Implications

This review critically examines the use of probiotics in post-weaning piglets, focusing on challenge situations, and proposes potential strategies to increase consistency in probiotic effects. Given the current lack of reproducibility commonly described with probiotic use, this approach could have significant positive effects upon the efficacy of probiotic products and economic viability of the swine industry.

Use of probiotics against pathogens

A vast amount of research is published yearly in relation to probiotic capacities to improve gastrointestinal health and fight digestive pathogens. It is worth mentioning that the interest of finding probiotic strategies to fight these pathogens not only exists in animal production, but it is also present in human medicine, which, in many cases, uses pigs as a One Health approach (Mardones et al., Reference Mardones, Hernandez-Jover, Berezowski, Lindberg, Mazet and Morris2017) or as a human model of disease (Meurens et al., Reference Meurens, Summerfield, Nauwynck, Saif and Gerdts2012). This fact enriches the amount of information available and may be useful for the pig industry. Table 1 recalls main scientific studies published to-date, assessing the use of probiotics against pathogens in piglet experimental models of disease.

Table 1 Pig in vivo scientific works evaluating the use of probiotics against digestive bacterial pathogens (Escherichia coli and Salmonella sp.)

N/A=not available.

¹ Daily mix: probiotic suspended on a daily basis and mixed with feed.

How to improve the use of probiotics in early life stages

In view of the present situation, it goes without saying that improving the use of probiotics in the swine industry relies on a drift from an empirical use to a more knowledge-based strategy. This section provides a few suggestions to be considered in the use of probiotics in early life stages, and in particular in post-weaning disorders. However, its aim is solely to provide a starting point for the reader to critically evaluate the use of probiotics, rather than a dissertation on their use.

To start with, assessment of the probiotic strains should be done in a wide range of health conditions. As commented by Bosi and Trevisi (Reference Bosi and Trevisi2010), the identification of strains with positive effects in a broad range of gut health situations, and even capable of working in different species is economically interesting for the additive industry. However, in some cases, although specific strains had demonstrated positive effects in a normal physiological situation, they were reported to be detrimental in challenge situations in piglets (see Table 1). Hence, in our opinion, it would be highly recommended to characterize the possible risks of using a probiotic in a disease context, building clear differences whether probiotic usage is intended as a therapy or as prophylaxis. For instance, in human studies, a clear distinction is made between research aimed at maintaining health and that which aimed to treat a disease, and this difference has important implications when designing trials and in regulatory affairs (Hill et al., Reference Hill, Guarner, Reid, Gibson, Merenstein, Pot, Morelli, Canani, Flint, Salminen, Calder and Sanders2014).

A second issue to address is the capacity of probiotics to modulate microbiota. As commented before, until today one major interest when using probiotics has been to replace antibiotics via production of in situ antimicrobial compounds or enzymes to cure infections (Patil et al., Reference Patil, Kumar, Verma and Baghel2015). Although some particular strains may have demonstrated effects here (Bhandari et al., Reference Bhandari, Xu, Nyachoti, Giesting and Krause2008; Cheikhyoussef et al., Reference Cheikhyoussef, Pogori, Chen and Zhang2008), their usefulness in this aspect is limited and spectacular improvements such as eliminating pathogen excretion are rarely reported (see Table 1). However, probiotics become much more powerful and valuable when we use them as ‘preventive’ health promoters and gut microbiota stabilizers (Simmering and Blaut, Reference Simmering and Blaut2001). There is an increasing amount of scientific publications supporting that probiotic effects in gut ecology and/or immune stimulation may provide support to keep animals healthy (Zhang et al., Reference Zhang, Xu, Liu, Lai, Ma, Wang and Zhu2010; Klaenhammer et al., Reference Klaenhammer, Kleerebezem, Kopp and Rescigno2012; Prieto et al., Reference Prieto, O’Sullivan, Tan, McLoughlin, Hughes, O’Donovan, Rea, Kent, Cassidy, Gardiner and Lawlor2014; Zacarías et al., Reference Zacarías, Reinheimer, Forzani, Grangette and Vinderola2014). In addition, new selection criteria based on the mechanisms of action of the strains can allow the apparition of other probiotics that have not been previously considered in animal production but can enhance gut health and make it more robust. Besides, to increase control on their effects, probiotic strategies should be more focused. Strains should be selected depending on the objectives being looked for, and not as if probiotics were beneficial for everything. Effects should target specifically to a site. Targeting, for example, M cells if applications seek to boost intestinal immunity by enhancing development of secretory IgA (Corthésy et al., Reference Corthésy, Gaskins and Mercenier2007), or targeting the hypothalamic–pituitary–adrenal axis if we want to improve animal well-being and reduce effects of common stressors (Hardy et al., Reference Hardy, Harris, Lyon, Beal and Foey2013; Zhou and Foster, Reference Zhou, Zhu, Zhang, Wang, Fan, Song, Yang, Jensen and Wang2015). In addition, some specific probiotic strains adapted to the colonic environment could be good candidates to fight gut dysbiosis (Corthésy et al., Reference Corthésy, Gaskins and Mercenier2007), but other strains could be better to enhance productive performance based on their enzymatic hydrolysis properties (Kim et al., Reference Kim, Kim, Rhee, Song, Lee, Kim, Lee, Lee and Park2007) or biosynthetic pathways for amino acids’ new synthesis (Pridmore et al., Reference Pridmore, Berger, Desiere, Vilanova, Barretto, Pittet, Zwahlen, Rouvet, Altermann, Barrangou, Mollet, Mercenier, Klaenhammer, Arigoni and Schell2004). Hence, further assessment and classification of commercial probiotics in relation to their mechanisms of action are desirable, to be able to implement strategies that are more precise and oriented to specific needs of these animals.

Another point to take into account is the variability in the response to a probiotic, depending on the host or the herd in which it is introduced. It has been described how a probiotic strategy may have ‘responder’ and ‘non-responder’ individuals in a homogenous group of animals, and also how different microbial environments can determine variability among herds (Klaenhammer et al., Reference Klaenhammer, Kleerebezem, Kopp and Rescigno2012; Arora et al., Reference Arora, Singh and Sharma2013; Starke et al., Reference Starke, Pieper, Neumann, Zentek and Vahjen2013). For instance, it has been described how the genetically determined different presence of sugar complexes along the host gut surface may facilitate the adhesion on the glycocalix of some enteropathogens, possessing specific colonization factors (such as E. coli F18 and K88) and, possibly, of commensal bacteria (Krogfelt, Reference Krogfelt1991; Lee et al., Reference Lee, Donaldson, Mikulski, Boyajian, Ley and Mazmanian2013). Moreover, the emerging ‘-omic’ technologies clearly open a window to refine our approach and understand better the interactions between a probiotic strain and the ecosystem in which it is going to be introduced. It is expected that by increasing our understanding in pig microbiome knowledge, we will identify key microbial groups of the piglets gut with an important role in maintaining a productive and disease-resistant ecosystems (Kim and Isaacson, Reference Kim and Isaacson2015). In addition, we will eventually be able to identify the most appropriate strain (or strains) to use as specific probiotic treatments for a particular situation depending on the targeted microbial ecosystem (Sanders et al., Reference Sanders, Guarner, Guerrant, Holt, Quigley, Sartor, Sherman and Mayer2013). For instance, two enterotype-like clusters have recently been identified in pig microbiota significantly correlated with performance (Ramayo-Caldas et al., Reference Ramayo-Caldas, Mach, Lepage, Levenez, Denis, Lemonnier, Leplat, Billon, Berri, Doré, Rogel-Gaillard and Estellé2016). Likewise, to correlate probiotic effects to specific enterotypes would reasonably reduce the variability of empirical use. On the other side, our understanding in probiotic interactions with the host and in particular with the intestinal cells gene expression has greatly improved in recent years. For example, a common mechanism for the anti-inflammatory activity of several probiotics has been described to be regulated by the micro-organisms pattern recognition receptors toll-like receptor 2 (TLR-2) (Villena et al., Reference Villena, Suzuki, Fujie, Chiba, Takahashi, Tomosada, Shimazu, Aso, Ohwada, Suda, Ikegami, Itoh, Alvarez, Saito and Kitazawa2012; Tomosada et al., Reference Tomosada, Villena, Murata, Chiba, Shimazu, Aso, Iwabuchi, Xiao, Saito and Kitazawa2013). In addition, it has been described how selective pressures among European pig populations have derived into specific TLR-2 gene variants (Darfour-Oduro et al., Reference Darfour-Oduro, Megens, Roca, Groenen and Schook2016). Overall, this is interesting because it provides a common mechanism for the anti-inflammatory activity of several probiotics (including different strains such as Lactobacillus spp. and Bifidobacterium spp.) (Tomosada et al., Reference Tomosada, Villena, Murata, Chiba, Shimazu, Aso, Iwabuchi, Xiao, Saito and Kitazawa2013). Moreover, it provides a potential biomarker for the screening and selection of new immune-regulatory strains, to be used efficiently at a population level to enhance immunity.

Furthermore, another possibility to potentiate probiotic effects would be to combine probiotics with complementary actions, with many beneficial examples reported in the bibliography (Casey et al., Reference Casey, Gardiner, Casey, Bradshaw, Lawlor, Lynch, Leonard, Stanton, Ross, Fitzgerald and Hill2007; Lessard et al., Reference Lessard, Dupuis, Gagnon, Nadeau, Matte, Goulet and Fairbrother2009; Zhou et al., Reference Zhou and Foster2015; Barba-Vidal et al., Reference Barba-Vidal, Castillejos, Roll, Cifuentes-Orjuela, Moreno Muñoz and Martín-Orúe2017b). Probiotic combinations can be multi-strain probiotics, containing more than one strain of the same species or closely related species (for instance, Lactobacillus acidophilus and L. casei), or multispecies probiotics, containing strains of different probiotic species that belong to one or more genera (e.g. L. acidophilus, Bifidobacterium longum and Enterococcus faecium) (Timmerman et al., Reference Timmerman, Koning, Mulder, Rombouts and Beynen2004). It has been suggested that the greater variety of probiotic genera present within a mixture may reduce its effectiveness, through mutual inhibition by the different species, antimicrobial compounds or competition for either nutrients or binding sites (Chapman et al., Reference Chapman, Gibson and Rowland2011 and Reference Chapman, Gibson and Rowland2012). However, multispecies probiotics have also been related to a broader spectrum of activity (e.g. inhibition of a wider variety of pathogenic bacteria), and if well-designed, a greater amount of synergism and symbiosis when different probiotic effects are combined (Timmerman et al., Reference Timmerman, Koning, Mulder, Rombouts and Beynen2004). Hence, although bacterial combinations have a high potential, beneficial properties of different strains are not always additive (Chapman et al., Reference Chapman, Gibson and Rowland2011). This is not an easy field of research and bacterial interactions inside the pig gut ecosystem should be further explored to be able to construct effective strategies. Still, unfortunately in vivo studies comparing single strains to probiotic combinations are still rare. Additional approaches to strengthen effects could be the addition of specific prebiotic substrates (symbiotic concept) to selectively improve the growth of the introduced strain (Shenderov, Reference Shenderov2011; Arboleya et al., Reference Arboleya, Stanton and Ryan2016) or to promote a microbiota more favorable for the probiotic to exert its action (Guerra-Ordaz et al., Reference Guerra-Ordaz, Gónzalez-Ortiz, La Regione, Woodward, Collins, Pérez and Martín-Orúe2014). Another option to improve and to specifically select the effects of a probiotic would be the genetic manipulation of the strain (Bjerre et al., Reference Bjerre, Cantor, Nørgaard, Poulsen, Blaabjerg, Canibe, Jensen, Stuer-Lauridsen, Nielsen and Derkx2016; Xu et al., Reference Xu, Yu, Zhang, Liu, Qiao, Cui, Jiang, Wang, Li and Tang2016). However, introduction of GMO in the animal feed is nowadays a very controversial issue.

The way a probiotic is administered to the piglets can also be a critical point to consider as, sometimes, reduced stability and viability of the probiotic cells can limit the use of the potentially most beneficial strains. Some bacterial genera are particularly sensitive to be introduced in the dry feed, as they cannot stand chemical–physical conditions of the feed or the manufacturing process (Angelis et al., Reference Angelis, Siragusa, Berloco, Caputo and Settanni2006). In this sense, the development of acclimatization procedures or protective coating to enable them to stand environmental aggression (Sewell, Reference Sewell2016) is a promising field of development for the use or probiotics as in-feed additives. Still, dry feed is not the only way a probiotic can be administered to piglets. Daily administration of fresh probiotic as a solid or liquid suspension by mixing it with the feed (top dressing) is a common procedure in research trials (see Table 1). However, although it may be a good strategy to increase the viability of probiotics when delivered, it is a highly time-consuming routine difficult to be implemented in commercial pig farms. Alternatively, fermented milk, suspension in milk or even suspension in water can be considered. For instance, Gebert et al. (Reference Gebert, Davis, Rehberger and Maxwell2011) supplemented a milk replacer with a Lactobacillus probiotic strain and saw positive effects on pre-weaning animals.

Besides, early dosing of probiotics in the pre-weaning period should be considered. Gut microbiota plays a critical role in the adaptation from a neonatal-immature gut to a functional adult system, resistant to adverse ecological shifts at challenges such as weaning (Lewis et al. Reference Lewis, Inman, Patel, Schmidt, Mulder, Miller, Gill, Pluske, Kelly, Stokes and Bailey2012). Hence, providing probiotics at this point could potentially permit the establishment of early and life-long health benefits (Kenny et al., Reference Kenny, Smidt, Mengheri and Miller2011). Sows should be given more importance here, as many studies have shown how introducing probiotics in the sow diet is an effective way to modify the gut ecosystem and the health of piglets (Alexopoulos et al., Reference Alexopoulos, Georgoulakis, Tzivara, Kyriakis, Govaris and Kyriakis2004; Bohmer et al., Reference Bohmer, Kramer and Roth-Maier2006; Apic et al., Reference Apic, Savic, Stancic, Zivkov-Balas, Bojkovski, Jovanovic, Radovic, Zvekic and Maksimovic2014; Siepert et al., Reference Siepert, Reinhardt, Kreuzer, Bondzio, Twardziok, Brockmann, Nöckler, Szabó, Janczyk, Pieper and Tedin2014; Kritas et al., Reference Kritas, Marubashi, Filioussis, Petridou, Christodoulopoulos, Burriel, Tzivara, Theodoridis and Pískoriková2015; Scharek-Tedin et al., Reference Scharek-Tedin, Kreuzer-Redmer, Twardziok, Siepert, Klopfleisch, Tedin, Zentek and Pieper2015). Alternatively, the introduction of probiotic strategies via ‘creep feed’ is increasingly being studied (Alexopoulos et al., Reference Alexopoulos, Georgoulakis, Tzivara, Kyriakis, Govaris and Kyriakis2004; Shim et al., Reference Shim, Verstegen, Kim, Kwon and Verdonk2005; Giang et al., Reference Giang, Viet, Ogle and Lindberg2010). Nevertheless, results of these experiments are largely variable, probably due to the fact that piglets usually ingest small or null quantities of them (Pajor et al., Reference Pajor, Fraser and Kramer1991).