Novel method based on chromogenic media for discrimination and selective enumeration of lactic acid bacteria in fermented milk products

Protective Cultures in Food Products: From Science to Market

An ultimate goal in food production is to guarantee food safety and security. Fermented food products benefit from the intrinsic capabilities of the applied starter cultures as they produce organic acids and bactericidal compounds such as hydrogen peroxide that hamper most food pathogens. In addition, highly potent small peptides, bacteriocins, are being expelled to exert antibiotic effects. Based on ongoing scientific efforts, there is a growing market of food products to which protective cultures are added exclusively for food safety and for prolonged shelf life. In this regard, most genera from the order Lactobacillales play a prominent role. Here, we give an overview on protective cultures in food products. We summarize the mode of actions of antibacterial mechanisms. We display the strategies for the isolation and characterization of protective cultures in order to have them market-ready. A survey of the growing market reveals promising perspectives. Finally, a comprehensive chapter discusses the current legislation issues concerning protective cultures, leading to the conclusion that the application of protective cultures is superior to the usage of defined bacteriocins regarding simplicity, economic costs, and thus usage in less-developed countries. We believe that further discovery of bacteria to be implemented in food preservation will significantly contribute to customer's food safety and food security, badly needed to feed world's growing population but also for food waste reduction in order to save substantial amounts of greenhouse gas emissions.

Fabrication of a novel hydrogel-based microfluidic chip and its application in pathogen analysis

Herein, we develop a novel hydrogel-based microfluidic chip, which can serve as a multifunctional analytical platform. The chip was fabricated through a newly developed hydrogel material, which shows satisfactory properties such as fast forming speed and good hydrophilicity. The chip mainly consists of two independent functional parts: a chromogenic layer and a microfluidic layer. The specially-designed toothed structure in the microfluidic layer can promote surface interactions and realize efficient enrichment of the target. The chromogenic layer contains chromogenic media, which can achieve rapid target identification through a simple visual readout. As a proof of concept, the proposed chip is employed for pathogen analysis. It shows satisfactory performance for efficient enrichment of Escherichia coli (E. coli) O157:H7. On the other hand, the visual detection limit of the chip for E. coli O157:H7 can reach 10 cfu mL^-1. It is believed that this work could provide a valuable reference for chip material exploitation and application.

A modified reinforced clostridial medium for the isolation and enumeration of Lactobacillus delbrueckii ssp. bulgaricus in a mixed culture

In this study, we modified reinforced clostridial medium (RCM) to selectively enumerate and isolate Lactobacillus delbrueckii ssp. bulgaricus, a probiotic and important starter culture in the dairy industry. The disparity in the reported carbohydrate fermentation pattern of L. delbrueckii ssp. bulgaricus was used to develop a growth medium not only selective for L. delbrueckii ssp. bulgaricus but significantly inhibitory to the growth of other lactic acid bacteria. A recently modified RCM (mRCM) was optimized for this study by the addition of 0.5% fructose, 0.5% dextrose, 1% maltose, and 0.25% sodium pyruvate while replacing lactose as a carbohydrate source. The cell recovery and bacterial counts of L. delbrueckii ssp. bulgaricus in tested products (pure L. delbrueckii ssp. bulgaricus strains, starter culture, probiotic supplements, and yogurt) using our mRCM with sodium pyruvate (mRCM-PYR) were significantly higher than in the recently modified RCM and the common de Man, Rogosa, and Sharpe (MRS) culture medium. The growth of other lactic acid bacteria (Streptococcus thermophilus, Lactobacillus acidophilus, Lactobacillus rhamnosus, and Lactobacillus reuteri) and Bifidobacteria was retarded in this modified medium compared with their growth in MRS and mRCM. This result is a significant improvement in the enumeration and differentiation of L. delbrueckii ssp. bulgaricus in mRCM-PYR compared with the results in MRS and mRCM where the high background growth of similar species interferes with the accuracy of bacterial population counts. Our results thus suggest that mRCM-PYR could be recommended as a reliable alternative growth medium for the selective enumeration and isolation of L. delbrueckii ssp. bulgaricus in a mixed culture.

Improving End-User Trust in the Quality of Commercial Probiotic Products

In a rapidly growing global probiotic market, end-users have difficulty distinguishing between high quality and poor quality products. This ambiguity threatens the trust consumers and healthcare providers have in probiotic products. To address this problem, we recommend that companies undergo third-party evaluations to certify probiotic quality and label accuracy. In order to communicate about product quality to end-users, indication of certification on product labels is helpful, although not all manufacturers choose to use this approach. Herein we discuss: third-party certification, the process of setting standards for identity, purity, and quantification of probiotics; some emerging methodologies useful for quality assessment; and some technical challenges unique to managing quality of live microbial products. This review provides insights of an Expert Panel engaged in this process and aims to update the reader on relevant current scientific methodologies. Establishing validated methodologies for all aspects of quality assessment is an essential component of this process and can be facilitated by established organizations, such as United States Pharmacopeia. Emerging methodologies including whole genome sequencing and flow cytometry are poised to play important roles in these processes.

Absolute Enumeration of Probiotic Strains Lactobacillus acidophilus NCFM® and Bifidobacterium animalis subsp. lactis Bl-04 via Chip-Based Digital PCR

The current standard for enumeration of probiotics to obtain colony forming units by plate counts has several drawbacks: long time to results, high variability and the inability to discern between bacterial strains. Accurate probiotic cell counts are important to confirm the delivery of a clinically documented dose for its associated health benefits. A method is described using chip-based digital PCR (cdPCR) to enumerate Bifidobacterium animalis subsp. lactis Bl-04 and Lactobacillus acidophilus NCFM both as single strains and in combination. Primers and probes were designed to differentiate the target strains against other strains of the same species using known single copy, genetic differences. The assay was optimized to include propidium monoazide pre-treatment to prevent amplification of DNA associated with dead probiotic cells as well as liberation of DNA from cells with intact membranes using bead beating. The resulting assay was able to successfully enumerate each strain whether alone or in multiplex. The cdPCR method had a 4 and 5% relative standard deviation (RSD) for Bl-04 and NCFM, respectively, making it more precise than plate counts with an industry accepted RSD of 15%. cdPCR has the potential to replace traditional plate counts because of its precision, strain specificity and the ability to obtain results in a matter of hours.

Flow cytometry: a versatile technology for specific quantification and viability assessment of micro-organisms in multistrain probiotic products

AIMS

Classical microbiology techniques are the gold standard for probiotic enumeration. However, these techniques are limited by parameters of time, specificity and incapacity to detect viable but nonculturable (VBNC) micro-organisms and nonviable cells. The aim of the study was to evaluate flow cytometry as a novel method for the specific quantification of viable and nonviable probiotics in multistrain products.

METHODS AND RESULTS

Custom polyclonal antibodies were produced against five probiotic strains from different species (Bifidobacterium bifidum R0071, Bifidobacterium longum ssp. infantis R0033, Bifidobacterium longum ssp. longum R0175, Lactobacillus helveticus R0052 and Lactobacillus rhamnosus R0011). Evaluation of specificity confirmed that all antibodies were specific at least at the subspecies level. A flow cytometry method combining specific antibodies and viability assessment with SYTO^® 24 and propidium iodide was applied to quantify these strains in three commercial products. Analyses were conducted on two flow cytometry instruments by two operators and compared with classical microbiology using selective media. Results indicated that flow cytometry provides higher cell counts than classical microbiology (P < 0·05) in 73% of cases highlighting the possible presence of VBNC. Equivalent performances (repeatability and reproducibility) were obtained for both methods.

CONCLUSIONS

This study showed that flow cytometry methods can be applied to probiotic enumeration and viability assessment. Combination with polyclonal antibodies can achieve sufficient specificity to differentiate closely related strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

Flow cytometry provides absolute and specific quantification of viable and nonviable probiotic strains in a very short time (<2 h) compared with classical techniques (>48 h), bringing efficient tools for research and development and quality control.

Subtilisin QK-2: secretory expression in Lactococcus lactis and surface display onto gram-positive enhancer matrix (GEM) particles

Background

Purified from the supernatant of Bacillus subtilis QK02 culture broth, Subtilisin QK-2 is a type of effective thrombolytic reagent that has great exploitable potential. However, the unbearable flavor that occurs with fermentation and the complicated methods that are required to obtain pure products limit the application of this enzyme. Lactic acid bacteria (LAB)-based delivery vehicles are promising as cheap and safe options for medicinal compounds. The secretory expression and surface display using LAB may popularize Subtilisin QK-2 more easily and conveniently with minimal adverse effects.

Results

Subtilisin QK-2 was expressed successfully in two forms using lactic acid bacteria. For the secretory expression in Lactococcus lactis, Subtilisin QK-2 was efficiently secreted into the culture using the promoter P_nisA and signal peptide SP_Usp. The expression levels were not different in L. lactis NZ9000 and NZ3900 without the effect of different selection markers. However, leaky expression was only detected in L. lactis NZ3900. The biological activity of this secreted Subtilisin QK-2 was enhanced by modulating the pH of medium to slightly alkaline during induction and by codon optimization of either the entire gene sequence (qk′) or only the propeptide gene sequence (qkpro′). For surface display onto gram-positive enhancer matrix (GEM) particles, n LysM repeats from the C-terminal region of the major autolysin AcmA of L. lactis were fused to either the C-terminus (n = 1, 3, 5) or the N-terminus (n = 1) of the Subtilisin QK-2. These fusion proteins were secreted into the culture medium, and the QK-3LysM was able to bind to the surface of various LAB GEM particles without a loss of fibrinolytic activity. Furthermore, the binding capacity significantly increased with a higher concentration of QK-3LysM. Compared to the free-form Subtilisin QK-2, the QK-3LysM displayed on the surface of GEM particles was more stable in the simulated gastric juice.

Conclusions

Combined with the safety and popularity of LAB, Subtilisin QK-2 may be easily applied worldwide to prevent and control thrombosis diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0478-7) contains supplementary material, which is available to authorized users.