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Dehydrated Culture Media

SENSITEST AGAR

Code: CM0409

A medium specially designed to give large, clear zones with all antibiotics, without the addition of lysed or whole blood.

Typical Formula*

gm/litre

Hydrolysed casein

11.0

Peptones

3.0

Sodium chloride

3.0

Glucose

2.0

Starch

1.0

Buffer salts

3.3

Nucleoside bases

0.02

Thiamine

0.00002

Agar

8.0

pH 7.4 ± 0.2 @ 25°C

 
* Adjusted as required to meet performance standards

Directions
Suspend 32g in 1 litre of distilled water and bring to the boil to dissolve the agar. Sterilise by autoclaving at 121°C for 15 minutes.

Description
Sensitest Agar CM0409 was developed in the Oxoid laboratories as an antimicrobial susceptibility testing medium that did not require the addition of lysed horse blood to overcome sulphonamide and trimethoprim antagonists.

Using hydrolysed casein as the major source of amino-nitrogen in the medium, it was possible to lower the peptone content to the minimum necessary to supply essential peptides and other growth factors. Careful control of the hydrolysis of the peptones ensures that antagonists to critical antibiotics do not arise.

Bell1 in a monograph on antimicrobial susceptibility testing chose Oxoid Sensitest Agar as the preferred medium, from those tested, on the following criteria:
1. Ability to support the growth of common Gram positive and Gram negative organisms under the conditions of test.
2. Ability to yield reproducible results.
3. Did not require the addition of lysed horse blood when a heavy inoculum method was employed.
4. Ability to demonstrate standard zones of inhibition with reference organisms and antibiotics.

In the final development of the CDS method Bell selected Sensitest Agar because of its superiority in sulphonamide testing, easier reconstitution of the dehydrated powder and stability of the powdered medium on storage.

Whilst the addition of 5% horse blood to the medium is required for demanding strains, e.g. Streptococcus pyogenes and Streptococcus pneumoniae, there is no significant difference in zone sizes from the addition of blood.

The agar used in the medium, has been specially processed to yield a gel that does not impede the diffusion of antimicrobials.
For further details of antimicrobial susceptibility testing see relevant section.

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 plates of agar at 2-8°C.

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

Quality control

Positive controls:

Expected results

Escherichia coli ATCC® 25922*Good growth; straw coloured colonies
Pseudomonas aeruginosa ATCC® 27853*Good growth; straw coloured colonies with green pigmentation
Staphylococcus aureus ATCC® 25923*Good growth; white coloured colonies
Enterococcus faecalis ATCC® 29212*

Good growth; white / grey coloured colonies

Negative control:

 
Uninoculated mediumNo change
* This organism is available as a Culti-Loop®

Precautions
As with other susceptibility testing media, Sensitest Agar should be used for rapidly growing aerobic organisms only. It should not be modified by the addition of carbohydrates or incubated in a CO2 enriched atmosphere.
If the medium is used for Bell’s CDS method then the specified discs and technique must be used. Refer to the monograph cited in the References.

Reference
1. Bell S. M. (1975) Supplement to Pathology (Journal of the Royal College of Pathology of Australia) Vol.7 No.4. pp. 1-48.

 
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