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UREA BROTH BASE

Code: CM0071

A liquid version of Christensen’s medium for the differentiation of urease-producing Enterobacteriaceae.

* Adjusted as required to meet performance standards

Directions
Add 0.9 g to 95 ml of distilled water. Sterilise by autoclaving at 115°C for 20 minutes. Cool to 55°C and aseptically introduce 5 ml of sterile 40% Urea Solution SR0020. Mix well and distribute 10 ml amounts into sterile containers.

Description
This is a liquid modification of Christensen medium¹. The modification is suitable for the differentiation of urease- producing organisms from members of the Salmonella and Shigella groups, during the routine examination of rectal swabs and faeces. Maslen noted that in the routine examination of faeces for Salmonella and Shigella organisms many non- lactose-fermenting colonies isolated were later found to belong to the urease-positive Proteae. He evolved this medium as a means whereby the latter organisms could be rapidly detected and eliminated - thus saving a considerable amount of time and media. Maslen claimed that the advantages of the fluid medium were:
1. A Pasteur pipette could be used to inoculate other diagnostic media.
2. Rapid growth ensued and it was possible to discern a clear-cut positive reaction within two to five hours at 35°C.
3. It was easier to detect any contamination during storage.

Technique
For the examination of faeces, specimens are cultured in enrichment and selective media in the usual manner. Discrete colonies are then picked off the surface of the solid selective media.

Inoculate tubes of Urea Broth with single colonies of non-lactose-fermenting organisms and incubate for 2 to 6 hours at 35°C. (Maslen states that the cultures should be incubated in a water bath in order to obtain the highest proportion of positive reactions within 5 hours.) Regard all organisms producing a pink coloration in the medium (i.e. due to the alkalinity caused by urea hydrolysis) as not belonging to the Salmonella or Shigella groups, and discard.

Inoculate all cultures showing no colour change (i.e. no urea hydrolysis) into `sugar’ peptone waters (Andrade Peptone Water CM0061) plus the appropriate carbohydrate and on to a Blood Agar Base CM0055 slope.

Incubate the new cultures, together with the Urea Broth, until the next morning. No further examination is necessary if the urea tube now shows an alkaline reaction (pink colour), otherwise continue the diagnostic tests - including slide agglutinations from the Blood Agar Base culture, if necessary.

Storage conditions and Shelf Life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared medium at 2-8°C.

Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Orange coloured solution

Quality control

* This organism is available as a Culti-Loop®

Precautions
It is preferable that the medium be used on the day of preparation. If not, examine the tubes carefully to ensure sterility.
After overnight incubation other members of the Enterobacteriaceae may show alkaline reactions.

Reference
1. Maslen L. G. C. (1952) Brit. Med. J. 2. 545-546.