Methods / Microbial enumeration
Microbial enumeration
What is the bioburden?
| Determinations | TAMC and TYMC, optionally specified organisms |
| Reported as | Colony forming units per gram or millilitre |
| Method suitability | Established per product, recovery within a factor of two |
| Sample | Two unopened units |
| Not a substitute for | USP <71> sterility |
Microbial enumeration counts viable organisms in a non-sterile product and reports total aerobic microbial count and total combined yeasts and moulds in colony forming units. It is a quantitative bioburden test under USP <61>. It is not a sterility test, it cannot be reported as one, and a result expressed only as pass or fail has discarded the information the test exists to produce.
Applied to: Supplements, Cosmetics, Non-sterile preparations. Reference: USP <61>, USP <62>.
Total aerobic microbial count is determined on soybean casein digest agar incubated at 30 to 35 degrees for three to five days. Total combined yeasts and moulds count uses Sabouraud dextrose agar at 20 to 25 degrees for five to seven days. Both are enumerations: the plates are counted and the result expressed per gram or per millilitre.
USP <62> extends this to tests for specified microorganisms, where the question is not how many but whether a particular organism is present at all. Escherichia coli, Salmonella species, Pseudomonas aeruginosa, Staphylococcus aureus, bile-tolerant Gram-negative bacteria and Candida albicans are the routine targets, with the selection driven by route of administration and the nature of the product.
| Determination | Medium | Temperature | Duration |
|---|---|---|---|
| TAMC | Soybean casein digest agar | 30 to 35 C | 3 to 5 days |
| TYMC | Sabouraud dextrose agar | 20 to 25 C | 5 to 7 days |
| Specified organisms | Selective media per organism | Per method | Per method |
The distinction is definitional. USP <61> is designed for products that are not required to be sterile and quantifies how much microbial life is present. USP <71> is designed for products that are required to be sterile and asks a binary question about growth across a fourteen-day incubation under aseptic conditions.
The two tests use different media, different incubation, different sample handling and different facilities. A laboratory offering enumeration under the name sterility is describing one test using the name of another, and the client is left believing an injectable preparation has been shown to be sterile when it has not.
This is not a pedantic distinction. It is the difference between a statement that can be relied on for a parenteral product and one that cannot.
Before enumeration means anything, the method has to be shown to work on that specific product. Many materials are inherently antimicrobial, and a preservative, a solvent residue or the active substance itself can suppress growth so that the plate count reflects the sample's chemistry rather than its bioburden.
Method suitability is established by inoculating the product with a defined low-level challenge, typically not more than 100 colony forming units of specified strains, and demonstrating recovery within a factor of two. Where recovery fails, the method is modified by dilution, filtration or neutralisation until it succeeds, and the modification is reported.
An enumeration result issued without a suitability determination behind it is unsupported, and the more antimicrobial the product, the more likely the clean result is an artefact.
| Sterility. A low count is not an absence. |
| Endotoxin, which persists after the organisms that produced it are dead. |
| Viruses, and organisms that will not grow on the media used. |
| Chemical identity, purity or content. |
Every method has a blind spot, and the blind spots are why a certificate lists what was not determined alongside what was. A reader who sees only results will read silence as reassurance.
Is TAMC and TYMC a sterility test?
No. TAMC and TYMC are quantitative enumeration tests under USP <61> for non-sterile products, reported in colony forming units. Sterility is USP <71>, a separate qualitative test with fourteen-day incubation under aseptic conditions.
Why report counts instead of pass or fail?
Because the count is the information. A product at 10 CFU per gram and one at 900 CFU per gram may both sit under a specification, but they say very different things about the manufacturing process.
What is method suitability and why does it matter?
It is a demonstration that the counting method can actually recover organisms from that specific product. Antimicrobial products suppress growth, and without a suitability check a clean plate may reflect the chemistry of the sample rather than its bioburden.
Which organisms are tested for under USP <62>?
Typically Escherichia coli, Salmonella species, Pseudomonas aeruginosa, Staphylococcus aureus, bile-tolerant Gram-negative bacteria and Candida albicans, selected according to route of administration and product type.
References
- USP <61>, Microbiological Examination of Non-Sterile Products: Microbial Enumeration Tests
- USP <62>, Tests for Specified Microorganisms
- European Pharmacopoeia 2.6.12 and 2.6.13
Sources
Related determinations
Updated 2026-09-01