An Introduction to
Bacterial Identification

Page 2: The API-20E® Enteric
Identification System

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BACTERIAL IDENTIFICATION:
• Page 1 – General Principles
• Page 2 – API-20E Enteric ID System
• Page 3 – Genotypic Identification

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 which is called the Analytical Profile Index, from which name the initials "API" are derived. The code can be fed into the manufacturer's database via touch-tone telephone, and the computerized voice gives back the identification, usually as genus and species. An on-line database can also be accessed for the identification. 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 necessarily constitute any commercial endorsement of this product. It is shown in our laboratory courses as a prime example of a convenient multi-purpose testing method one may encounter out there in the "real world."

In the following photos:

In the first set of reactions:

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.

These general microbiology pages have copyright by John Lindquist
and found their permanent sanctuary on www.jlindquist.net circa 2001.
Copies found elsewhere are neither authorized nor up to date.
Page content was last modified on 6/1/10 at 9:00 PM, CDT.
John Lindquist, Department of Bacteriology,
University of Wisconsin – Madison