In the CFU vs. strains question, strain identity usually matters more. CFU (colony-forming units) tells you how many live bacteria a serving is designed to deliver. The strain, written as genus, species and a code such as La-14, tells you which bacteria they are, and research results belong to specific strains. A good label gives you both, and a higher CFU number is not automatically a better probiotic. Here is how to read each one.
Key takeaways
- CFU stands for colony-forming units, the standard count of live, viable bacteria in a probiotic.
- A strain is named in three parts: genus, species and an alphanumeric strain code.
- Products with higher CFU counts are not necessarily more effective than products with lower counts, according to the NIH Office of Dietary Supplements.
- Research findings belong to the strain that was studied, so a strain code on the label is worth more than a bigger number.
What does CFU mean on a probiotic label?
CFU stands for colony-forming units, the standard count of live, viable bacteria in a probiotic. In the lab, a sample is spread on a growth plate, and each live cell that can multiply forms a visible colony that can be counted. Dead cells do not form colonies, so CFU is a measure of living bacteria, not of weight.
Labels write the number in two ways: as words ("40 billion CFU") or in scientific shorthand, where 1 x 109 means 1 billion and 1 x 1010 means 10 billion. The NIH Office of Dietary Supplements (ODS) notes that many probiotic supplements contain 1 to 10 billion CFU per dose, while some contain 50 billion or more.
Because probiotics are alive, the count can fall over time, especially with heat and moisture. ODS advises looking for products labeled with the CFU at the end of shelf life rather than at the time of manufacture. Storage matters too; our guide on whether probiotics need to be refrigerated covers heat, humidity and the bathroom cabinet.
What is a probiotic strain code?
A strain is named in three parts: genus, species and an alphanumeric strain code. Take Lactobacillus acidophilus La-14:
- Genus: Lactobacillus - the broad family group.
- Species: acidophilus - one kind within that group.
- Strain code: La-14 - one specific, deposited line of that species, the way a single named variety differs from others of the same plant.
Two products can both say L. acidophilus and contain different strains. Only the code tells you which one is inside. You may also see newer names on some labels: in 2020, scientists split the old Lactobacillus group into many new genera, so Lactobacillus plantarum is now often written Lactiplantibacillus plantarum. The strain code does not change with the new name.
CFU vs. strains: which matters more?
Research findings belong to the strain that was studied, so a strain code on the label is worth more than a bigger number. The National Center for Complementary and Integrative Health (NCCIH) puts it plainly: if one kind of Lactobacillus has a particular effect, that does not mean another kind of Lactobacillus, or any Bifidobacterium, will do the same. ODS adds that because the effects of probiotics can be specific to certain strains, recommendations for their use in the clinic need to be strain-specific.
CFU still matters. A strain has to arrive in large enough live numbers to do anything at all, and the amount studied for one strain is not a guide for another. Think of it this way:
| Question | CFU answers | Strain code answers |
|---|---|---|
| What is it? | How many live bacteria per serving | Which exact bacteria they are |
| Can you compare products with it? | Only roughly; counting methods and shelf-life dates differ | Yes; the same code means the same strain |
| Does it link to research? | Not by itself | Yes; studies are run on named strains |
| Is bigger better? | Not necessarily | Not applicable; fit and evidence matter |
Is a higher CFU count always better?
No. Products with higher CFU counts are not necessarily more effective than products with lower counts, according to the NIH Office of Dietary Supplements. A bigger number can make a label look stronger without saying anything about whether the strains are the right ones, whether they survive the trip through the stomach, or whether they are still alive on the day you open the bottle. More strains are not automatically better either; a long list can simply mean less of each. What helps more is knowing which strains are inside, how the formula protects them, and how it was tested.
How to read CFU and strains on a real label
Here is the label of our own Probiotic 40+ as a worked example. It lists 40 billion CFU per 2-capsule serving across four named strains: Lactobacillus acidophilus La-14, Bifidobacterium lactis Bl-04, Lactobacillus plantarum Lp-115 and Lactobacillus paracasei Lpc-37. The strains sit in a proprietary blend. Blend totals are on the label; the amounts inside each blend are not. In other words, you can see the 40 billion CFU total and every strain code, but not the count for each strain.
The directions say to take two capsules once daily (for best results, one during the day and one in the evening) and not to exceed two capsules per day. The CAUTION panel on the label reads: "Do not exceed recommended dose. Pregnant or nursing mothers, children under the age of 18, and individuals with a known medical condition should consult a physician before using this or any dietary supplement." NCCIH notes that the risk of harmful effects from probiotics is greater in people with severe illnesses or compromised immune systems, so talk with your doctor first if that applies to you.
The same label lists MAKTREK (Bi-Pass Technology), which the product page says is there so more of the bacteria survive stomach acid, plus two prebiotics: complex marine polysaccharide (60 mg) and fructooligosaccharide (24 mg). The label's other ingredients are hypromellose (vegetable capsule), rice maltodextrin and L-leucine. The product page adds that the rice maltodextrin is corn-derived and the L-leucine is sourced from duck feathers, so the product is not suitable for a plant-based diet. Every batch is third-party tested for purity and potency; our standards page explains how.
Use this checklist on any probiotic label:
- Full strain names. Genus, species and a strain code for every organism.
- CFU per serving. And how many capsules make a serving.
- Shelf-life wording. Whether the count is stated at manufacture or through the date on the bottle.
- Delivery and storage. How the bacteria are protected and how to store the bottle.
- Other ingredients. Fillers and sources that matter for your diet.
- Testing. Whether an outside lab checks each batch.
Frequently asked questions about CFU vs. probiotic strains
Is 40 billion CFU too much?
Many probiotic supplements contain 1 to 10 billion CFU per dose and some contain 50 billion or more, according to the NIH Office of Dietary Supplements. The right amount is the one on the label you are following. For Probiotic 40+, the directions say to take two capsules once daily and not to exceed two capsules per day, and the CAUTION panel on its label reads: "Do not exceed recommended dose. Pregnant or nursing mothers, children under the age of 18, and individuals with a known medical condition should consult a physician before using this or any dietary supplement." The National Center for Complementary and Integrative Health notes that the risk of harmful effects from probiotics is greater in people with severe illnesses or compromised immune systems, so talk with your doctor first if that applies to you.
Do more strains mean a better probiotic?
Not automatically. Each strain has its own research record, and a long list can mean smaller amounts of each. Named strains with a clear total count are more useful than a long list of species names.
What does the letter-and-number code after a probiotic name mean?
It is the strain designation, such as La-14 or Bl-04. It identifies one specific line of bacteria within a species, so you can match the label to research on that exact strain.
Does the CFU count drop over time?
It can. Probiotics are living organisms and some die during storage, especially with heat and moisture. The NIH Office of Dietary Supplements suggests choosing products that state the count at the end of shelf life, and storing the bottle as the label says.
Should I choose a probiotic by CFU or by strain?
Start with the strains, then check the count. Strain codes tell you which bacteria you are getting and link them to research; the CFU tells you how many. Our guide to how long probiotics take to work covers what to expect after you start.
Related reading from our Wellness Hub
- Inside Probiotic 40+: 40 Billion CFU and Four Strain Codes Explained
- How Long Do Probiotics Take to Work?
- Do Probiotics Need to Be Refrigerated? Storage, Heat and Shelf Life
- Prebiotics vs. Probiotics vs. Synbiotics: A Plain-English Guide
Sources
- NIH Office of Dietary Supplements - Probiotics fact sheet for health professionals
- NCCIH - Probiotics: usefulness and safety
- Hill C, et al. Expert consensus document. The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol, 2014 (PubMed)
- Zheng J, et al. A taxonomic note on the genus Lactobacillus: Description of 23 novel genera, emended description of the genus Lactobacillus Beijerinck 1901, and union of Lactobacillaceae and Leuconostocaceae. Int J Syst Evol Microbiol, 2020 (PubMed)
Individual results vary.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
