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Dehydrated Culture Media

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CHINA BLUE LACTOSE AGAR

Code: CM0209

A standard, non-inhibitory solid medium for enumeration and differentiation of bacteria in dairy products.

Typical Formula*

gm/litre

Peptone

5.0

`Lab-Lemco’ powder

3.0

Lactose

10.0

Sodium chloride

5.0

China blue

q.s.

Agar

12.0

pH 7.0 ± 0.2 @ 25°C

 
* Adjusted as required to meet performance standards

Directions
Suspend 35g in 1 litre of distilled water. Boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes.

Description
China Blue Lactose Agar was formulated by Brandl and Sobeck-Skal1. A standard, non-inhibitory solid medium for the differentiation and enumeration of bacteria in milk, proposed by the Methodenkommission fÏr Milchwirtschaft2. The china blue serves as a pH indicator, to differentiate between lactose fermenters and non-lactose fermenters, but does not suppress the growth of cocci; therefore this medium may be used for the detection of streptococci and staphylococci as well as the coli-aerogenes group.

Plates may be streak-inoculated or decimal dilutions of milk may be added to the molten, cooled medium in a pour-plate technique.
After 18 hours incubation at 35°C colony appearances are:-

Colour

Micro-organisms

Blue

Lactose fermenters e.g. Escherichia coli and coliform bacteria: 3-4 mm diameter
Staphylococci: 1 mm diameter
Streptococci: 0.5 mm diameter

Colourless

Non-lactose fermenters e.g.
Salmonella, Serratia, Proteus species and others

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 / pale blue coloured, free-flowing powder
Prepared medium: Blue coloured gel       

Quality control

Positive controls:Expected result
Enterococcus faecalis ATCC® 29212 *Good growth; pale blue colonies   
Escherichia coli ATCC® 25922 *Good growth; light blue coloured colonies with dark centres
Negative control:  
Uninoculated mediumNo change
* This organism is available as a Culti-Loop®

Precautions
It is important to remember that Gram positive and negative cocci and bacilli can grow on this medium. Always confirm the organism morphology and Gram reaction.

References
1. Brandl E. and Sobeck-Skal E. (1963) Milchwiss. Ber. 13. 1-9.
2. Methodenbuch Band VI. Verband Deutscher Landwirtschaftlicher Untersuchungs und Forschungsanstalten. 1970.

 
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