Antibiotic Medium No. 9

Antibiotic Medium No. 9

Official Base-Layer Agar for Polymyxin B & Colistin Cylinder-Plate Assays

Catalog Number: AS-1108

Overview

Antibiotic Medium No. 9 is the official standardized base-layer agar used in the microbiological cylinder-plate (cup-plate) assay of polymyxin B and colistin (polymyxin E). It is designated as “Medium I” in USP <81>, European Pharmacopoeia 2.7.2, Japanese Pharmacopoeia (JP 17), and WHO guidelines.

When used in combination with Antibiotic Medium No. 10 as the seeded overlay inoculated with Bordetella bronchiseptica, this medium produces exceptionally large, sharp, and highly reproducible inhibition zones that are specific to polymyxin antibiotics. The formulation is enriched and slightly more alkaline than general-purpose base agars to maximize polymyxin diffusion and assay sensitivity.

Key Features & Benefits

Optimized nutrient content and pH for polymyxin diffusion

Official base layer for polymyxin B and colistin assays

Used exclusively with Antibiotic Medium No. 10 as the seed layer

Produces the clearest and largest inhibition zones for polymyxins

Globally harmonized formulation (USP / EP / JP / WHO)

Typical Applications

Pharmaceutical Quality Control
Compendial potency assay of polymyxin B sulfate

Compendial potency assay of colistin sulfate (polymyxin E)

Routine QC testing of aminoglycoside–polymyxin formulations

Cylinder-Plate Diffusion Assays
Assay of polymyxin-containing parenteral products

Assay of ophthalmic polymyxin formulations

Assay of topical polymyxin preparations

Principle of the Medium

Antibiotic Medium No. 9 functions as a nutritionally rich and diffusion-optimized base layer that supports uniform antibiotic migration. When overlaid with seeded Antibiotic Medium No. 10, polymyxin antibiotics diffuse radially from cylinders or cups, forming inhibition zones whose diameter is directly proportional to antibiotic potency.

Typical Composition (per litre)

Peptone (meat or casein) – 9.0 g

Beef Extract – 2.3 g

Yeast Extract – 4.5 g

D-Glucose – 1.5 g

Agar – 17.0 g

Final pH: 7.2 ± 0.1 at 25 °C (after sterilisation)

Preparation

Suspend 34.3 g of dehydrated medium in 1 L purified water

Heat with frequent agitation and boil for 1–2 minutes until fully dissolved

Sterilise by autoclaving at 121 °C for 15 minutes

Cool to 60–65 °C

Pour 20–25 mL into sterile 90–100 mm Petri dishes to form a uniform base layer
(approximately 5–6 mm thickness)

Allow plates to solidify on a perfectly level surface

Overlay with seeded Antibiotic Medium No. 10 containing
Bordetella bronchiseptica ATCC 4617

Storage & Stability

Dehydrated medium: Store below 30 °C in a cool, dry place

Prepared base-layer plates: Store at 2–8 °C and use within 7 days

Mandatory Combination

Base layer: Antibiotic Medium No. 9

Seed layer: Antibiotic Medium No. 10 + Bordetella bronchiseptica ATCC 4617

Intended Use

For pharmaceutical quality-control and official compendial antibiotic assays only.
Not for clinical diagnostic, food, or veterinary use.

Quality & Compliance

Complies with USP <81>, EP 2.7.2, JP 17, and WHO requirements

Manufactured under controlled conditions for batch-to-batch consistency

Suitable for regulated pharmaceutical laboratories

Customs & Trade Information

HS / AHECC Code: 3821.00.00
Prepared culture media for development or maintenance of microorganisms