How to tell if a probiotic is alive? A complete guide to viability

October 19, 2025 •

6 min read

According to the National Institutes of Health, manufacturers are not required to list the number of viable microorganisms on probiotic labels, making it hard for consumers to assess product quality. Learning how to tell if a probiotic is alive is crucial for ensuring you receive the intended health benefits from your supplement.

Quick Summary

This guide helps you identify high-quality probiotics by teaching you to decode labels, understand viability metrics, and recognize signs of an effective supplement. Learn to distinguish viable products from those that offer little benefit.

Key Points

Check for Guaranteed Potency: A quality probiotic guarantees its CFU count through the expiration date, not just at the time of manufacture.
Understand Viability Metrics: Learn the difference between CFU and the more precise AFU (Active Fluorescent Units) to assess potency.
Prioritize the Delivery System: Look for acid-resistant capsules or microencapsulation, which protect bacteria from stomach acid.
Monitor Your Body's Response: The most reliable sign of an effective probiotic is improvement in digestion, mood, energy, and immunity.
Verify Strain Specificity: Look for products listing the full strain name, as benefits are often strain-specific and backed by clinical research.
Confirm Proper Storage: Ensure the product is stored according to its label, as probiotics are sensitive to heat and moisture.

Why Probiotic Viability Matters

Probiotics, by definition, are "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host". This means that for a probiotic product to be effective, its bacteria must be alive and in sufficient numbers to survive the journey through the digestive system and colonize the gut. The health benefits associated with probiotics—such as improved digestion, enhanced immune function, and better mood—are directly tied to the activity of these live cultures. A probiotic supplement containing a large number of dead bacteria will not provide the same benefits as a supplement with a high count of viable, active microorganisms. However, some research suggests that dead bacterial cells, or postbiotics, may offer some immunological benefits, though they do not fit the traditional definition of probiotics.

Deciphering the Probiotic Label: CFU vs. AFU

One of the first steps in determining if a probiotic is alive is to carefully read the product label. The potency of a probiotic is typically measured in either Colony-Forming Units (CFUs) or Active Fluorescent Units (AFUs).

Colony-Forming Units (CFUs)

CFU is the traditional method for measuring probiotic potency. It counts the number of viable, active cells that can grow and form a colony on a culture plate under specific lab conditions. While widely used, the CFU method has several limitations:

Underestimation: It can underestimate the total number of viable cells because some bacteria, called "viable but non-culturable" (VBNC), are alive but won't grow on a culture plate.
Inaccuracy: The count can vary depending on the lab conditions and the plating process itself.
Time of Measurement: Many companies list the CFU count "at time of manufacture," which doesn't account for natural die-off during storage and transportation. Reputable brands will guarantee potency "through expiration".

Active Fluorescent Units (AFUs)

AFU is a more modern and precise measurement method, often utilizing flow cytometry. This technology counts all viable cells, including VBNC cells, by using fluorescent markers to assess membrane integrity. The benefits of AFU include:

Higher Accuracy: Provides a more complete count of all viable bacteria, offering a more precise reflection of the product's potency.
Faster Results: It is a faster process than traditional plate counting.
Strain Differentiation: Allows for the identification of different strains within a mixed-culture product.

Comparison of Viability Metrics


Feature	CFU (Colony-Forming Units)	AFU (Active Fluorescent Units)
Method	Plate counting (culturing)	Flow cytometry (fluorescent markers)
Cells Measured	Culturable (replicating) cells only	All viable cells, including viable but non-culturable (VBNC)
Accuracy	Can underestimate potency; variable results	More precise and comprehensive count
Process Time	Slower; involves waiting for colonies to form	Rapid, high-throughput analysis
Label Claim	Often "at time of manufacture"	Guaranteed potency "through expiration"

How to Assess a Probiotic's Viability

Beyond the label, several factors can affect a probiotic's ability to remain alive and effective. Look for these signs of a high-quality product.

The Shelf-Life Guarantee

This is perhaps the most important indicator. Probiotics are sensitive to heat, moisture, and time. A trustworthy manufacturer will guarantee the CFU count through the product's expiration date, rather than just at the time of manufacture. This is a promise that the company has accounted for the natural die-off rate and has formulated the product to remain potent for its entire shelf life.

The Stability and Storage Factor

How a probiotic is stored significantly impacts its viability. Some strains require refrigeration, while others are engineered to be shelf-stable. Check the label for specific storage instructions. A product stored improperly, such as in direct sunlight or a warm location, will likely have reduced viability. When purchasing, look for products that are kept in appropriate conditions, such as refrigerated if required.

The Delivery System

For an oral probiotic to be effective, the bacteria must survive the harsh, acidic environment of the stomach. Many high-quality supplements use advanced delivery systems to protect the live cultures:

Acid-Resistant Capsules: These are designed to withstand stomach acid and dissolve later in the small intestine, where the bacteria can be released safely.
Microencapsulation: Individual bacterial cells are coated with a protective layer, shielding them from stomach acid and environmental stressors.

The Most Reliable Indicator: Your Body's Response

While lab metrics are valuable, the most meaningful sign of an alive and active probiotic is the positive changes you experience in your health. Results vary depending on the strain and your health goals, but look for improvements in:

Digestion: Reduced bloating, less gas, and more regular bowel movements are common signs.
Energy and Mood: A healthy gut-brain axis can lead to more stable energy levels and a calmer mindset.
Immunity: Some strains can support the immune system, leading to fewer sick days.
Skin Health: The gut-skin axis means that a healthier gut can lead to clearer, more radiant skin.

Live vs. Dead Probiotics: Is There a Difference?

As mentioned earlier, some companies produce products using heat-killed or inactivated bacteria. These are sometimes called "paraprobiotics" or "postbiotics". While they don't fit the definition of a probiotic because they are not alive, some studies suggest they may still offer benefits, particularly related to immune modulation. For example, heat-killed bacteria or their components have been shown to influence the immune system. However, the overwhelming majority of clinical research demonstrating efficacy for gastrointestinal issues has focused on viable, live probiotics. If your goal is to support gut flora, viable cells are paramount.

At-Home 'Tests' for Probiotic Activity

Some viral articles and online forums suggest simple at-home tests, like the milk test, to check for probiotic viability. This test involves adding a probiotic capsule's contents to milk and observing if it thickens or curdles, a sign of fermentation.

Caveat: The milk test is largely unreliable and should not be used as a definitive indicator. Many probiotic strains are not designed to ferment milk and will show no activity, even if perfectly viable. The test is only relevant for specific dairy-fermenting strains and is not accurate for the wide variety of probiotic species on the market. The best way to evaluate a product is by its labeling and your body's response, not an unscientific kitchen experiment.

What to Look for in a High-Quality Probiotic

To ensure you are getting a high-quality, viable product, keep the following in mind:

Specific Strains: Look for products that list the full strain designation (e.g., Bifidobacterium longum BB536), not just the genus and species. Specific strains have specific, researched benefits.
Clinical Evidence: Reputable brands will use strains that have been studied in human clinical trials for the benefits you seek.
Guaranteed Potency: Ensure the company guarantees the potency through expiration, not just at the time of manufacture.
Advanced Delivery: Opt for products with proven delivery systems, like acid-resistant capsules, that protect the bacteria from stomach acid.
Third-Party Testing: Choose brands that use third-party testing to verify purity and potency.
Appropriate Storage: Follow the product's storage instructions, whether that requires refrigeration or if it's shelf-stable.

For further reading on the scientific criteria for probiotics, the International Scientific Association for Probiotics and Prebiotics (ISAPP) offers valuable resources.

Conclusion

Determining if a probiotic is alive and effective is more complex than a simple at-home test. The best indicators are found on the product label itself and in your body's response to the supplement. By looking for guaranteed potency through expiration, a robust delivery system, and specific, clinically-researched strains, you can confidently choose a product that delivers a sufficient dose of viable, active cultures. Ultimately, the most reliable proof is a noticeable improvement in your digestive health and overall well-being, confirming the live microbes are doing their job.

Frequently Asked Questions

Not necessarily. While CFU count indicates potency, a high CFU number doesn't guarantee effectiveness if the bacteria are dead or can't survive transit through the gut. Factors like the specific strain, delivery system, and guaranteed viability through expiration are more important.

While a milk test is a common DIY suggestion, it is not reliable. It only tests for milk-fermenting bacteria, and many effective probiotic strains will not cause milk to curdle, even if they are perfectly viable. Use label information and your body's response instead.

CFU (Colony-Forming Units) is a traditional measure counting bacteria that can multiply in a lab dish, potentially underestimating the total viable count. AFU (Active Fluorescent Units), using modern technology like flow cytometry, provides a more precise count of all viable cells, including those that are viable but non-culturable.

It depends on the specific product. Some probiotic strains are shelf-stable and can be stored at room temperature, while others require refrigeration to maintain viability. Always check the product label for specific storage instructions.

The time it takes to notice effects varies. Many people report digestive improvements within a few weeks, while effects on mood or immunity may take longer, often 4 to 6 weeks or more. Consistent, daily use is key.

While the clinical definition of a probiotic requires it to be alive, some studies show that dead bacteria or their components (postbiotics) can still have beneficial effects, especially related to immune modulation. However, the most well-documented benefits for gut health are from viable organisms.

If you don't notice improvements after consistent use (4+ weeks), consider switching brands or strains. You might need a different strain for your specific health goals or a product with a more effective delivery system. Consult a healthcare provider knowledgeable about probiotics for personalized advice.