Fast screening of Bifidobacterium longum sublethal stress conditions in a novel two-stage continuous culture strategy

Stress Response in Bifidobacteria

Bifidobacteria naturally inhabit diverse environments, including the gastrointestinal tracts of humans and animals. Members of the genus are of considerable scientific interest due to their beneficial effects on health and, hence, their potential to be used as probiotics. By definition, probiotic cells need to be viable despite being exposed to several stressors in the course of their production, storage, and administration. Examples of common stressors encountered by probiotic bifidobacteria include oxygen, acid, and bile salts. As bifidobacteria are highly heterogenous in terms of their tolerance to these stressors, poor stability and/or robustness can hamper the industrial-scale production and commercialization of many strains. Therefore, interest in the stress physiology of bifidobacteria has intensified in recent decades, and many studies have been established to obtain insights into the molecular mechanisms underlying their stability and robustness. By complementing traditional methodologies, omics technologies have opened new avenues for enhancing the understanding of the defense mechanisms of bifidobacteria against stress. In this review, we summarize and evaluate the current knowledge on the multilayered responses of bifidobacteria to stressors, including the most recent insights and hypotheses. We address the prevailing stressors that may affect the cell viability during production and use as probiotics. Besides phenotypic effects, molecular mechanisms that have been found to underlie the stress response are described. We further discuss strategies that can be applied to improve the stability of probiotic bifidobacteria and highlight knowledge gaps that should be addressed in future studies.

Ligilactobacillus salivarius functionalities, applications, and manufacturing challenges

Ligilactobacillus salivarius is a lactic acid bacteria that has been gaining attention as a promising probiotic. Numerous strains exhibit functional properties with health benefits such as antimicrobial activity, immunological effects, and the ability to modulate the intestinal microbiota. However, just a small number of them are manufactured at an industrial scale and included in commercial products. The under exploitation of L. salivarius strains that remain in the freezer of companies is due to their incapacity to overcome the environmental stresses induced by production and stabilization processes.The present study summarizes the functionalities and applications of L. salivarius reported to date. It aims also at providing a critical evaluation of the literature available on the manufacturing steps of L. salivarius concentrates, the bacterial quality after each step of the process, and the putative degradation and preservation mechanisms. Here, we highlight the principal issues and future research challenges for improving the production and long-term preservation at the industrial scale of this microorganism, and probably of other probiotics.Key points• L. salivarius beneficial properties and commercialized products.• Production conditions and viability of L. salivarius after stabilization processes.• Prospects for identifying preservation mechanisms to improve L. salivarius stability.

Turbidimetric definition of growth limits in probiotic Lactobacillus strains from the perspective of an adaptation strategy

The application of an adaptation strategy for probiotics, which may improve their stress tolerance, requires the identification of the growth range for each parameter tested. In this study, 4 probiotics (Lactobacillus acidophilus, Lacticaseibacillus casei, Lacticaseibacillus rhamnosus, and Lactiplantibacillus plantarum) were grown under different pH, NaCl, and sucrose concentrations at 25°C, 30°C, and 37°C. Turbidimetric growth curves were carried out and lag phase duration, maximum growth rate, and amplitude (i.e., the difference between initial and stationary phase optical density) were estimated. Moreover, cell morphology was observed, and cell length measured. The growth response, as well as the morphological changes, were quite different within the 4 species. The L. acidophilus was the most sensitive strain, whereas L. plantarum was shown to better tolerate a wide range of stressful conditions. Frequently, morphological changes occurred when the growth curve was delayed. Based on the results, ranges of environmental parameters are proposed that can be considered suboptimal for each strain, and therefore could be tested. The quantitative evaluation of the growth kinetics as well as the morphological observation of the cells can constitute useful support to the choice of the parameters to be used in an adaptation strategy, notwithstanding the need to verify the effect on viability both in model systems and in foods.

Commensal Obligate Anaerobic Bacteria and Health: Production, Storage, and Delivery Strategies

In the last years several human commensals have emerged from the gut microbiota studies as potential probiotics or therapeutic agents. Strains of human gut inhabitants such as Akkermansia, Bacteroides, or Faecalibacterium have shown several interesting bioactivities and are thus currently being considered as food supplements or as live biotherapeutics, as is already the case with other human commensals such as bifidobacteria. The large-scale use of these bacteria will pose many challenges and drawbacks mainly because they are quite sensitive to oxygen and/or very difficult to cultivate. This review highlights the properties of some of the most promising human commensals bacteria and summarizes the most up-to-date knowledge on their potential health effects. A comprehensive outlook on the potential strategies currently employed and/or available to produce, stabilize, and deliver these microorganisms is also presented.

Impact of the fermentation parameters pH and temperature on stress resilience of Lactobacillus reuteri DSM 17938

This study was undertaken to investigate the impact of culture pH (4.5-6.5) and temperature (32-37 °C) on the stress resilience of Lactobacillus reuteri DSM 17938 during freeze-drying and post freeze-drying exposure to low pH (pH 2) and bile salts. Response-surface methodology analysis revealed that freeze-drying survival rates [Formula: see text] were linearly related to pH with the highest survival rate of 80% when cells were cultured at pH 6.5 and the lowest was 40% when cells were cultured at pH 4.5. The analysis further revealed that within the chosen temperature range the culture temperature did not significantly affect the freeze-drying survival rate. However, fermentation at pH 4.5 led to better survival rates when rehydrated cells were exposed to low pH shock or bile salts. Thus, the effect of pH on freeze-drying survival was in contrast to effects on low pH and bile salts stress tolerance. The rationale behind this irreconcilability is based on the responses being dissimilar and are not tuned to each other. Culturing strain DSM 17938 at pH values higher than 5.5 could be a useful option to improve the survivability and increase viable cell numbers in the final freeze-dried product. However, the dissimilar responses for the process- and application parameters tested here suggest that an optimal compromise has to be found in order to obtain the most functional probiotic product possible.

Biochemical Engineering Approaches for Increasing Viability and Functionality of Probiotic Bacteria

The literature presents a growing body of evidence demonstrating the positive effect of probiotics on health. Probiotic consumption levels are rising quickly in the world despite the fluctuation of their viability and functionality. Technological methods aiming at improving probiotic characteristics are thus highly wanted. However, microbial metabolic engineering toolbox is not available for this kind of application. On the other hand, basic microbiology teaches us that bacteria are able to exhibit adaptation to external stresses. It is known that adequately applied sub-lethal stress, i.e., controlled in amplitude and frequency at a given stage of the culture, is able to enhance microbial robustness. This property could be potentially used to improve the viability of probiotic bacteria, but some technical challenges still need to be overcome before any industrial implementation. This review paper investigates the different technical tools that can be used in order to define the proper condition for improving viability of probiotic bacteria and their implementation at the industrial scale. Based on the example of Bifidobacterium bifidum, potentialities for simultaneously improving viability, but also functionality of probiotics will be described.

Scalable temperature induced stress for the large-scale production of functionalized Bifidobacteria

The application of sub-lethal stresses is known to be an efficient strategy to enhance survival of probiotic bacteria during drying processes. In this context, we previously showed that the application of heat stress upon the entry into stationary phase increased significantly the viability of Bifidobacterium bifidum. However, this heat shock has been considered only in small-scale bioreactor and no information is available for a possible scaling-up strategy. Five different operating scales (0.2, 2, 20, 200 and 2000 L) have thus been tested and the results showed that the viability of B. bifidum increases from 3.15 to 6.57 folds, depending on the scale considered. Our observations pointed out the fact that the heat stress procedure is scalable according to the main outcome, i.e., increases in cell viability, but other factors have to be taken into account. Among these factors, dissolved carbon dioxide seems to play a significant role, since it explains the differences observed between the test performed at laboratory scale and in industrial conditions.

Probiotics from research to market: the possibilities, risks and challenges

Probiotic foods can affect large parts of the population, while therapeutic applications have a less wide scope. While commercialization routes and regulatory requirements differ for both applications, both will need good scientific support. Today, probiotics are mainly used for gastrointestinal applications, their use can easily be extended to skin, oral and vaginal health. While most probiotics currently belong to food-grade species, the future may offer new functional microorganisms in food and pharma. This review discusses the crosstalk between probiotic producers, regulatory people, medical care and healthcare workers, and the scientific community.