The antimicrobial effectiveness test in United States Pharmacopeia chapter <51> is the most widely quoted way of showing that a preservative system works. It is also widely misread. This post walks through what the test involves and how to read the table on an EsoDant data sheet.

The procedure in brief

  1. Five reference organisms are grown up separately: Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli, Candida albicans and Aspergillus brasiliensis. Between them they represent Gram-negative and Gram-positive bacteria, a yeast and a mould.
  2. Each organism is inoculated into a separate portion of the product at a high level, typically 105 to 106 colony-forming units per gram or millilitre.
  3. The inoculated samples are stored at room temperature and counted at intervals – on EsoDant sheets you will see the initial count, day 14 and day 28.
  4. The counts are compared with the acceptance criteria for the product category.

What "Category 2" means

USP <51> divides products into four categories. Category 2 covers topically used products made with aqueous bases or vehicles – which is where creams, lotions, shampoos and washes fall. For Category 2 the criteria are:

  • Bacteria: not less than a 2.0 log reduction from the initial count at 14 days, and no increase from the 14-day count at 28 days.
  • Yeast and mould: no increase from the initial count at 14 and 28 days.

"No increase" is defined as not more than 0.5 log units higher than the previous value.

Reading the EsoDant table

On the EsoDant 9010 sheet, for example, P. aeruginosa goes from 8 × 107 at inoculation to 3 × 104 at day 14 – more than a 3-log reduction, comfortably past the 2-log requirement – and then NI at day 28. NI simply means no increase, i.e. the criterion for the 28-day reading was met. The same pattern across all five organisms is what lets the sheet state that the product conforms to USP <51>, Category 2.

The test was run on the preservative as supplied, by an independent laboratory. That tells you the material is potent. It does not tell you what happens in your formulation.

Why your result can differ

A preservative's performance in a finished product depends on the product:

  • Ethoxylated surfactants and solubilisers can trap phenoxyethanol in micelles, lowering the free fraction that reaches microbes.
  • pH outside the stated range weakens some chemistries – isothiazolinones above pH 8, for instance.
  • Heat during processing can destroy part of the dose before the product is even filled.
  • Water activity, packaging and the raw-material bioburden all shift the starting point.

This is why every EsoDant data sheet says the same thing: the use levels are a starting point, and the manufacturer should confirm efficacy in the final formulation by challenge testing. We are happy to help interpret the results, or to suggest a use-level adjustment if a first pass comes back marginal.

See the full USP <51> data on the EsoDant 9010 sheet →