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USP 51

Antimicrobial Effectiveness Test

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USP 51 – Antimicrobial Effectiveness Test

USP 51 test method determines the antimicrobial efficacy of preservatives that offer protection from microbial contamination. USP 51 test is performed on aqueous-based products including medical, cosmetics and personal care products.

Antimicrobial preservatives play a vital role to maintain the quality and integrity of products by limiting the growth of microorganisms. Thereby, USP 51 is established to check the effectiveness of antimicrobial preservatives against test microorganisms, this in turn ensures the product’s safety for its intended use.

USP 51 – Antimicrobial Effectiveness Test Method

  1. Test microorganisms from stock culture are procured into a sterile vessel either by centrifugation (for broth medium) or washing (for solid medium).
  2. These test microorganisms are resuspended in sterile saline and maintained at a microbial count of ~1 X 10^8 CFU/ml.
  3. The test sample is placed in different sterile containers and is exposed to each of the test organisms separately.
  4. Initial microbial concentration in the test sample is determined using plate count methods.
  5. Inoculated test samples are incubated at 22.5 ± 2.5°C and viable count of microorganisms is determined at 7, 14, and 28 days respectively.
  6. Antimicrobial preservative effectiveness of the test product is determined by comparing microbial growth at specific time intervals to the initial concentration of test microorganisms.

At Microbial Investigations (MIS), we have outstanding testing facilities to perform antimicrobial preservative challenge tests – USP 51. Additionally, we do perform microbial enumeration of non-sterile products using tests such as USP 61.

Our microbiology experts design and conduct antimicrobial assays and supervise microbiological activities on a regular basis. Our constant approach is to achieve reliable and reproducible results that can help our clients to develop high-quality products.

Contact us to get a quote on USP 51 or know more about our microbial testing services.


USP 51 test determines the efficacy of antimicrobial preservatives used to prevent microbial contamination and spoilage.

USP 51 test can be applied to aqueous-based products such as injections, eye drops, syrups, cosmetics, pet care, and personal care products.

The turnaround time for USP 51 test is 4 to 5 weeks.

At Microbe Investigations, we test for the USP 51 using the following strains : Pseudomonas aeruginosa (ATCC 15442), Escherichia coli (ATCC 8739), Staphylococcus aureus (ATCC 6538P), Enterococcus hirae (ATCC 10541), Klebsiella pneumoniae (ATCC 4352), MRSA (ATCC 33591), Salmonella enterica (ATCC 10708), Candida Albicans (MTCC 3017) and Aspergillus niger (ATCC-16888).

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