1. Introduction
One of the most profitable and productive agricultural industries is the poultry industry. Recent advancements in nutrition, genetics, housing management, chicken health and welfare have allowed it to flourish, resulting in a potential growth of egg production of 8.51% and broiler production of 7.52% [
1]. The use of antibiotics as growth promoters has resulted in high levels of poultry output worldwide; these antibiotics have impacted chickens’ intestinal flora and immune systems to aid in controlling infections [
2,
3]. Concern for global health security and the environment due to the emergence of antibiotic-resistant bacteria and antibiotic residues in meat and other livestock products has led many countries to restrict the use of antibiotics in animal feed [
4]. This has encouraged nutritionists and feed manufacturers around the world to search for alternatives to antibiotic growth promoter (AGPs) that can maintain efficient poultry production while ensuring that poultry meat and eggs are safe for consumption. Possible replacements for AGPs include feeding prebiotics, probiotics, synbiotics, enzymes, herbs, essential oils, acidifying feed with organic acids and postbiotics [
5].
While probiotics have many positive health benefits, their functionality and effectiveness are subject to debate. Recent findings suggest that for a variety of animal species, probiotics need to be tailored more specifically in order to maximize their beneficial effects. Furthermore, certain strains of probiotic bacteria were discovered to have antibiotic-resistant genes which can be transmitted to gut microflora and the environment [
6,
7]. Additionally, studies showed that some probiotics can have a detrimental effect on the host by causing local inflammation in healthy hosts and exacerbating tissue inflammation in those with inflammatory bowel disease [
8]. The ‘postbiotic’ has emerged which extends the scope of the probiotic concept beyond its inherent viability [
9]. The term ’postbiotic’ refers to the soluble factors (stabilized bacteria, cellular products, or metabolic by-products) secreted by living microbes or released after microbial lysis [
10], which are mainly derived from
Lactobacillus,
Bifidobacterium,
Streptococcus, fecal bacteria [
11,
12], and
Saccharomyces cerevisiae yeast [
13,
14]. Recent research suggests that postbiotics offer various health benefits through immune system modulation (cell wall compounds may strengthen immunity), increased adhesion to intestinal cells (which restricts pathogen growth), and secretion of various metabolites [
11,
15]. Non-viable micro-organisms or microbial cell extracts have an additional advantage over probiotic-supplemented feed preparations, as the viability of probiotics may differ and dead cells may outnumber the live cells [
16]. Moreover, these non-viable microbes and extracts can significantly reduce shelf life of the poultry products [
17]. The present study was conducted to detect the effects of
Bacillus subtilis as a probiotic and
Saccharomyces cerevisiae fermentation product (SCFP) as a postbiotic on the growth performance, carcass characteristics, gut microflora and immunity of broiler chickens as an alternative to antimicrobials in poultry production system to minimize the effect on global health security.
4. Discussion
The solution-based approach to increase poultry production, to reduce production cost and to decrease negative environmental impact is the priority for poultry researchers. Modern poultry production systems are associated with numerous stressors, such as change of feed, high stocking density and processing in the hatchery, which reduce bird immunity and increase bacterial pathogen colonization—affecting not only bird health and growth, but also compromising food safety [
30]. Use of antibiotics in sub-therapeutic doses in poultry feed was considered as one approach to control gut pathogens. Currently, non-therapeutic use of antibiotics in poultry is facing reduced social acceptance as it may generate antimicrobial-resistant commensals compromising food safety and quality. The European Union, and the United States FDA, banned the non-therapeutic use of antibiotics in livestock and poultry long ago [
31,
32], but cessation of non-therapeutic antibiotic usage in poultry farming was correlated with reduced growth and increased mortality of the birds due to bacterial infections such as colibacillosis, salmonellosis and necrotic enteritis [
33]. Replacement of antibiotics with a suitable alternative without hampering the growth, immunity and health of the birds is a pressing research question.
Saccharomyces cerevisiae is considered the most promising candidate either as a probiotic (live yeast form) or as prebiotic in the poultry diet which showed remarkable improvement of growth performance, modulation of bird immune system, repairing the gastrointestinal tract and reducing the gut pathogen colonization [
34]. So, the present study was conducted to evaluate the effects of postbiotic (
Saccharomyces cerevisiae fermentation product, SCFP) along with a probiotic (
Bacillus subtilis) on the growth performance, immunity, gut health, and carcass characteristics of broiler chickens.
Feeding with SCFP (T
3 group) significantly improved average daily feed intake (ADFI) and average daily gain (ADG) of chickens compared to the T
1 (control) and T
2 (probiotic) groups from 1 to 14 days of age. Similarly, feeding with yeast hydrolysate significantly improved ADFI, ADG, and body weight during the starter and grower phase of the experimental birds compared to the control groups [
34,
35]. It could be explained with the increased villi height associated with better absorption of nutrients, increasing the secretion of auxiliary digestive enzymes and anti-inflammatory effects of yeast hydrolysate in animals [
36,
37]. In contrast, a few studies [
38,
39] reported improvement of body weight gain during the later phase (after 21 days) of the growth with the feeding of yeast hydrolysate, associated with presence of gut microbiota-secreting short chain fatty acids (SCFAs) and improved metabolic activities. Although not evaluated, the findings of present study could be correlated with the presence of SCFA-forming beneficial gut microbiota during the starter and grower phase of the growth. Significant improvement of FCR in SCFP-fed birds (T
3), compared to the control (T
1) groups across the entire period of the experiment (1–42 days), is supported with the earlier findings [
38,
39]. The meta-analysis of the findings [
40] suggested inclusion of yeast or yeast products (less than 10 g/kg of diet) could improve growth and FCR of the birds.
The absence of statistically significant differences in slaughter body weight, eviscerated carcass weight, dressing percentage, weight of breast, frame, thigh, drumstick, wing, neck, gizzard, liver, heart, spleen, and bursa between the treatment groups is corroborative with the earlier studies [
35,
41]. Addition of probiotics in the diet helps in the detoxification process, which might be the reason for the normal size of the liver in the treatment groups [
42].
Dietary addition of SCFP in the experimental birds did not alter the concentration of glucose, total protein, albumin, triglyceride and uric acid in serum, which confirmed the absence of adverse side effects in the studied birds [
35]. In agreement with earlier reports [
43,
44], the present study also confirmed significant reduction of blood cholesterol concentration in SCFP-treated birds compared to the control or probiotic-fed groups. Lower serum concentration of cholesterol in the birds is associated with production of eggs with a low cholesterol level, which is especially popular among health-conscious consumers [
45]. Although the present study was conducted in broilers, it can be conducted in layers in future to observe the production of eggs with low cholesterol. During stress, the hypothalamus pituitary–adrenal axis secretes corticosterone as the major hormone, which can depress humoral immunity and decrease production of antibodies against sheep red blood cells [
46]. Reduced level of corticosterone in T3 group of birds during day 28 of the experiment can be correlated with increased antibody titer against NDV and IBDV. Further, corticosterone was found to be associated with upregulation of CCLi2 mRNA expression in splenic lymphocytes which attract active lymphocytes from the peripheral blood to the spleen. Corticosterone-associated upregulation of CXCLi1 and CXCLi2 mRNA expression in peripheral lymphocytes instead attracts heterophiles from bone marrow which are mostly immature [
47]. The replacement of matured lymphocytes with immature heterophiles in the peripheral blood circulation was found to be responsible for decreased phagocytic activity. In the present study, a reduced corticosterone level in the T3 and T2 groups in comparison to T1 was found to be associated with increased (non-significant) phagocytic activity of the lymphocytes.
The present study revealed that dietary supplementation of SCFP had no significant effect on hemoglobin, total leukocyte count, difference leukocyte count and ratio of heterophil and lymphocyte, which was also observed in a previous study in which dietary supplementation of
Saccharomyces cerevisiae with
Nigella sativa did not find any significant effect on blood biochemical profile in broiler chickens [
48].
The effect of SCFP dietary supplementation on poultry gut microflora revealed a significant reduction of total
E. coli, pathogenic
E. coli (EHEC) and
Salmonella in comparison to the probiotic-fed group and control birds. Reduction of
E. coli and
Salmonella colonization was also observed in earlier studies in the birds fed with the yeast products which could be explained by exclusion of the pathogens due to competition for a carbon source in the gut, binding of the pathogens with a surface of yeast-produced functional carbohydrates instead of intestinal receptors—which prevent activation of pro-inflammatory cytokines-based signaling pathways—and production of enzymes to disintegrate bacterial toxins [
30,
49].
Saccharomyces cerevisiae was found to be more effective against Gram-negative pathogens such as
E. coli and
Salmonella due to its capacity to disintegrate the bacterial outer membrane—which is found only in Gram-negative bacteria, causing increased permeability and depolarization of the cytoplasmic membrane [
50]. Agglutination of pathogens expressing mannose specific type-1 fimbriae (such as
E. coli and
Salmonella) by the yeasts is another possible mechanism [
51].
Dietary treatments did not have a notable impact on the counts of
Lactobacillus in the pre-cecal digesta. Similarly, feeding with dried yeast culture [
48] and other prebiotics [
52] did not reveal significant modulation on
Lactobacillus count in broiler chickens.
Lactobacillus itself can act as a probiotic by preventing colonization of gut pathogens and the lactic acid produced by the lactobacilli is used by butyric acid producers, increasing the digestibility of the birds [
53]. Hence, in the present study, maintenance of lactobacilli in the treatment groups, compared to the control group, seems to be beneficial.
One of the noteworthy findings of the present study is significant reduction of antimicrobial-resistant pathogens (ESBL-producing
Enterobacteriaceae) in the treatment groups in comparison to the control group.
Bacillus subtilis probiotic strains earlier showed in vitro antimicrobial effect against ESBL-producing
E. coli, although failed to prevent gut colonization of ESBL-bacteria when studied in vivo [
54]. There is no report on the efficacy of SCFP on ESBL-producing
Enterobacteriaceae to compare the present finding. The present study revealed maximum occurrence of
blaCTM-Type followed by
blaSHV-Type and
blaTEM-Type in the studied birds which is supportive of earlier studies. The CTX-M is considered as the major ESBL determinant in apparently healthy poultry, whereas the SHV and TEM determinants are predominant in poultry with subclinical infections [
55].
The villi height in the duodenum, jejunum and ileum was significantly increased in the birds supplemented with SCFP and probiotic compared to the control group, which confirms the earlier observations [
34,
56]. In addition, the ratio between villi height and crypt depth was significantly increased in the ileum of SCFP-fed group compared to the birds supplemented with probiotic and the control group.
Saccharomyces cerevisiae has a trophic effect on ileal and jejunal villi compared to the duodenum as detected in the present study, which is consistent with earlier observations [
57]. The ileum is the primary site for amino acid absorption and longer ileal villi implies higher nutritional utilization reflected in better growth performance.
On day 28, antibody titers against both the IBD and NDV vaccine were significantly higher in the SCFP (T
3) group compared to the probiotic (T
2) and control (T
1) groups. The oligosaccharides present in the yeast hydrolysate can activate the macrophages and the cytokines are released to generate the acquired immune response [
35]. As with mammals, the immune response in birds after vaccination is characterized with the generation of IgM first (up to day 30 post-vaccination) followed by IgY [
58]. The previous study explored dietary supplementation of yeast products to promote the production of IgM in the birds vaccinated against NDV [
39], which is the reason for the higher antibody titer in the T3 group compared to the others on day 28. The effect of yeast supplementation on the generation of IgY is still unclear and it might explain the absence of variations in all the groups in antibody titer on day 35. However, significantly higher antibody titer against IBDV in the birds fed with SCFP was not detected earlier as the earlier studies with SCFP focused on NDV only. In India, both the ND and IBD are considered as major viral infections, producing constraints in optimum production [
59] for which the study objective took an inclusive approach to consider both.
The present study could not find modulation of cell-mediated immune response in the studied birds, which was more pronounced in challenge studies—especially with intracellular pathogens (for example,
Coccidia) fed with yeast hydrolysate and was also dependent on the dosage of the yeast products [
60].