The Use of Lactic Acid Bacteria as a Probiotic in Swine Diets

As the resistance of pathogens to antibiotics and the possibility of antibiotic residues in animal products attract increasing attention, the interest in the use of alternatives to in-feed antibiotics has been growing. Recent research with Lactic acid bacteria (LAB) in pigs suggests that LAB provide a potential alternative to antibiotic strategies. LAB include Lactobacillus species, Bifidobacterium spp, Bacillus spp, and some other microbes. LAB can adjust the intestinal environment, inhibit or kill pathogens in the gastrointestinal tract and improve the microbial balance in the intestine, as well as regulate intestinal mucosal immunity and maintain intestinal barrier function, thereby benefiting the health of pigs. The related mechanisms for these effects of LAB may include producing microbicidal substances with effects against gastrointestinal pathogens and other harmful microbes, competing with pathogens for binding sites on the intestinal epithelial cell surface and mucin as well as stimulating the immune system. In this review, the characteristics of LAB and their probiotic effects in newborn piglets, weaned piglets, growing pigs and sows are documented.


Introduction
Antibiotics have been widely used for growth promotion and prevention of diarrhea in farm animals [1]. As a common additive used in livestock feeds, antibiotics contribute to an improved economic efficiency. However, the negative effects of antibiotics have become increasingly prominent. Consumers are increasingly concerned about antibiotic residues in meat products [2]. Furthermore, it has been suggested that the continuous use of antibiotics may increase bacterial resistance, which can threaten the health of both animals and humans [3]. Therefore, the use of antibiotics as growth promoters has been banned in many countries, such as the European Union and Japan. In addition, other countries including China and the United States are banning or planning to ban the inclusion of antibiotics in swine diets.
Lactic acid bacteria (LAB) have been suggested to be an alternative strategy to antibiotic growth promoters [4]. LAB comprise a group of gram-positive, acid-tolerant, generally non-sporulating, non-respiring rod shaped (bacillus), or spherical (coccus) bacteria that are associated by their common metabolic and physiological characteristics. These bacteria produce lactic acid as the major metabolic end-product of carbohydrate fermentation. This trait has linked LAB with food fermentation, as acidification inhibits the growth of spoilage agents. Furthermore, lactic acid and other metabolic products contribute to the organoleptic and textural profile of a food item [5]. LAB includes various major genera, including Lactobacillus spp, Bifidobacterium spp, Lactococcus spp, Lactosphaera spp, Leuconostoc spp, Melissococcus spp, Oenococcus spp, Pediococcus spp, Streptococcus spp, and Enterococcus spp [6,7].
In recent years, multiple reports have described the beneficial effects of LAB, such as regulation of the intestinal microflora, inhibition or prevention of pathogens in the gastrointestinal tract (GIT), enhancement of intestinal mucosal immunity and maintaining intestinal barrier function [8][9][10][11][12]. The aim of this review is to systematically review and update the evidence on the efficacy of using LAB in pig diets.

Applications of Lactic Acid Bacteria in Pigs
Studies in LAB applied to replace antibiotics in pigs have noted LAB have a lot of beneficial effects. We have constructed a summary table (Table 1) from some literatures, in order to provide a visualized overview of the reported LAB trains used in pigs.

Applications of LAB in Neonatal Piglets
The neonatal period is a critical time in piglet ontogeny, due to the fact that the GIT and immune system have yet to fully develop [13]. These deficiencies result in low disease resistance in piglets and make them vulnerable to invasion by pathogenic microorganisms. A serious infection or stress reaction in the neonate has negative impact on piglets, thus affecting the whole process of individual development [14].
Supplementation of LAB in neonatal piglets can regulate the formation of the piglet gut microflora, thus benefiting the health of piglets [13,14]. Liu et al. [15] found that oral administration of L. fermentum I5007 in formula-fed piglets (dosed daily with 6 × 10 9 CFU/mL of L. fermentum I5007 dissolved in 3 mL of 0.1% peptone water once a day for 14 days vs. just 0.1% peptone water in control group) favored intestinal development and reduced the number of potentially enteropathogenic Escherichia spp and Clostridium spp in neonatal piglets. This was consistent with an earlier study showing that piglets provided with LAB (enteral feeding for 2 days with formula with LAB vs. porcine colostrum or formula groups) had a lower density of the potential pathogen Clostridium perfringens [14]. In addition, commensal Lactobacillus bacteria were more closely associated with enterocytes along the villus-crypt in piglets treated with LAB [14].
LAB have been shown to enhance intestinal barrier function [15,16]. A dysfunction in the intestinal barrier plays a major role in the pathophysiology of a variety of gastrointestinal disorders. Previous research demonstrated that various strains of Lactobacillus were responsible for different modulations of cell layer integrity and could attenuate the barrier disruption (rearrangement of ZO-1) caused by Salmonella LPS administration [17]. Another study indicated that oral administration of L. fermentum I5007 decreased the mRNA expression of the inflammatory cytokine IL-1β and increased the concentrations of butyrate [15], which help strengthen the intestinal barrier and defend against pathogenic microbes [18]. Wang et al. [19] also studied this strain and revealed that L. fermentum I5007 improved weight gain and feed conversion, decreased the occurrence of diarrhea, enhanced T-cell differentiation and induced cytokine expression in the ileum of piglets.

Applications of LAB in Weaned Piglets
During weaning, piglets are faced with a considerable amount of psychological stress induced by changes in feed and the environment. LAB can relieve weaning stress, prevent diarrhea and promote growth of piglets during and after weaning [10,20,21]. In addition to improving the intestinal microbiota of piglets, inclusion of Enterococcus faecium significantly improved growth and feed conversion of weaning pigs [22,23]. LAB complexes, such as a combination of E. faecium, L. acidophilus, Pediococcus pentosaceus and L plantarum (a basal diet without antibiotics or probiotics was used as control, and the other 3 groups were fed the control diet supplemented with 600 ppm of one of three different LAB complexes) increased feed intake and weight gain and improved feed conversion [24]. Yu et al. [25] demonstrated that L. fermentum I5007 (a basal diet with L. fermentum supplementation as the experimental group vs. a basal diet without antibiotics or L. fermentum as the control group) colonized and adhered to the GIT epithelium forming a protective membrane against pathogenic microbes while at the same time modulating immunity along with promoting the expression of MUC2 and MUC3. In addition, L. fermentum I5007 exhibited additional effects in alleviating weaning stress syndrome by enhancing the levels of proteins involved in energy metabolism, lipid metabolism, cell structure and mobility, protein synthesis, immune response [26], and improved the anti-oxidative defence system [27], thereby facilitating cellular proliferation and depressing apoptosis.

Applications of LAB in Growing-Finishing Pigs
As growing-finishing pigs have a mature GIT, with high digestive enzyme activity, immune capacity and disease resistance, the influence of LAB in growing-finishing pigs is relatively limited. Supplementation of a LAB mixture (based on Bacillus lichenformis and B. subtilis, probiotic application group fed a basal diet with LAB mixture vs. control group fed a basal diet with antibiotic used as) improved weight gain and reduced mortality of growing-finishing pigs [28]. Ohashi et al. [29] evaluated the effect of feeding yoghurt, prepared with L. bulgaricus strain 2038 (three female pigs fistulated at the cecum were fed 250 g of this yoghurt for 2 weeks; the whole experiment was divided to pre-administration period, administration period and post-administration period), on indigenous lactobacilli in the pig cecum and found that continuous consumption of this strain will stimulate the growth of some indigenous lactobacilli and alter the composition of the lactobacilli. L. plantarum ZJ316 (the control group was fed a diet supplemented with the antibiotic mequindox, three groups with different L. plantarum levels and a group with a mixture of mequindox and L. plantarum) was also found to improve pig growth and pork quality. The probiotic mechanism was related to the inhibition of the growth of opportunistic pathogens and promotion of increased villus height [30].

Applications of LAB in Sows
Although there are relatively few studies about the application of LAB in sows, it is very important to conduct research in this field. In a recent study, the effects of L. johnsonii XS4 (control group received basal diet and experiment group received the same diet supplemented with L. johnsonii XS4, from 90th day of pregnancy to the weaning day at 25th day of lactation) on reproductive performance, gut environment, and blood biochemical and immunological indexes of sows were investigated. The results showed that administration of L. johnsonii XS4 in diets towards the end of pregnancy and during lactation had positive effects on the performance of sows, increasing litter weight at birth, 20-day litter weight, the number of piglets at weaning and weaning litter weight, along with a significant increase in serum IgG levels and a decrease in alanine aminotransferase concentrations [31]. Lactina, a mixture of Streptococcus thermophiles, E. faecium, L. bulgaricus, L. acidophilus, L. helveticus and L. plantarum, supplemented both to sow and piglet diets, increased complement activity in piglets at 5 days of age compared with a control group, while the addition of Lactina to sows only or to piglets only did not produce any significant effects [32]. Another probiotic mixture of B. licheniformis and B. subtilis (normal feed plus the probiotic mixture vs. untreated control group) was shown to improve sow feed intake and decrease sow weight loss during the sucking period [8].

Supplementation Stage and Optimum Dose of LAB in Pigs
Many studies have been conducted on optimal supplementation strategies in pigs. The effects of L. plantarum ZJ316 on pig growth at a dose of 1 × 10 9 CFU/day were more pronounced than a dose of 5 × 10 9 CFU/day or 1 × 10 10 CFU/day [30]. Zhu et al. [33] reported the effects of L. rhamnosus ATCC7469 on serum IL-17 production and intestinal T-cell responses in pigs challenged with E. coli were dose-dependent, showing that serum concentrations of IL-17 and the percentage of ileal intraepithelial CD3 + CD4 − CD8 + cells increased in the high-dose (1 × 10 11 CFU/mL) piglets, but not the low-dose (1 × 10 9 CFU/mL) piglets [33]. Furthermore, Yu et al. [25] fed weaned piglets with diets containing 3.2 × 10 6 CFU/g, 5.8 × 10 7 CFU/g or 2.9 × 10 8 CFU/g of L. fermentum I5007. Their results showed that a dose of 5.8 × 10 7 CFU/g maximized the digestibility of crude protein among the different concentrations of L. fermentum.
The supplementation stages of L. reuteri I5007 has also been studied by oral administration (1.7 × 10 10 CFU/day for each piglet) either daily for 4 days starting on day 1 or every 4th day from day 1 to 17. The data showed that piglets in the prolonged duration of treatment (every 4th-day group) had the highest abundance of mRNA for TGF-β and the lowest for IFN-γ [34].

Survival and Adhesion within the Gastrointestinal Tract
To behave as a probiotic, LAB must first be able to survival passage though the upper GIT, meaning that LAB must have the characteristics of resistance to increased acidity from inorganic acid production (e.g., hydrochloric acid) and pancreatic enzymes [42]. The most commonly used probiotics are strains of LAB such as Lactobacillus, and Bifidobacterium, which are known to withstand gastric acid, bile salts and pancreatic secretions, to adhere to colonic mucosa and readily colonize the intestinal tract [43]. For example, Lactobacillus reuteri I5007, initially known as L. fermentum I5007, was selected from over 7000 native Lactobacilli colonies according to criteria including tolerance to heat, low pH, and bile salts, as well as storage stability and antagonism to pathogenic agents [44]. Charteris et al. [36] found that Lactobacillus and Bifidobacterium showed a moderate tolerance to acid pH during 1.5 h of incubation which was decreased after 2 h [45]. Previous studies have pointed out that acid resistance appeared to be mediated by membrane ATPases as described for L. acidophilus [46] and bile resistance was mediated by bile salt hydrolysis in L. reuteri [47].
Secondly, LAB as probiotics should have the potential to adhere to intestinal epithelial cells [42]. Adhesion of a probiotic strain to the GIT is important for bacterial colonization, pathogen exclusion, and interaction with host cells for the protection of epithelial cells or immune modulation [48]. L. reuteri I5007 showed strong adhesion to porcine intestinal mucus and several cell lines such as Caco-2 cells, IPEC-J2 cells and IEC-6 cells [49,50]. Other LAB strains also have the capacity to adhere to mucus and the intestinal epithelial cells [50][51][52]. Mechanisms of adherence to an epithelial surface involve receptor-specific binding and charge as well as hydrophobic interaction. LAB commonly express cell surface hydrophobicity, contact angle and adhesion to xylene [53]. This may facilitate adhesion to mucus. Furthermore, Cell Surface Proteins have been shown to mediate adhesion to mucus by various LAB [54]. Interestingly, LAB showed no host specificity in adhesion to intestinal mucus, but differed between the different compartments of the GIT [55].

Antibacterial and Bactericidal Effects
One of the most important modes of action of LAB is antimicrobial activity through inhibition of the adhesion of pathogenic bacteria [42]. The lactic acid produced by LAB contributes to an acidic environment in the GIT which partly influences growth of pathogenic microorganisms. What's more, LAB commonly produce bacteriocins which are peptides with bactericidal activity usually against strains of closely related species and can inhibit growth or adhesion of harmful bacteria. A protein secreted from L. acidophilus was reported to inhibit the gastric pathogen Helicobacter pylori and supplementation of Saccharomyces boulardii to rumen fluid eradicated Escherichia coli O157:H7 [46,56]. Li et al. [41] reported that L. fermentum I5007 had a strong competitiveness against both E. coli K88 and Salmonella typhimurium and could adhere to Caco-2 cells and porcine intestinal mucosa [50].
LAB can inhibit pathogenic bacteria by competing for nutrients in the gut or for binding sites on the intestinal epithelium [57]. As most intestinal pathogens must adhere to the intestinal epithelium to colonize in the intestine and produce diseases [58], some LAB strains have been chosen as probiotics specifically based on their ability to adhere to the intestinal epithelium and thus compete with pathogens for binding sites [59].
Another mechanism to inhibit pathogens in the gut is via increasing production of intestinal mucins which may protect the epithelial cells by functioning as a physicochemical barrier. L. plantarum 299v was shown to increase mRNA expression of MUC2 and MUC3 in HT29 intestinal cells, and this led to inhibition of adhesion of enteropathogenic E. coli [60].

Antioxidation and Immunomodulation
Some LAB strains produce antioxidants and influence the immue system. It is well known that oxidative damage forms part of the pathogenesis for many chronic diseases. Bifidobacterium longum ATCC 15708 and L. acidophilus ATCC 4356 inhibited linoleic acid peroxidation and scavenged free radicals. L. fermentum I5007 also demonstrated the ability to scavenge free radicals in vitro [61]. LAB provides defense by inducing anti-inflammatory cytokines and reducing proinflammatory cytokines from intestinal epithelial cells [62,63], but certain LAB will enhance the gut inflammatory immune response [64]. For instance, L. lactis and L. bulgaricus induced an increase in IgA + cells entering the IgA cycle but not CD4 + cells. However, L. casei and L. plantarum were able to increase IgA − cells and CD4 + cells [65]. In addition, L. casei Shirota induced production of the proinflammatory cytokine IL-12 with subsequent production of IFN-γ in murine splenocytes [66]. The properties of immunomodulation appear to be strain dependent.

Safety
The industrial importance of the LAB is evidenced by their "Generally Regarded as Safe (GRAS)" status. Studies of LAB both in vitro and in vivo indicate that they are safe for livestock and human consumption [67,68]. However, plasmids in some strains of LAB have been shown to encode for antibiotic resistance genes [49,69,70]. For instance, L. reuteri ATCC 55730, a commercially available probiotic strain, was demonstrated to carry potentially transferable resistance traits for tetracycline and lincomycin. However it has been replaced by L. reuteri DSM 17938, in which the two resistance plasmids have been removed without losing any probiotic characteristics [70]. At the same time, the taxonomy of several LAB has been reconstructed during the last decade, and the use of modern polyphasic taxonomy has reclassified several probiotic strains [6,49,71]. Generally, LAB strains carry a very low risk of causing infection. Many related products have been traditionally used over generations, and have been proven to be safe.

Conclusions
In conclusion, the available data from studies and applications of LAB in pigs clearly indicate that LAB have great potential as alternatives to in-feed antibiotics. However, LAB are not a single entity. Different LAB strains even of the same species may have different metabolic effects which in turn affect performance and the immune system of the host. Therefore, randomized, double-blind, placebo-controlled, case-controlled studies on the efficacy of LAB preparations, as well as optimal supplementation stages and doses, are needed.