |
| MAN | Fermentative or oxidative degradation of D - mannitol with acid formation | blue or blue-green | yellow |
| IN O | Fermentative or oxidative degradation of inositol with formation of acid | blue or blue-green | yellow |
| SOR | Fermentative or oxidative degradation of D - sorbitol with acid formation | blue or blue-green | yellow |
| RHA | Fermentative or oxidative degradation of L - rhamnose with acid formation | blue or blue-green | yellow |
| SAC | Fermentative or oxidative degradation of sucrose with acid formation | blue or blue-green | yellow |
| MEL | Fermentative or oxidative degradation of melibiose with acid formation | blue or blue-green | yellow |
| AMY | Fermentative or oxidative degradation of amygdalin with acid formation | blue or blue-green | yellow |
| ERA | Fermentative or oxidative degradation of L - arabinose with formation of acid | blue or blue-green | yellow |
| OX | Oxidase test | (look there) | (look there) |
Individual evidence
- ↑ a b c d e Roland Süßmuth, Jürgen Eberspächer, Rainer Haag, Wolfgang Springer: Biochemical-microbiological internship . 1st edition. Thieme Verlag, Stuttgart / New York 1987, ISBN 3-13-685901-4 , p. 78-85 .
- ↑ a b c P. B. Smith, KM Tomfohrde, DL Rhoden, A. Balows: API system: a multitube micromethod for identification of Enterobacteriaceae . In: Applied Microbiology . tape 24 , no. 3 , September 1972, p. 449-452 , PMID 4562482 , PMC 376540 (free full text).
- ↑ HK Geiss, HD Piotrowski, V. Hingst: Evaluation of API 20 NE in routine diagnostics of nonfermenting gram-negative rod-shaped bacteria . In: Central Journal for Bacteriology, Microbiology and Hygiene. Series A: Medical Microbiology, Infectious Diseases, Virology, Parasitology . tape 259 , no. 1 , February 1985, p. 35-42 , doi : 10.1016 / s0176-6724 (85) 80005-5 , PMID 3890425 .
- ^ Species identifiable by the various identification systems. In: API & ID 32 Identification Databases. bioMérieux sa, April 2015, accessed on January 18, 2020 .
Web links
- German Collection of Microorganisms and Cell Cultures (DSMZ): API test finder. In: Website BacDive . Retrieved January 18, 2020 .
- API (Analytical Profile Index) 20E Test - Procedure, Uses and Interpretation. In: Website Microbiology Info.com . April 12, 2019, accessed January 18, 2020 .