An Introduction to
Bacterial Identification
Page 2: The API-20E® Enteric
Identification System
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The API-20E test kit for the identification of enteric bacteria (bioMerieux, Inc., Hazelwood, MO) provides an easy way to inoculate and read tests relevant to members of the Family Enterobacteriaceae and associated organisms. A plastic strip holding twenty mini-test tubes is inoculated with a saline suspension of a pure culture (as per manufacturer's directions). This process also rehydrates the dessicated medium in each tube. A few tubes are completely filled (CIT, VP and GEL as seen in the photos below), and some tubes are overlaid with mineral oil such that anaerobic reactions can be carried out (ADH, LDC, ODC, H2S, URE).
After incubation in a humidity chamber for 18-24 hours at 37°C, the color reactions are read (some with the aid of added reagents), and the reactions (plus the oxidase reaction done separately) are converted to a seven-digit code. The code is fed into the manufacturer's database via touch-tone telephone, and the computer voice gives back the identification, usually as genus and species. The reliability of this system is very high, and one finds systems like these in heavy use in many food and clinical labs.
Note: Discussion and illustration of the API-20E system here does not constitute any necessary commercial endorsement of this product. It is shown in Bacteriology 102 and 324 as a prime example of a convenient multi-purpose testing method one may encounter out there in the "real world."
In the following photos:
- Note especially the color reactions for amino acid decarboxylations (ADH through ODC) and carbohydrate fermentations (GLU through ARA).
- The amino acids tested are (in order) arginine, lysine and ornithine. Decarboxylation is shown by an alkaline reaction (red color of the particular pH indicator used).
- The carbohydrates tested are glucose, mannitol, inositol, sorbitol, rhamnose, sucrose, melibiose, amygdalin and arabinose. Fermentation is shown by an acid reaction (yellow color of indicator).
- Hydrogen sulfide production (H2S) and gelatin hydrolysis (GEL) result in a black color throughout the tube.
- A positive reaction for tryptophan deaminase (TDA) gives a deep brown color with the addition of ferric chloride; positive results for this test correlate with positive phenylalanine and lysine deaminase reactions which are characteristic of Proteus, Morganella and Providencia.
In the first set of reactions:
- Culture "5B" (isolated from an early stage of sauerkraut fermentation) is identified as Enterobacter agglomerans which has been a convenient dumping ground for organisms now being reassigned to better-defined genera and species including the new genus Pantoea. This particular isolate produces reddish (lactose +), "pimply" colonies on MacConkey Agar which exude an extremely viscous slime as may be seen here.
- Culture "8P44" is identified as Edwardsiella hoshinae. The CDC had identified this culture (in 1988) as the ultra-rare Biogroup 1 of Edwardsiella tarda which may not be in the API-20E database. This system probably would not be able to differentiate between these two organisms.
 |
culture
no. | O
N
P
G | A
D
H | L
D
C | O
D
C | C
I
T | H
2
S | U
R
E | T
D
A | I
N
D | V
P | G
E
L | G
L
U | M
A
N | I
N
O | S
O
R | R
H
A | S
A
C | M
E
L | A
M
Y | A
R
A | identification |
| 8101 | + | – | + | + | – | – | – | – | + | – | – | + | + | – | + | + | + | + | – | + | Escherichia
coli |
| 5B | + | – | – | – | + | – | – | – | – | + | – | + | + | + | + | + | – | – | + | + | Enterobacter
agglomerans |
| 8P44 | – | – | + | + | – | + | – | – | + | – | – | + | + | – | – | – | + | – | – | + | Edwardsiella
hoshinae |
 |
culture
no. | O
N
P
G | A
D
H | L
D
C | O
D
C | C
I
T | H
2
S | U
R
E | T
D
A | I
N
D | V
P | G
E
L | G
L
U | M
A
N | I
N
O | S
O
R | R
H
A | S
A
C | M
E
L | A
M
Y | A
R
A | identification |
| 8030 | + | – | + | – | + | – | + | – | – | + | – | + | + | + | + | + | + | + | + | + | Klebsiella
pneumoniae |
| 8068 | – | – | – | – | – | + | + | + | + | – | + | + | – | – | – | – | + | – | + | – | Proteus
vulgaris |
| 8P14 | – | – | + | + | – | + | – | – | – | – | – | + | + | – | + | + | – | + | – | + | Salmonella
sp. |
