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Dehydrated Culture Media

Sector: Clinical

LYSINE DECARBOXYLASE BROTH (TAYLOR MODIFICATION)

Code: CM0308 (Tablets)

To detect lysine decarboxylase production by salmonellae and some other Enterobacteriaceae.

Typical Formula*

gm/litre

Yeast extract

3.0

Glucose

1.0

L-lysine

5.0

Bromocresol purple

0.016

pH 6.1 ± 0.2 @ 25°C

* Adjusted as required to meet performance standards 

Directions
Add 1 tablet to 5ml of distilled water in an appropriate bottle. Sterilise by autoclaving at 121°C for 15 minutes.

Description
Lysine Decarboxylase Broth is a diagnostic medium which distinguishes salmonellae (and some other Enterobacteriaceae) by a distinct biochemical reaction.

Taylor’s modification of the medium1 shows an improved performance over the formulation described by Falkow2. This is achieved by omitting peptone from the medium, thus eliminating false positives caused by Citrobacter freundii and its paracolon types. These organisms utilise peptone as a nitrogen source, produce an alkaline reaction masking the absence of lysine decarboxylase.

Taylor’s modification shares the advantages of Falkow’s formulation over that of Moller3 in that it does not require anaerobic culture and low pH and it is relatively easy to control.

During the initial stages of incubation, fermentation of glucose by the organism, with acid production results in a colour change to yellow. On further incubation, if lysine is decarboxylated to cadaverine, there will be an alkaline reaction. The colour then changes to purple (positive). If the colour remains yellow, the reaction is negative.

Decarboxylase reactions of various members of the enterobacteriaceae on lysine

Organism

Lysine Decarboxylation

Salmonella species

+

Salmonella paratyphi A

-

Shigella species

-

Escherichia coli(including late-lactose variants
Alcalescens-Dispar) V

V

Citrobacter species (including the Bethesda-Ballerup group)

-

Providencia species

-

Proteus species

-

Serratia species

V

Klebsiella species

V

Enterobacter species

V


Technique
The medium is inoculated with the organism and incubated for 24 hours at 35°C.
Results after 24 hours:
Purple colour - positive reaction
Yellow colour - negative reaction

Storage conditions and Shelf life
Store the tablets at 2-8°C and use before the expiry date on the label.
Store the prepared medium at 2-8°C.

Appearance
Dehydrated medium: blue/grey tablets
Prepared medium: blue/grey solution

Quality control

Positive control:

Expected results

Salmonella typhimurium ATCC® 14028 *

Turbid growth, purple medium

Negative control:

 

Citrobacter freundii ATCC® 8090 *

Turbid growth, yellow medium

* This organism is available as a Culti-Loop®

Precautions
Use light inocula for the tubes of lysine medium.
Note the negative reaction of Salmonella paratyphi A.
Do not read the test after less than 24 hours incubation. Some organisms may require prolonged incubation for up to 4 days.

References
1. Taylor W. I. (1961) Appl. Microbiol. 9. 487-490.
2. Falkow S. (1958) Amer. J. Clin. Path. 29. 598-600.
3. Moller V. (1955) Acta Path. Microbiol. Scand. 36. 158-172.

 
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