Lactic Acid Bacteria Counting Test: Methods, Standards, and Applications

Lactic acid bacteria (LAB) counting test is a microbiological enumeration method used to quantify viable probiotic and fermentative microorganisms in food, dietary supplements, and feed products. Accurate LAB enumeration is essential for verifying label claims, ensuring product quality, and meeting regulatory requirements under standards such as GB 4789.35 and ISO 15214.

Technical Overview

Lactic acid bacteria (LAB) are a group of Gram-positive, non-spore-forming, facultative anaerobic or anaerobic bacteria that produce lactic acid as the main metabolic end product of carbohydrate fermentation. This group primarily includes genera such as Lactobacillus, Bifidobacterium, Lactococcus, Streptococcus, Pediococcus, and Leuconostoc. Because viable LAB content directly determines the functional and commercial value of fermented foods and probiotic products, quantitative enumeration is a core quality control and regulatory compliance requirement.

A lactic acid bacteria counting test determines the number of colony-forming units (CFU) per gram or per milliliter of sample using selective culture media and controlled incubation conditions. In many jurisdictions, minimum viable counts are legally defined. For example, Chinese national food safety standards require yogurt to contain at least 1,000,000 CFU/g (10⁶ CFU/g) of lactic acid bacteria during the shelf life, while probiotic dietary supplements commonly declare counts ranging from 10⁸ to 10¹¹ CFU per serving.

Enumerating LAB presents unique technical challenges: many strains are oxygen-sensitive, require anaerobic or microaerophilic incubation, grow slowly compared to common indicator organisms, and are often present in high numbers alongside other viable microorganisms in acidic products. Therefore, validated culture media, strict anaerobic handling, and standardized procedures are critical for accurate and reproducible results.

Testing Samples

Fermented Dairy Products

Yogurt, drinking yogurt, kefir, cultured buttermilk, sour cream, and cheese are the most common sample types. These products contain mixed starter cultures such as Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, and Lactobacillus acidophilus, often combined with probiotic adjuncts such as Bifidobacterium species.

Probiotic Dietary Supplements

Capsules, tablets, sachets, powders, and gummies containing freeze-dried or microencapsulated LAB cultures. These samples typically require pre-treatment to dissolve or neutralize carriers, and enumeration is often performed both at release and at the end of shelf life to verify stability.

Fermented Plant-Based Foods and Beverages

Kombucha, fermented plant-based yogurts, kimchi, sauerkraut, fermented soy products, sourdough starters, and fermented fruit or vegetable juices contain diverse LAB populations that may be enumerated for process control or research purposes.

Animal Feed and Agricultural Products

Silage inoculants, probiotic feed additives, and fermentation starters used in livestock production are tested to confirm viable microbial content and batch-to-batch consistency.

Clinical and Research Samples

Fecal samples, oral swabs, and vaginal specimens may be analyzed in research settings to study microbiome composition and the intestinal survival of ingested probiotic strains.

Testing Items

Typical enumeration targets and testing parameters include:

• Total lactic acid bacteria count — enumeration of all presumptive LAB in the sample.
• Lactobacillus acidophilus count — selective enumeration using MRS-salicin or MRS-sorbitol agar under anaerobic conditions.
• Bifidobacterium count — selective enumeration on modified MRS-LP agar or RB agar with anaerobic incubation.
• Streptococcus thermophilus count — enumerated on M17 agar or SYL agar under aerobic incubation.
• Lactococcus lactis and other mesophilic LAB — enumerated per ISO 15214 using MRS agar at 30 °C.
• Species or strain identification — confirmation by MALDI-TOF MS, 16S rRNA sequencing, or strain-specific PCR when regulatory labeling or IP protection requires verification.
• Viability and stability assessment — viable counts measured at multiple time points during accelerated and real-time shelf-life studies.

Testing Methods

Culture-Based Plate Count (Reference Method)

The conventional plate count method remains the regulatory gold standard. In China, GB 4789.35-2016 specifies the enumeration of lactic acid bacteria in foods: the sample is homogenized in sterile diluent, serially diluted, plated onto MRS agar or selective variants, and incubated anaerobically at 36 °C ± 1 °C for 48–72 hours (72 hours for Bifidobacterium). Colonies are counted and results reported as CFU/g or CFU/mL.

Internationally recognized culture standards include:

• ISO 15214-1 — Horizontal method for enumeration of mesophilic lactic acid bacteria, colony-count technique at 30 °C.
• ISO 15214-2 — Colony-count technique at 42 °C (thermophilic LAB).
• ISO 20128 / IDF 180 — Enumeration of presumptive Lactobacillus acidophilus on a selective medium.
• ISO 29981 / IDF 220 — Enumeration of presumptive Bifidobacterium.
• GB 4789.35 and GB 4789.34 — Chinese national standards for LAB and Lactobacillus acidophilus enumeration.

Membrane Filtration Method

For liquid samples with expected low counts, the sample is passed through a 0.45 µm membrane filter, which is then placed on selective agar for incubation. This improves detection sensitivity.

Molecular and Rapid Methods

Real-time quantitative PCR (qPCR), PMA-qPCR (which distinguishes viable from dead cells), flow cytometry (fluorescent staining reported as active fluorescent units, AFU), and impedance microbiology are increasingly used for rapid screening and high-throughput stability studies. However, regulatory acceptance still generally favors culture-based CFU values for label claims, so rapid methods are typically validated against the plate count method.

Procedure Summary

• Sample preparation and homogenization in sterile peptone-salt diluent.
• Serial tenfold dilution under aseptic conditions.
• Inoculation onto selective media (MRS, MRS-bile, M17, or species-specific variants).
• Anaerobic incubation (anaerobic jars, chambers, or gas-generating sachets) at 36–42 °C for 48–72 hours.
• Colony counting, calculation, and confirmation of typical colonies by microscopy and biochemical or molecular identification.
• Result expression as CFU/g or CFU/mL with associated measurement uncertainty.

Applications

• Product release testing — verifying that fermented dairy products, probiotic supplements, and functional beverages meet declared viable counts before market release.
• Regulatory compliance — demonstrating conformity with national food safety standards and probiotic labeling requirements in target markets.
• Shelf-life and stability studies — tracking viable count decline over time under refrigerated, ambient, or accelerated storage conditions to establish expiration dates.
• Process validation — evaluating the impact of fermentation parameters, drying, microencapsulation, acidification, and packaging on culture viability.
• Research and development — screening new strains, optimizing growth media, and assessing survival through simulated gastric and intestinal digestion.
• Supplier and import control — verifying the quality of incoming bulk cultures and finished goods in trade and supply chain management.

FAQ

What is the typical turnaround time for an LAB counting test?

Culture-based enumeration generally requires 3 to 5 working days, including anaerobic incubation (2–3 days) and colony confirmation. Molecular rapid methods can deliver results within 1–2 days but require method validation.

What is the reporting unit and detection limit?

Results are reported as CFU/g or CFU/mL. For standard plate counting, the reliable quantitative range is typically 10–300 colonies per plate, corresponding to a detection limit of approximately 10 CFU/g when the lowest dilution is plated. Lower levels can be detected using membrane filtration or larger inoculum volumes.

Why are anaerobic conditions required?

Many LAB, especially Bifidobacterium species, are sensitive to oxygen and fail to form visible colonies under aerobic incubation. Anaerobic jars, anaerobic chambers, or oxygen-scavenging sachets are used to maintain the required low-redox environment.

What is the difference between CFU and AFU?

CFU (colony-forming units) measures the number of cells capable of reproducing on agar, determined by culture. AFU (active fluorescent units) is determined by flow cytometry based on membrane integrity staining. AFU counts are often higher than CFU counts because they include viable but non-culturable cells; most regulations still specify CFU.

How should probiotic samples be shipped to the laboratory?

Refrigerated products should be shipped with cold packs at 2–8 °C, while freeze-dried supplements can generally be shipped at ambient temperature. Samples should be transported promptly, and extreme temperature excursions avoided, since improper handling can understate the true viable count.

Can the test identify the specific probiotic strain declared on the label?

Plate counting quantifies presumptive target organisms, but species- or strain-level verification requires complementary methods such as MALDI-TOF mass spectrometry, 16S rRNA sequencing, or strain-specific PCR typing, which accredited third-party laboratories can provide alongside enumeration.