The USP Antimicrobial Effectiveness Test (AET) is a product quality test which is designed to demonstrate the effectiveness of added antimicrobial preservatives of a product. 

Antimicrobial effectiveness must be demonstrated for aqueous-based, multiple-dose topical and oral dosage forms and for other dosage forms such as ophthalmic, otic, nasal, irrigation, and dialysis fluids also. 

As all of the useful antimicrobial agents are toxic substances, for maximum protection of patients, the concentration of the preservative should be below a level that may be toxic to human beings based on the recommended dosage of the medicinal product. Antimicrobial preservatives should not be used as a substitute for good manufacturing practices and the antimicrobial preservatives used in the product must be declared on the label.

Testing of Products

1. Product Categorizes

For the purpose of testing, compendial articles have been divided into four categories. The criteria of antimicrobial effectiveness for these products are a function of the route of administration. It is expected that formulations containing preservatives will meet minimal efficacy standards, whether packaged as multi-doses or unit doses.


2. Procedure

The procedure can be conducted either in five original containers if a sufficient volume of product is available in each container and if the product container can be entered aseptically (i.e., needle and syringe through an elastomeric rubber stopper), or in five sterile, capped bacteriological containers [inert relative to the antimicrobial agent(s)] of suitable size into which a sufficient volume of product has been transferred. Inoculate each container with one of the prepared and standardized inocula, and mix. The volume of the suspension inoculum used is between 0.5% and 1.0% of the volume of the product to minimize potential effects on the product. The concentration of test microorganisms that is added to the product (Category 1, 2, or 3) is such that the final concentration of the test preparation after inoculation is between 1 × 105 and 1 × 106 cfu/mL of the product. For Category 4 products (antacids), the final concentration of the test preparation after inoculation is between 1 × 103 and 1 × 104 cfu/mL of the product.

The initial concentration of viable microorganisms in each test preparation is estimated based on the concentration of microorganisms in each of the standardized inocula as determined by the plate-count method. Incubate the inoculated containers at 22.5 ± 2.5°. Sample each container at the appropriate intervals. Record any changes observed in appearance at these intervals. Determine, by the plate-count procedure, the number of cfu present in each test preparation for the applicable intervals (see General Procedures in Microbial Enumeration Tests 〈61〉). Plate counts will be conducted using a minimum of duplicate plates, with the cfu averaged before determination of deduced cfu/mL. If membrane filtration is used, duplicate membrane filters will be used for each estimate. Using the calculated concentrations of cfu/mL present at the start of the test, calculate the change in log10 values of the concentration of cfu/mL for each microorganism at the applicable test intervals, and express the changes in concentration in terms of log reductions. The log reduction is defined as the difference between the log10 unit value of the starting concentration of cfu/mL in the suspension and the log10 unit value of cfu/mL of the survivors at that time point.

Culture Conditions for Inoculum Preparation

Criteria for Tested Microorganisms

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