Lactic acid bacteria (LAB) viable count determination is a core microbiological experiment used to quantify culturable colony-forming units (CFU) in probiotic supplements, fermented dairy products, beverages, starter cultures, and feed additives. Accredited third-party laboratories perform this test in accordance with recognized standards such as GB 4789.35-2016 and the ISO 15214 series to verify label claims, support quality control, and demonstrate regulatory compliance. This article explains the technical principle, applicable sample types, testing items, laboratory methods, and practical applications of the LAB viable count determination experiment.
Technical Overview
Lactic acid bacteria (LAB) are a functional group of Gram-positive, non-spore-forming, catalase-negative bacteria that ferment carbohydrates into lactic acid. The group primarily includes Lactobacillus, Lactococcus, Streptococcus, Pediococcus, Leuconostoc, and Weissella, while Bifidobacterium is frequently enumerated alongside LAB in commercial probiotic testing due to its common inclusion in probiotic formulations.
The viable count determination experiment measures the number of metabolically active, culturable cells capable of forming visible colonies on selective agar media under defined atmospheric and temperature conditions. Results are reported as colony-forming units per gram or per milliliter (CFU/g or CFU/mL). Because only living cells are enumerated, this experiment is the definitive tool for verifying probiotic potency, monitoring fermentation quality, and confirming that products retain the minimum viable counts required by regulation throughout their shelf life.
The experiment is typically performed according to the following methodological framework:
- GB 4789.35-2016: National food safety standard for the examination of lactic acid bacteria in foods, including enumeration of total LAB, Lactobacillus delbrueckii subsp. bulgaricus, and Streptococcus thermophilus
- GB 4789.34-2016: National food safety standard for the examination and identification of Bifidobacterium
- ISO 15214 series: Horizontal method for the enumeration of mesophilic lactic acid bacteria by colony-count technique at 30 °C
- ISO 7889: Enumeration of characteristic microorganisms in yogurt by colony-count technique at 37 °C
- ISO 20128: Enumeration of presumptive Lactobacillus acidophilus on a selective medium
- ISO 29981: Enumeration of presumptive Bifidobacterium by colony-count technique at 37 °C
- ISO 19344: Enumeration of lactic acid bacteria in milk products by flow cytometry
- ISO 6887 series and ISO 7218: General rules for the preparation of test samples, initial suspensions, and decimal dilutions, and for microbiological examination
Testing Samples
Probiotic Dietary Supplements
Single-strain and multi-strain products including powders, capsules, tablets, sachets, gummies, and effervescent formulations. These products typically declare high potency levels, often ranging from 10^8 to over 10^11 CFU per serving, requiring extended dilution series during analysis.
Fermented Dairy Products
Set and stirred yogurt, drinking yogurt, fermented milk, kefir, sour cream, and ripened or fresh cheese, in which LAB function as characteristic starter organisms and contribute to texture, flavor, and shelf stability.
Lactic Acid Bacteria Beverages
Live (unpasteurized) and non-live (pasteurized) lactic acid bacteria drinks, probiotic juice-based beverages, and fermented plant-based dairy alternatives such as soy, oat, or coconut yogurts.
Other Fermented Foods
Traditional fermented vegetables such as sauerkraut, kimchi, and pickles, as well as fermented soy products, sourdough, fermented sausages, and fermented teas, where LAB counts serve as quality and process-control indicators.
Starter Cultures and Direct Vat Inoculation Products
Concentrated freeze-dried, deep-frozen, or protected liquid starter cultures used by dairy and food manufacturers, which require precise potency verification before release.
Animal Feed and Feed Additives
Probiotic premixes, silage inoculants, and direct-fed microbial products for livestock, poultry, and aquaculture, where viable count determines declared efficacy and dosage.
Research and Bio-Ingredient Samples
Laboratory fermentation broths, cell concentrates, fermented botanical extracts, and postbiotic development samples supplied in sterile containers for strain screening and process optimization studies.
Testing Items
The scope of testing is tailored to the product matrix, strain composition, and regulatory purpose. Common testing items include:
- Total lactic acid bacteria viable count (CFU/g or CFU/mL)
- Enumeration of Lactobacillus delbrueckii subsp. bulgaricus in yogurt and fermented milk
- Enumeration of Streptococcus thermophilus in yogurt and fermented milk
- Enumeration of presumptive Lactobacillus acidophilus
- Enumeration of the Lactobacillus casei group (L. casei, L. paracasei, L. rhamnosus)
- Enumeration of Lactobacillus plantarum and other declared single strains
- Enumeration of presumptive Bifidobacterium species
- Multi-strain composition analysis and individual strain ratio verification
- Viability retention and shelf-life stability testing under defined storage conditions
- Complementary hygiene indicator testing, such as yeasts and molds, coliforms, and total aerobic plate count
- Strain identity confirmation by Gram staining, catalase testing, carbohydrate fermentation profiling, or 16S rRNA gene sequencing
Testing Methods
Sample Preparation and Serial Dilution
Following the general rules of ISO 6887-1 and GB 4789.35-2016, a representative test portion (commonly 25 g or 25 mL) is homogenized in 225 mL of sterile diluent such as 0.85% saline or buffered peptone water using a paddle blender. Ten-fold serial dilutions are then prepared, with the dilution range adjusted to the expected microbial load. For oxygen-sensitive or light-sensitive strains, handling under reduced-oxygen conditions and prompt processing minimize viability loss before analysis.
Selective Plating and Incubation
Selected dilutions are inoculated in duplicate onto appropriate culture media using the pour-plate or surface-spread technique. Typical media include de Man Rogosa Sharpe (MRS) agar for Lactobacillus and total LAB enumeration, M17 agar for Streptococcus thermophilus, acidified MRS agar for Lactobacillus delbrueckii subsp. bulgaricus, and transoligosaccharide propionate agar with antibiotic supplement for presumptive Bifidobacterium. Plates are incubated at 36 ± 1 °C (or 30 °C for the ISO 15214 mesophilic protocol) for 48 to 72 hours. Lactobacillus and Bifidobacterium plates are incubated anaerobically in anaerobic jars or chambers, while Streptococcus thermophilus is typically incubated aerobically or under microaerophilic conditions as specified by the method.
Colony Counting, Calculation, and Confirmation
After incubation, plates yielding 30 to 300 characteristic colonies are counted, duplicate counts are averaged, and the result is multiplied by the corresponding dilution factor to obtain the viable count expressed as CFU/g or CFU/mL. Representative colonies are confirmed by Gram staining, cellular morphology, and the catalase test, since LAB are Gram-positive and catalase-negative. Where required for label claims or dispute resolution, confirmatory identification is performed using carbohydrate fermentation profiles or molecular methods such as 16S rRNA sequencing.
Most Probable Number (MPN) Method
For products expected to contain low levels of LAB, such as pasteurized beverages or environmental samples, the MPN technique may be applied. Multiple replicate tubes of MRS broth are inoculated with decimal dilutions, incubated anaerobically, and positive tubes showing growth and acid production are used to derive the MPN value from standard statistical tables, with confirmation from positive tubes.
Rapid and Alternative Methods
Flow cytometry in accordance with ISO 19344 enables rapid enumeration by fluorescent viability staining and is particularly valuable for high-throughput stability studies. Quantitative PCR combined with viability dyes, automated colony counters, and impedance-based systems can serve as screening tools, although culture-based plate counts remain the reference approach for regulatory submissions and label verification in most jurisdictions.
Quality Control and Result Reliability
Each analytical run includes media sterility controls, diluent blanks, growth promotion testing with certified reference strains, and duplicate plating. Incubators and anaerobic systems are continuously monitored for temperature and atmosphere. Typical inter- and intra-laboratory reproducibility for culture-based LAB enumeration falls within 0.25 to 0.5 log10 units, and this measurement uncertainty is considered when comparing results against declared potency.
Applications
Lactic acid bacteria viable count determination supports a wide range of commercial, regulatory, and scientific objectives:
- Verification of probiotic label claims, such as guaranteed CFU counts at end of shelf life, for dietary supplements and functional foods
- Regulatory compliance demonstration, including the minimum requirement of 1,000,000 CFU/g (mL) of lactic acid bacteria at the end of shelf life for fermented milk under GB 19302-2010 and the corresponding requirement of 1,000,000 CFU/mL for live lactic acid bacteria beverages under GB/T 21732-2008
- Shelf-life and stability studies under real-time and accelerated storage conditions, enabling data-driven expiry dating and cold-chain strategy decisions
- Incoming material inspection and batch release testing in dairy plants, supplement manufacturers, and starter culture production facilities
- Quality troubleshooting, such as investigating viability loss caused by moisture ingress, processing stress, or incompatible excipients
- Research applications including strain screening, survival testing in simulated gastric and intestinal fluids, fermentation optimization, and postbiotic development
- Import and export inspection, e-commerce platform compliance verification, and independent third-party testing for consumer confidence
- Verification of silage inoculants and direct-fed microbial feed additives in animal nutrition programs
FAQ
What is the difference between viable count and total cell count?
Viable count enumerates only cells capable of growth and colony formation on culture media, whereas total cell count includes dead and viable-but-non-culturable cells. Because probiotic efficacy is tied to living cells, viable count expressed in CFU is the internationally recognized metric for potency.
Why can one colony represent more than one cell?
Many lactic acid bacteria grow in chains or clusters, so a single colony may originate from a chain of several cells. This is why results are reported as colony-forming units rather than absolute cell numbers.
How should samples be packaged and shipped to the laboratory?
Refrigerated products should be shipped at 2–8 °C with cold packs, frozen products with dry ice, and freeze-dried powders or capsules in sealed, moisture-proof containers protected from heat. A sufficient quantity, typically at least 100 g or several retail units, should be submitted, and testing should commence as soon as possible after receipt.
What is the typical turnaround time?
Standard plate-count enumeration usually requires 5 to 7 working days, accounting for dilution, incubation of 48 to 72 hours, counting, and confirmation. Tests involving strain identification by molecular methods or full shelf-life protocols require additional time.
What detection range can be reported?
Depending on the dilution scheme, culture-based enumeration can cover approximately 10 CFU/g (mL) to above 10^11 CFU/g (mL), which accommodates both low-level fermented foods and high-potency probiotic concentrates.
Do dead bacteria affect the result?
Dead cells do not form colonies and are excluded by culture-based methods. Flow cytometry with viability staining or PCR with viability dyes can be added when differentiation of live and dead populations is scientifically or commercially relevant.
Why do laboratory results sometimes differ from label claims?
Declines during storage, exposure to heat or humidity, strain-specific sensitivity to oxygen or acid, and normal measurement uncertainty can all contribute to differences. Consistent testing at defined time points by an accredited third-party laboratory provides the most reliable basis for verifying declared potency.