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Dehydrated Culture Media

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MAXIMUM RECOVERY DILUENT (PEPTONE SALINE DILUENT)

Code: CM0733

a protective and isotonic diluent for maximal recovery of micro-organisms (ISO/DIS 6649)

Typical Formula*

gm/litre

Peptone

1.0

Sodium chloride

8.5

pH 7.0 ± 0.2 @ 25°C

 
* Adjusted as required to meet performance standards

Directions
Dissolve 9.5g in 1 litre of distilled water. Dispense into the final containers and sterilise by autoclaving at 121°C for 15 minutes.

Description
Maximum Recovery Diluent combines the protective effect of peptone in the diluting solution with the osmotic support of physiological saline1,2. The low concentration of peptone does not cause multiplication of the organisms within 45 minutes (@ 20-25°C) of dilution of the sample. The isotonic strength of the diluent ensures recovery of organisms from various sources which may be vulnerable in distilled water or aqueous suspensions.

Technique

  1. Prepare the medium according to the directions. For the IS0 method3 distribute the medium into 90ml volumes or into 9ml volumes.
  2.  Put 10g of the test sample into a sterile blender jar or sterile plastic bag.
  3. Add 90ml of sterile Maximum Recovery Diluent.
  4. Operate the blender according to its speed for sufficient time to give a total number of 15,000-20,000 revolutions. Alternatively operate a peristaltic type blender (Stomacher) for 2 minutes.
  5. Within 15 minutes transfer 1ml of the macerate to 9ml of sterile diluent and mix well (10-1 dilution).
  6. Prepare additional decimal dilutions in the same way.
  7. Aseptically transfer 1ml of each dilution of the initial suspension in duplicate to the centre of a sterile Petri dish.
  8. Prepare pour plates with the medium of choice.
  9. Allow the agar to solidify and incubate in a manner appropriate to the agar medium.

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 room temperature.

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

Quality control

The diluent is inoculated with each organism, separately, and held at 20 -25°C. Counts of colony forming units are obtained at 0 minutes and 45 minutes from inoculation, using suitable nutrient medium. The results are compared, using the time "0" count as the base bench mark.

 

 Positive controls:Expected results
Escherichia coli ATCC® 25922*±50% original count
Escherichia coli ATCC® 8739*±50% original count
Staphylococcus aureus ATCC® 25923±50% original count

 References
1.
Straker R. P. and Stokes J. L. (1957) Appl. Microbiol. 5. 21-25.
2. Patterson J. W. and Cassells J. A. (1963) J. Appl. Bact. 26. 493-497.
3. ISO/DIS 6649. Meat and Meat Products-Detection and Enumeration of Clostridium perfringens.

 
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