Assessing Viability and Stress Tolerance of Probiotics—A Review

Abstract

The interest in probiotics has increased rapidly the latest years together with the global market for probiotic products. Consequently, establishing reliable microbiological methods for assuring the presence of a certain number of viable microorganisms in probiotic products has become increasingly important. To assure adequate numbers of viable cells, authorities are enquiring for information on viability rates within a certain shelf-life in colony forming units (CFU). This information is obtained from plate count enumeration, a method that enables detection of bacterial cells based on their ability to replicate. Although performing plate count enumeration is one manner of assessing viability, cells can still be viable without possessing the ability to replicate. Thus, to properly assess probiotic viability, further analysis of a broader group of characteristics using several types of methods is proposed. In addition to viability, it is crucial to identify how well the cells in a probiotic product can survive in the gastrointestinal tract (GIT) and thus be able to mediate the desired health benefit while passing through the human body. A broad spectrum of different assay designs for assessing probiotic gastric tolerance have been used in research and quality control. However, the absence of any consensus on how to assess these qualities makes it difficult to compare between laboratories and to translate the results into in vivo tolerance. This review presents and discusses the complexity of assuring that a probiotic is suitable for beneficial consumption. It summarizes the information that can be subtracted from the currently available methods for assessment of viability and stress tolerance of a probiotic, hereby altogether defined as “activity.” Strengths and limitations of the different methods are presented together with favorable method combinations. Finally, the importance of choosing a set of analyses that reveals the necessary aspects of probiotic activity for a certain product or application is emphasized.

Viability droplet digital polymerase chain reaction accurately enumerates probiotics and provides insight into damage experienced during storage

“…While the concept of viability is heavily debated, the probiotic industry has a long history of utilizing traditional PC methods for the enumeration of viable probiotics ( Kiefer et al, 2020 ; Wendel, 2022 ). For this reason, v-ddPCR methods were compared to PC, to assess their ability to accurately enumerate viable probiotics (stability) in short-term and accelerated storage studies.…”

Natural sources and encapsulating materials for probiotics delivery systems: Recent applications and challenges in functional food development

“…Largest fraction of commercial strains of probiotics known today have established thermal susceptibility, as the optimal temperature for probiotic viability evolved to be the human body temperature. Mechanistic introspection of this declined viability due to low versus high temperatures exposure manifests via exacerbated membrane porosity leading to leakage of intra-cellular contents, and thermal stress mediated inactivation of critical molecular machinery (polymerases), respectively ( 54 – 56 ).…”

“…BSH positive bacteria are more tolerant to bile exposure. Besides BSH, bile salt efflux system also exist in various probiotic bacteria as a mechanism to cope with bile stress during passage via the small intestine ( 54 , 86 , 87 ). An interesting correlation between bile tolerance and antibiotic susceptibility of lactobacilli has also been reported, bile exposure renders lactobacilli susceptible to clinical regimes of antibiotics ( 88 ).…”

Non-inhibitory levels of oxygen during cultivation increase freeze-drying stress tolerance in Limosilactobacillus reuteri DSM 17938

“…Online flow cytometry (FCM) using a previously developed OnCyt module (Besmer et al, 2014), and a k-means clustering-based gating method, were performed to get a detailed picture of effects on cell proliferation and viability. In parallel, the effect of low oxygen on acid tolerance, bile tolerance, and 5′ NT activity, which are important characteristics and markers for probiotic activity (Ruiz et al, 2013;Wang et al, 2018;Pang et al, 2022;Wendel, 2022), were also investigated.…”