Antibiotic Agar No. 11, is a specialized medium used for determining the potency of antibiotics through microbiological assay techniques. Conforming to the specifications of The United States Pharmacopeia (USP), it plays a critical role in pharmaceutical quality control and research.
Composition and Principle
The typical formulation includes Beef Extract, Yeast Extract, Casein Peptone, Peptone, Dextrose, Agar, with a final pH of 7.9 ± 0.1. This medium serves as a base for two types of assays: the Cylinder Plate Assay and the Turbidimetric Assay. The Cylinder Plate Assay involves measuring the inhibition zone caused by antibiotic diffusion from a cylinder placed on the inoculated agar surface. The Turbidimetric Assay is based on the inhibition of microbial culture growth in a fluid medium containing a uniform antibiotic solution.
To prepare, 30.5 g of the powder is suspended in 1 liter of distilled or deionized water, boiled for complete dissolution, and autoclaved at 121°C for 15 minutes. The pH is adjusted to 8.3 ± 0.1 after cooling the base to 45-50°C by adding NaOH.
For the Cylinder Plate Assay, a 24-48 hour agar slant culture is washed with sterile purified water, and the culture is diluted to the desired concentration. The assay uses Petri dishes and cylinders with specific dimensions. The base layer is solidified before overlaying the seed layer containing the test organism. The Turbidimetric Assay involves preparing dilutions of the antibiotic reference standards and the test samples in tubes, which are then incubated, and growth is halted using formalin. Light transmittance is measured using a spectrophotometer.
Interpretation of Results
The results interpretation requires referring to appropriate procedures detailed in pharmacopeial methods. For the Cylinder Plate Assay, the size of the inhibition zone is compared with that of known standards, while for the Turbidimetric Assay, the microbial growth is compared against reference standards.
The dehydrated medium should be stored at 10-25°C, away from light, until the expiry date on the label or signs of deterioration or contamination appear.
Warnings and Precautions
While not classified as hazardous, it is designed for in vitro diagnostic use and must be used only by properly trained operators. Consultation of the Safety Data Sheet is recommended for correct use.
Antibiotic Agar No. 11 is an indispensable tool in pharmaceutical quality control for determining the potency of antibiotics. Its versatility in accommodating both Cylinder Plate and Turbidimetric Assays, combined with its compliance with pharmacopeial standards, makes it a reliable and essential medium in pharmaceutical microbiology.