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ISO-SENSITEST AGAR

Code: CM0471

a semi defined medium designed for antimicrobial susceptibility testing

*Adjusted as required to meet performance standards

Directions
Suspend 31.4g 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
Oxoid Iso-Sensitest™ Agar CM0471 was developed specifically for antimicrobial susceptibility tests. Its formulation was carefully constructed to give a reproducible, semi-defined medium in which the undefined components were kept to a minimal level. However, it allows the growth of the great majority of micro-organisms without further supplementation.

It has been designed to overcome the objections made about Mueller-Hinton media by various workers^{1,2,3,4,5,6,7}.
These objections were:
1. Different MIC values in the broth and agar versions of the medium.
2. Agar versions showing antagonistic effects towards tetracycline.
3. High levels of sulphonamide and trimethoprim antagonists.
4. Poor reproducibility with different manufacturers’ peptones.
5. Poor growth-supporting ability for streptococci and variable growth rates with gram-positive organisms generally.

Some mutant strains which are totally dependent on thymine and thymidine for their growth will, however, not grow in Oxoid `Iso-Sensitest Agar’, which has these two compounds at very low levels, as they are the naturally occurring antagonists of trimethoprim. Care must be taken to recognise these strains^8,9,10.

Oxoid Iso-Sensitest Agar, was developed from Oxoid Sensitest Agar (CM0409) which has been used successfully in several centres throughout the world^11,12,13. The role of metal ions as antagonists to certain antibiotics has been described by many workers^{14,15,16,17,18,19}. A stabilised mineral content in sensitivity test media is, therefore, important.

Reller, Schoenknecht, Kenny & Sherris³ demonstrated the contribution of cations provided in media by ordinary agars. A considerable difference in mineral content between the agar and broth media was shown. Only by using an agar, which is specially processed to remove the free anions and cations, can a stabilised mineral content be obtained.

Oxoid Iso-Sensitest Agar has stability in mineral content and the ability to produce optimum antimicrobial zones of inhibition. The amino-nitrogen base of acid-hydrolysed casein and special peptones has been supplemented with defined growth factors. Careful preparation of the nutrients ensures that trimethoprim and sulphonamide antagonists are at very low levels.

THE ADDITION OF LYSED HORSE ERYTHROCYTES TO THE MEDIUM IS NOT REQUIRED WHEN CARRYING OUT ANTIMICROBIAL SUSCEPTIBILITY TESTS WITH THESE COMPOUNDS.

SUPPLEMENTAL NUTRIENTS FOR NUTRITIONALLY DEPENDENT ORGANISMS
Some pathogenic organisms may be nutritionally dependent because of intrinsic demands for special growth factors, e.g. streptococci, Neisseria, Haemophilus, Campylobacter etc. Other organisms may exhibit mutant characteristics, e.g. dwarf Staph. aureus, thymidine-dependent Escherichia coli.

Supplemental nutrients can be added to Iso-Sensitest Agar to obtain or improve growth of these organisms²⁰.

Certain supplements interfere with antimicrobial activity and tests must be made to measure their effect.

Iso-Sensitest Agar with the addition of 10% of horse blood has been recommended as a suitable medium when testing susceptibility of Helicobacter pylori²¹. Although Mueller-Hinton agar performed nearly as well, Iso-Sensitest Agar was preferred because its contents are better defined.

Iso-sensitest Agar is recommended by the BSAC for use within their Disc Diffusion Method for Antimicrobial Susceptibility Testing^22.

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 and use as freshly as possible.

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

Quality control

* This organism is available as a Culti-Loop®

Note: Please refer to relevant standards for further quality control testing.

References
1. Ericsson H. M. and Sherris J. C. (1971) Acta. Pathol. Microbiol. Scand. Suppl. 217. 1-90.
2. Garrod L. P. and Waterworth P. M. (1971) J. Clin. Path. 24. 779-789.
3. Reller L. B., Schoenknecht F. D., Kenny M. A. and Sherris J. C. (1974) J. Infect. Dis. 130. 454-463.
4. Duncan I. B. R. (1974) Antimicrob. Agents & Chemotherapy 5. 9-15.
5. Yourassowsky E., Vanderlinden M. P. and Schoutens E. (1974) J. Clin. Path. 27. 897-901.
6. Neussil H. (1976) Chemotherapy Vol.2. 33-40.
7. Bridson E. Y. (1976) Arztl. Lab. 22. 373-376.
8. Tanner E. I. and Bullin C. H. (1974) J. Clin. Path. 27. 565-568.
9. Thomas M. and Bond L. (1973) Med. Lab. Technol. 30. 277-279.
10. Barker J., Healing D. and Hutchinson J. G. P. (1972) J. Clin. Path. 25. 1086-1088.
11. Reynolds A. V., Hamilton-Miller J. M. T. and Brumfitt W. (1974) B.M.J. (ii) 778.
12. Stewart Sheila M., Anderson Isobel M. E. and Malcolm Margaret G. G. (1975) J. Clin. Path. 28. 195-197.
13. Bell S. M. (1975) Pathology 7. Suppl. 1-48.
14. Garrod L. P. and Waterworth P. M. (1969) J. Clin. Path. 22. 534-538.
15. Gilbert D. N., Kutscher E., Ireland P., Barnett J. A. and Sandford J. P. (1971) J. Infect. Dis. 124. (suppl.), S37-S45.
16. Zimelis V. M. and Jackson G. G. (1973) J. Infect. Dis. 127. 663-669.
17. Davis S. D., Ianetta A., Wedgewood R. J. (1971) J. Infect. Dis. 124. 610-612.
18. Brenner V. C. and Sherris J. C. (1972) Antimicrob. Agents Chemother. 1. 116-122.
19. Traub W. H. (1970) Appl. Microbiol. 20. 98-102.
20. Acar J. F. (1980) Antibiotics in Laboratory Medicine, Lorian V. (Ed.) Williams and Wilkens, Baltimore, USA, 48-51.
21. Hartzen S.H., Andersen L.P., Bremmelgaard A. et al (1997) Antimicrob. Ag. and Chemother. 41. 2634-2639.
22. British Society for Antimicrobial Chemotherapy (2002) Disc Diffusion Method for Antimicrobial Susceptibility Testing. Version 2.1.1. BSAC.

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