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Glossary

What are microbial limits for dietary supplements?

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

Microbial limits are the highest counts of bacteria, yeasts and molds a supplement batch is allowed to contain, often with tests for specific organisms such as Salmonella and E. coli. FDA's 2007 rule makes each maker set limits on contamination for every finished batch (21 CFR 111.70), and labs report counts as colony forming units.1,2,3

Key facts

  • FDA's rule defines microorganisms as yeasts, molds, bacteria, viruses and similar organisms of public health or sanitary concern, including those that make a product decompose or signal filth.4
  • The aerobic plate count, a method in FDA's lab manual, is meant to show the overall level of microorganisms in a product.3
  • Most mold species can grow at a water activity of 0.85 or less, drier than most yeasts can manage.5
  • Supplements have been recalled for microbiological contamination, one of the problems FDA cited when it wrote its 2007 rule.6
  • In a Polish survey, 6.5% of food supplement samples had microbiological impurities.7

What gets counted

  • Total aerobic count: a broad count of microorganisms that grow in air, as an overall measure of microbial load.3
  • Yeasts and molds: fungi that can spoil products and, in some cases, produce toxins called mycotoxins.5
  • Specific organisms such as Salmonella and E. coli, which FDA's lab manual has dedicated methods for.2

Counts are reported in colony forming units (CFU): the colonies that grow on a plate, divided by the amount of sample spread on it.3

Why plant ingredients need watching

Botanicals come from crops, which molds can invade in the field and in storage. Reviews find that supplements made with plant materials commonly carry a range of bacteria and fungi, some of them potentially harmful.5,8

An older study of eight kinds of botanical supplements found bacteria, fungi or both in most samples. It recorded only whether microbes were present, not how many.9

Molds can leave mycotoxins behind. FDA's manual notes these are stable compounds that processing and cooking do not destroy, and a 2025 review found herbs, spices and plant extract supplements can carry several at once.5,10

How makers keep counts low

FDA's rule requires manufacturing under conditions that guard against microbial growth, using means such as sterilizing, pasteurizing, freezing or refrigeration, and control of pH, humidity or water activity.11

Water activity is a measure of the free moisture in a material. Because molds tolerate fairly dry conditions, keeping powders and capsules sealed and away from humidity matters even for dry products.4,5

The rule has each maker set its own microbial limits rather than listing numbers. Researchers in Poland described the same gap under their food law.1,7

Probiotics are a special case

A probiotic contains live microbes on purpose, so its count of the labeled strains is a measure of strength, not contamination. Testing it means counting and identifying the intended organisms and checking that no harmful ones are present.12

Who should be most careful

Serious fungal infections from contaminated food or supplements appear to be rare. A CDC review found they occur mainly in people with weakened immune systems, and three of the publications it identified involved supplements.13

Guidelines for people at high risk recommend avoiding some foods that may carry high levels of fungi. If your immune system is weakened, ask your doctor before starting any supplement.13

Questions people ask

Are microbes in a supplement always a problem?

Not always. Probiotics contain live microbes on purpose, and studies find some bacteria or fungi in many plant based products. Limits set how many are acceptable and which organisms should be absent.9,12

What does CFU mean on a lab report?

Colony forming unit. It is the unit for microbial counts from plate methods, and on a contaminant test a lower number is better. On a probiotic label it counts the live organisms you are meant to get.3,12

Is there one legal microbial limit for all supplements?

Not in FDA's rule, which has each maker establish limits on contamination for its own products.1

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Sources

  1. 21 CFR 111.70. What specifications must you establish? Opened 2026-10-07.
  2. U.S. Food and Drug Administration. Bacteriological Analytical Manual (BAM). Opened 2026-10-07.
  3. Zhang G, Ferguson M, Blodgett RJ. Bacteriological Analytical Manual, Chapter 3: Aerobic Plate Count. U.S. Food and Drug Administration, January 2026 edition. Opened 2026-10-07.
  4. 21 CFR 111.3. What definitions apply to this part? Opened 2026-10-07.
  5. Tournas V, Stack ME, Mislivec PB, Koch HA, Bandler R. Bacteriological Analytical Manual, Chapter 18: Yeasts, Molds and Mycotoxins. U.S. Food and Drug Administration, April 2001 edition. Opened 2026-10-07.
  6. U.S. Food and Drug Administration. Backgrounder on the Final Rule for Current Good Manufacturing Practices (CGMPs) for Dietary Supplements. June 22, 2007. Opened 2026-10-07.
  7. Ratajczak M, Kubicka MM, Kamińska D, Długaszewska J. Microbiological quality of food supplements. Acta Pol Pharm. 2015;72(2):383-387. Opened 2026-10-07.
  8. Ratajczak M, Kaminska D, Światły-Błaszkiewicz A, Matysiak J. Quality of dietary supplements containing plant-derived ingredients reconsidered by microbiological approach. Int J Environ Res Public Health. 2020;17(18):6837. Opened 2026-10-07.
  9. Raman P, Patino LC, Nair MG. Evaluation of metal and microbial contamination in botanical supplements. J Agric Food Chem. 2004;52(26):7822-7827. Opened 2026-10-07.
  10. Kanabus J, Bryła M, Leśnowolska-Wnuczek K, Waśkiewicz A, Twarużek M. Mycotoxins occurrence in herbs, spices, dietary supplements, and their exposure assessment. Toxins (Basel). 2025;18(1):20. Opened 2026-10-07.
  11. 21 CFR 111.365. What precautions must you take to prevent contamination? Opened 2026-10-07.
  12. Zawistowska-Rojek A, Zaręba T, Tyski S. Microbiological testing of probiotic preparations. Int J Environ Res Public Health. 2022;19(9):5701. Opened 2026-10-07.
  13. Benedict K, Chiller TM, Mody RK. Invasive fungal infections acquired from contaminated food or nutritional supplements: a review of the literature. Foodborne Pathog Dis. 2016;13(7):343-349. Opened 2026-10-07.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.