Probiotic biomarkers and models upside down: From humans to animals

Highlights

• Poultry models and human cells yield similar results in poultry probiotics testing.

• Human cell lines can be used to test most of the parameters of candidate probiotics.

• Such cell models render probiotics R&D inexpensive, fast, and reproducible.

Abstract

Probiotics development for animal farming implies thorough testing of a vast variety of properties, including adhesion, toxicity, host cells signaling modulation, and immune effects. Being diverse, these properties are often tested individually and using separate biological models, with great emphasis on the host organism. Although being precise, this approach is cost-ineffective, limits the probiotics screening throughput and lacks informativeness due to the ‘one model - one test - one property’ principle. There is а solution coming from human-derived cells and in vitro systems, an extraordinary example of human models serving animal research. In the present review, we focus on the current outlooks of employing human-derived in vitro biological models in probiotics development for animal applications, examples of such studies and the analysis of concordance between these models and host-derived in vivo data. In our opinion, human-cells derived screening systems allow to test several probiotic properties at once with reasonable precision, great informativeness and less expenses and labor effort.

Introduction

In 2019, global poultry production reached 125.64 million tons and sustained stable growth observed over recent years - in contrast to other meat types. Already in 2018, poultry outrun pork and became number one meat produced worldwide (Shahbandeh, 2020).

At the same time, poultry meat is the major cause of diseases in humans transmitted by contaminated foods and the primary source of spreading antibiotic resistance. The two key pathogens responsible for this are Salmonella and Campylobacter spp. (Sabo et al., 2020), with the former affecting one out of every eighty Europeans and the latter being the main bacterial cause of community-acquired gastroenteritis in Europe in 2016 (Allerberger, 2016). As for food-transferred antibiotic resistance, animals account for 70 % of the antibiotic consumption worldwide and further increase is expected by 2030 in response to the growing meat demand from developing countries. As a result of the antibiotic usage in agricultural practice, food-induced antibiotic resistance is spreading causing death of 700,000 people per year (Tarradas et al., 2020).

Pathogenic bacteria of farming animals compete with healthy, non-pathogenic gut microbiota for colonization of the host gastrointestinal tract (GIT). Based on this fundamental principle, probiotics were suggested and have been used as an alternative to low-dose antibiotics for farming animals since 1970s (Chaves et al., 2017), although much earlier studies laid the microbiological and physiological ground for this technology (Vanbelle et al., 1990). The 2006’s European ban on the use of growth promoting antibiotics in agricultural practice, gave a great stimulus for developing novel probiotics as feed additives to optimize gut health and animal performance (Sun et al., 2012).

While there have already been effective undefined probiotic cultures on the market, novel probiotic cultures featured by improved cost-efficacy and production scaling up are required under the increasing demand (Nava et al., 2005).

Today, in the area of probiotic development, selecting biomarkers of candidate probiotics efficacy, performance, and exact modes of action remains is one of the most challenging objectives (Tarradas et al., 2020). Despite knowing and understanding the modes of action of some probiotic strains, researchers are still often unable to summarize the specific effects that a probiotic should have on the host to maximize productivity and promote optimal health and welfare (Tarradas et al., 2020). These gaps in the knowledge make it difficult to study and select probiotics (Tarradas et al., 2020) and suggest the need for a broad-spectrum properties screening for every candidate probiotic for it to become an effective applied tool.

Fundamental knowledge of the gut functioning and pathogens-induced dysfunction helps in rationalizing this approach and in designing screening approaches (Nava et al., 2005; Chaves et al., 2017; Hossain et al., 2017). Accordingly, probiotics properties testing should range from assessing probiotic survival in the GIT (Hossain et al., 2017) to probiotic-host cells physical interactions analysis (Nava et al., 2005) and toxicity screening (Hossain et al., 2017) and down to host cells signaling pathways and gross endocrine effects modulation assays (Chaves et al., 2017).

In most cases, probiotic properties are tested on different models and separately due to economic and labor input limitations. Yet, current experience in the field allows us to suggest a novel approach of using human cell models with a view of ‘multi-dimensional’ farming animals’ probiotics testing. This approach is based on two assumptions/approximations. First, although being distinct enough, human and poultry gut physiology and microbiota interactions are not entirely different, even at the molecular level. Second, in a sensible number of studies, animal farming probiotics properties were tested on poultry and human models in parallel, and the correlation is known.

The present review aims at focusing on these studies and suggesting a probiotics properties screening pipeline supported by experimental evidence. To do so, we’ll start with reiterating the specific interactions and influences of probiotic organisms on the host cells and organisms. These interactions are the subject of numerous excellent reviews (Cox, Dalloul, 2015; El-Hack et al., 2018; Sood et al., 2020), we only mention them because of further considerations on the models used and to be used in probiotic research.