ANTIBIOTIC ASSAY MEDIUM NO. 10 (as per USP) (PART – I)

This medium is used as seed agar for assay of Polymyxin B, Colistimethate sodium, Colistin and Carbenicillin. The medium composition is in accordance to USP and CFR and numerically identical with the name assigned by Groove and Randall.
Polymixins are reported to have slow diffusion in agar giving smaller zone of inhibition. Hence the reduced agar concentration (1.2%) in this medium improves the diffusion of polymixin in the medium.
Polysorbate 80 is reported to function synergistically with polymixins on spheroplasts of Pseudomonas aeruginosa. Polysorbate 80 enhances the penetration of Polymyxin to the cytoplasmic membrane and hence is an appropriate ingredient in the medium used for assay of Polymyxin. Freshly prepared plates should be used for antibiotic assays. Test organisms are inoculated in sterile seed agar pre-cooled to 40-45?C and spread evenly over the surface of solidified base agar. All conditions in the microbiological assay must be controlled carefully. The use of standard culture media in the test is one of the important steps for good results.

for antibiotic assay of Carbenicillin, Colistimethate sodium and Polymyxin-B