Last reviewed: October 2026 by Dr. Farrukh Mehmood, Ph.D., 16+ years in pharmaceutical QC/QA
Key takeaways
- The total microbial count of a non-sterile product has two parts: TAMC (total aerobic microbial count, on Soybean-Casein Digest Agar at 30–35 °C for 3–5 days) and TYMC (total combined yeasts and moulds count, on Sabouraud Dextrose Agar at 20–25 °C for 5–7 days).
- The usual starting point is 10 g or 10 mL of product in 90 mL of diluent, which gives a 1:10 (10−1) dilution.
- CFU/g = mean colonies ÷ (volume plated in mL × dilution). With 1 mL of the 1:10 dilution, multiply the mean count by 10.
- Count plates with the highest number of colonies below 250 (TAMC) or below 50 (TYMC).
- Acceptance criteria come from USP <1111>. A limit of 103 CFU/g means a maximum acceptable count of 2,000.
The total microbial count test measures the number of viable aerobic bacteria and fungi in a non-sterile pharmaceutical product or raw material. It is one of the most common tests in a QC microbiology lab and is described in USP <61> (Ph. Eur. 2.6.12), with acceptance criteria in USP <1111> (Ph. Eur. 5.1.4). This guide covers the method step by step, the counting rules, a worked calculation, the full acceptance criteria table and a free checklist.
Free Total Microbial Count checklist (PDF)
Materials, step-by-step procedure and review points on one page.
TAMC and TYMC: what is counted
| TAMC | TYMC | |
|---|---|---|
| Full name | Total aerobic microbial count | Total combined yeasts and moulds count |
| Medium | Soybean-Casein Digest Agar (SCDA / TSA) | Sabouraud Dextrose Agar (SDA) |
| Incubation | 30–35 °C for 3–5 days | 20–25 °C for 5–7 days |
| What is counted | All colonies on SCDA, including any fungi | All colonies on SDA, including any bacteria |
| Counting limit per plate | Fewer than 250 colonies | Fewer than 50 colonies |
If bacteria growing on SDA would push the TYMC over its limit, USP <61> allows Sabouraud Dextrose Agar containing antibiotics.
Materials
- Diluent: buffered sodium chloride–peptone solution pH 7.0, phosphate buffer pH 7.2, or Soybean-Casein Digest Broth. Add neutralizers or surfactants (for example polysorbate 80) if the product requires them.
- Soybean-Casein Digest Agar and Sabouraud Dextrose Agar that have passed the growth promotion test
- Sterile Petri dishes (9 cm), pipettes, dilution bottles and a balance
- Water bath at about 45 °C for molten agar
- Incubators at 30–35 °C and 20–25 °C
- Biosafety cabinet or laminar air flow unit
- Membrane filtration unit (0.45 µm), if the product requires it
Sample preparation
- Water-soluble products (tablets, capsules, powders, syrups): dissolve or suspend 10 g or 10 mL in diluent to make 100 mL. This is the 10−1 dilution.
- Non-fatty products insoluble in water: suspend 10 g or 10 mL in diluent to 100 mL. A surfactant such as 1 g/L polysorbate 80 helps wetting.
- Fatty products (ointments, creams): dissolve in isopropyl myristate sterilized by filtration, or mix with a minimal quantity of sterile polysorbate 80 heated to no more than 40 °C (45 °C in exceptional cases), then dilute with warm diluent.
- Transdermal patches: remove the liners, place the adhesive side on sterile gauze and extract in diluent containing polysorbate 80.
- Make further tenfold dilutions (10−2, 10−3) if high counts are expected. See the serial dilution guide.
Method 1: Pour plate method
- Pipette 1 mL of the 10−1 dilution into each of at least two sterile Petri dishes for TAMC, and two more for TYMC.
- Add 15–20 mL of molten SCDA (TAMC plates) or SDA (TYMC plates), cooled to not more than 45 °C.
- Swirl gently to mix the sample and agar evenly, and let it set.
- Invert and incubate: SCDA at 30–35 °C for 3–5 days, SDA at 20–25 °C for 5–7 days.
- Include a negative control: diluent plated the same way.
Method 2: Surface-spread method
- Use pre-poured, surface-dried plates of SCDA and SDA (15–20 mL of medium in a 9 cm dish).
- Spread not less than 0.1 mL of the dilution over the surface of each plate with a sterile spreader. Use at least two plates per medium per dilution.
- Incubate and count as for the pour plate method.
Remember that a 0.1 mL spread plate represents ten times less sample than a 1 mL pour plate. Include the plated volume in the calculation.
Other methods: membrane filtration and MPN
Membrane filtration is used for products with antimicrobial activity, or when low limits need a large sample volume. A quantity equivalent to 1 g of product is filtered through a 0.45 µm membrane. The membrane is rinsed to remove the inhibitory substance, then transferred to SCDA (TAMC) or SDA (TYMC). The most probable number (MPN) method is the least precise and accurate. Use it for TAMC only when no other method is suitable. It is not used for TYMC.
Method suitability: prove the method recovers organisms
Before routine testing, show that the product does not inhibit growth under the test conditions. Inoculate the prepared sample with not more than 100 CFU each of S. aureus ATCC 6538, P. aeruginosa ATCC 9027, B. subtilis ATCC 6633, C. albicans ATCC 10231 and A. brasiliensis ATCC 16404. The mean count recovered must not differ by more than a factor of 2 from the control without product. If recovery fails, increase the dilution, add neutralizers or switch to membrane filtration. Repeat the suitability test whenever the method or the product formulation changes.
How to calculate TAMC and TYMC
CFU per g (or mL) = mean colonies per plate ÷ (volume plated in mL × dilution of that tube)
With 1 mL plated from the 10−1 dilution: CFU/g = mean colonies × 10. With 0.1 mL plated from the 10−1 dilution: CFU/g = mean colonies × 100.
Worked example: uncoated tablet (non-aqueous oral preparation)
10 g of crushed tablets is dispersed in 90 mL of buffered sodium chloride–peptone solution (10−1). 1 mL is pour-plated in duplicate on SCDA and SDA.
| Count | Plate 1 | Plate 2 | Mean | × dilution factor | Result | USP <1111> maximum | Verdict |
|---|---|---|---|---|---|---|---|
| TAMC (SCDA) | 23 | 27 | 25 | × 10 | 250 CFU/g | 2,000 CFU/g | Complies |
| TYMC (SDA) | 4 | 2 | 3 | × 10 | 30 CFU/g | 200 CFU/g | Complies |
No growth: if no colonies appear on plates of the 10−1 dilution (1 mL plated), report the result as less than 10 CFU/g.
TAMC and TYMC calculator
Result per g or mL. Maximum acceptable counts: 10¹ → 20, 10² → 200, 10³ → 2,000, 10⁴ → 20,000. Check counts are within the countable range (below 250 for TAMC, below 50 for TYMC) before relying on the result.
Acceptance criteria for non-sterile dosage forms (USP <1111>)
| Route of administration | TAMC (CFU/g or CFU/mL) | TYMC (CFU/g or CFU/mL) | Specified microorganisms |
|---|---|---|---|
| Non-aqueous preparations for oral use | 10³ | 10² | Absence of Escherichia coli (1 g or 1 mL) |
| Aqueous preparations for oral use | 10² | 10¹ | Absence of Escherichia coli (1 g or 1 mL) |
| Rectal use | 10³ | 10² | – |
| Oromucosal use | 10² | 10¹ | Absence of Staphylococcus aureus and Pseudomonas aeruginosa (1 g or 1 mL) |
| Gingival use | 10² | 10¹ | Absence of S. aureus and P. aeruginosa (1 g or 1 mL) |
| Cutaneous use | 10² | 10¹ | Absence of S. aureus and P. aeruginosa (1 g or 1 mL) |
| Nasal use | 10² | 10¹ | Absence of S. aureus and P. aeruginosa (1 g or 1 mL) |
| Auricular use | 10² | 10¹ | Absence of S. aureus and P. aeruginosa (1 g or 1 mL) |
| Vaginal use | 10² | 10¹ | Absence of P. aeruginosa, S. aureus and Candida albicans (1 g or 1 mL) |
| Transdermal patches (limits for one patch, including adhesive layer and backing) | 10² | 10¹ | Absence of S. aureus and P. aeruginosa (1 patch) |
| Inhalation use (special requirements apply to liquid preparations for nebulization) | 10² | 10¹ | Absence of S. aureus, P. aeruginosa and bile-tolerant Gram-negative bacteria (1 g or 1 mL) |
| Ph. Eur. special provision: oral dosage forms containing raw materials of natural (animal, vegetal or mineral) origin for which antimicrobial pretreatment is not feasible | 10⁴ | 10² | Additional limits for bile-tolerant Gram-negative bacteria and absence of Salmonella, E. coli and S. aureus. See Ph. Eur. 5.1.4 for the quantities. |
For non-sterile substances (APIs and excipients) for pharmaceutical use, the general criteria are TAMC 10³ CFU/g and TYMC 10² CFU/g, unless the monograph says otherwise.
How to interpret 10¹, 10² and 10³
| Acceptance criterion | Maximum acceptable count |
|---|---|
| 10¹ CFU | 20 |
| 10² CFU | 200 |
| 10³ CFU | 2,000 |
| 10⁴ CFU | 20,000 |
Testing for the specified microorganisms (for example the absence of E. coli) is a separate test described in USP <62>. It uses enrichment broths and selective media, not the TAMC/TYMC plates.
Documentation and reporting
- Record the sample details, the weight or volume taken, the diluent and its volume, the dilutions plated, the medium batch numbers (with GPT status), and the incubator IDs with start and end times.
- Record each plate count, the mean, the calculated CFU/g or CFU/mL, and the countable-range check.
- Report the TAMC and TYMC separately, with units, against the specification.
- Report negative results as “less than” the detection limit (for example <10 CFU/g), not as zero.
- A result above the limit is an OOS result. Investigate it according to your OOS procedure.
Common mistakes
- Spreading 1 mL on a surface plate. It cannot be absorbed evenly. Spread plates take about 0.1 mL.
- Forgetting to correct for the plated volume or the dilution.
- Counting crowded plates (more than 250 colonies on TSA, or more than 50 on SDA) instead of using a higher dilution.
- Skipping method suitability for a new product, so antimicrobial activity goes undetected and counts are falsely low.
- Using media without a passing growth promotion test.
- Reporting “0 CFU/g” instead of “<10 CFU/g”.
Frequently asked questions
What is the difference between TAMC and TYMC?
TAMC is the total count of colonies on Soybean-Casein Digest Agar after 3–5 days at 30–35 °C. It is mainly bacteria, but fungi growing on it are also counted. TYMC is the total count on Sabouraud Dextrose Agar after 5–7 days at 20–25 °C, mainly yeasts and moulds.
What does a limit of 10³ CFU/g mean?
Under USP <1111>, a criterion of 10³ CFU/g is interpreted as a maximum acceptable count of 2,000 CFU/g.
Why multiply the colony count by 10?
Because 10 g of product in 90 mL of diluent is a 1:10 dilution. 1 mL of it contains 0.1 g of product, so the colonies on that plate come from 0.1 g. Multiplying by 10 converts the count to CFU per gram.
Pour plate or spread plate: which is better?
Both are pharmacopoeial. The pour plate tests a larger volume (1 mL), so it has a lower detection limit. The spread plate keeps colonies on the surface, which makes them easier to see and pick for identification.
Is the total microbial count the same as the test for specified microorganisms?
No. USP <61> counts all aerobic bacteria and fungi. USP <62> checks for the absence of specific objectionable organisms such as E. coli, Salmonella, P. aeruginosa, S. aureus and C. albicans.
Related guides
- Growth promotion test (GPT) of culture media
- Serial dilution method in microbiology (with calculator)
- Microbiological testing of primary packaging material
- OOS procedure in pharma
References
- USP General Chapter <61> Microbiological Examination of Nonsterile Products: Microbial Enumeration Tests.
- USP General Chapter <62> Microbiological Examination of Nonsterile Products: Tests for Specified Microorganisms.
- USP General Chapter <1111> Microbiological Examination of Nonsterile Products: Acceptance Criteria for Pharmaceutical Preparations and Substances for Pharmaceutical Use.
- Ph. Eur. 2.6.12 and 5.1.4 (harmonized equivalents).
Part of our QC microbiology guide, which maps every microbiology test, guide and tool on AskPharmaTutor in the order a lab uses them.
