1. Introduction
Weaning pigs are very susceptible to environmental stress and gastrointestinal disorders due to the inflammation caused by pathogenic bacteria such as
Salmonella and
Escherichia coli [
1,
2]. In previous decades, antibiotic growth promoters have been used therapeutically on farms to improve the growth performance, health, and well-being of weaning pigs, and for the prevention of diseases [
3]. Even though they have presented many beneficial effects for these animals, the use of antibiotics must be reduced due to the development of resistance by several strains of pathogenic bacteria [
2,
3]. Nowadays, many studies are seeking to develop a feeding strategy to reduce and minimize the use of antibiotics in weaning pigs. The alternatives to antibiotics, such as probiotics, promise a similar beneficial effect to antibiotics in weaning pigs [
1,
2,
4,
5]. Previous studies found that application of probiotics could reduce inflammation by pathogenic bacteria in the gut of weaning pigs, and then increase the pigs’ performance and health [
2,
3,
4]. Moreover, the probiotics could promote the development of the intestinal tract in weaning pigs [
6].
In general, three main cultured microbes are commonly used as additives in pigs: lactic acid bacteria (LAB),
Bacillus, and yeast [
2,
5,
7]. Several strains of cultured LAB and
Bacillus are able to produce antimicrobial substances [
8,
9], which can help to improve the performance of weaning pigs. Additionally, strains of cultured
Bacillus or yeast were also reported to produce digestive enzymes [
10,
11]. Selected microbes consisting of LAB (
Pediococcus acidilactic and
Lactobacillus plantarum),
Bacillus subtilis, and
Saccharomyces cerevisiae were isolated from different sources and selected because of their ability to produce antimicrobial substances or digestive enzymes such as protease, amylase, and lipase. A mixture of these microbes as a feed additive was reported to improve the meat quality of Korean native chicken and Hanwoo beef cattle [
12,
13]. However, the effects of these microbes on weaning pigs have not been tested yet. In the present study, the mixture of these selected microbes was applied to weaning pigs as a microbial additive to study the dual antimicrobial and digestive-enzyme activities. Antimicrobial activity might have beneficial effects by improving immune response and decreasing inflammation of the gut by pathogenic bacteria. On the other hand, the digestive activity might have a positive effect by increasing nutrient absorption in the intestinal tract. In our hypothesis, dietary microbial additives are expected to produce dual activities to improve not only growth performance and blood metabolites, but also fecal microflora in weaning pigs. Therefore, the present study aimed to investigate the effects of microbial additives producing antimicrobial and digestive-enzyme activity on the growth performance, blood metabolites, and fecal microflora of weaning pigs.
4. Discussion
The actual counts of LAB were reported to be lower than the original counts, while the counts of
Bacillus and yeast were similar to the original counts. The decreases in LAB counts could have occurred normally due to the storage time. Nevertheless, the actual counts of LAB,
Bacillus, and yeast were in the recommended range as feed additives for animals according to a previous study, which found such range to be 10
6 to 10
8 cfu/g [
2]. In the present study, the initial body weight was not different statistically among treatments, which indicated a similar condition of the piglets when the experiment was started. Generally, the growth performances of weaning pigs were not affected by supplementary microbial additives, but a supplementation level of 1.0% tended to result in the highest feed efficiency. Similar to the present study, Suo et al. [
5] reported that supplementary microbial additives presented a quadratic pattern for feed efficiency of pigs according to the doses of application. According to Pan et al. [
20], microbial additives are capable of enhancing immune responses and attenuating intestinal damage, thus improving weaning pig performance. Moreover, microbial additives were reported to have beneficial and antibiotic effects. This indicated that microbial additives were an alternative feed additive to replace the use of antibiotics. Choi et al. [
21] reported that dietary microbial additives containing a mixture of
L. acidophilus,
B. subtilis, and
S. cerevisiae improved ADG, ADFI, and feed efficiency of weaning pigs. In the study, dietary yeast culture as an additive also was effective in increasing digestibility and growth performance of weaning pigs [
22]. The growth performances of weaning pigs increased with supplementation of a single LAB, such as
L. acidophilus, in diet [
23]. In contrast, Nguyen et al. [
7] reported that a microbial additive containing a mixture of
Bacillus spp. had no effects on growth performance of weaning pigs, but improved fecal bacteria due to the presence of antimicrobial substance from
Bacillus spp. Similar to Nguyen et al. [
7], Xuan et al. [
24] also reported that microbial additives containing
S. cerevisiae and
Bacillus spp. had no effects on growth performance of weaning pigs. Based on those previous studies, effects of microbial additives on growth performances of weaning pigs could be varied depending on the microbe species and strain. There are several factors that affect the effectiveness of microbial additives to improve the growth performance of pigs, including microbial strains, doses, environment, physiological condition, and duration of treatment [
2,
25].
The concentration of IgG in blood could reflect the immune response of the animal, with a higher IgG concentration in blood indicating a better immune response. Liu et al. [
26] reported that dietary direct-fed microbes, prebiotics, yeast, or plant extract potentially could improve the immune response of pigs, even though the result might not be consistent in every trial, depending on the animal condition, doses, and environment. The results for IgG in the present study were in agreement with previous studies that reported a similar improvement of IgG by using supplementary microbial additives [
6,
27]. In addition, this result supported the microbial count in feces, in which the populations of
Salmonella and
E. coli were reported to decrease linearly with an increasing supplementation level (
Table 6). The mechanism of microbial additives to enhance immune response is not fully understood, but their ability to modify the microbial ecosystem in the gut might improve the immune response of weaning pig [
6,
26].
Supporting the results for feed efficiency, supplementary microbial additives at 0.1% improved insulin and glucose concentrations in the blood. The increased concentrations of insulin and glucose in the blood might be a response to increased energy absorption in the intestine [
2,
5,
25]. Microbial additives were reported to increase the length of intestinal villi, which increased the nutrient-absorption surface in the small intestine and apparent digestibility [
2,
5,
25]. The other reason for higher glucose and insulin could be due to the activity of digestive enzymes from
B. subtilis SK877,
B. subtilis BBG-B20, and
S. cerevisiae BBG-Y6, which increased the nutrient digestibility of weaning pigs. Antimicrobial activity produced by
P. acidilactic BBG-L1 and
L. plantarum SK3121 could also have a role in effectively inhibiting pathogenic bacteria in the digestive tract to assist in better absorption of nutrients by weaning pigs, according to Liao and Nyachoti [
2]. On the other hand, IGF-1 concentration in blood had a positive correlation with the growth performance of weaning pigs [
28]. IGF-1 was highest with supplementation at 1.0%, which supported the results of insulin, blood glucose, and feed efficiency. However, supplementation at 1.5% had similar results to 0% and 0.5% on insulin, IGF-1, and blood glucose, which indicated a saturated condition when it was supplemented at a high dose. In some cases, an overdose of microbial additives can even reduce immune response and decrease growth performance [
5,
25,
26,
29]. The high concentration of BUN in blood reflected the excretion of nitrogen by the animals, which indicated a low utilization of nitrogen for protein synthesis [
30,
31]. Devi and Kim [
32] reported that supplementary
Enterococcus faecium had no effect on BUN concentration in the blood of weaning pigs, while Liu et al. [
33] reported that supplementary
L. brevis decreased BUN concentration. The reason for the increase of BUN concentration at higher supplementation levels of microbial additives was unclear in the present study. It might have been caused by higher absorption of nutrients, including nitrogen, at higher supplementation levels of microbial additives, which would support the results for feed efficiency.
A population of complex microbes in feces could indicate such population in the gut. In fecal microflora, increasing the supplementation level of microbial additives increased the LAB count, but decreased
Salmonella and
E. coli counts linearly after 21 d of feeding due to the presence of antimicrobial activity by
P. acidilactic BBG-L1 and
L. plantarum SK3121. These results were in agreement with several previous studies [
4,
7,
23,
34], which also reported a beneficial effect of microbial additives to inhibit pathogenic bacteria in feces. A population of pathogenic bacteria in feces has a positive correlation with their population in the gut. High populations of
Salmonella and
E. coli in the gut of weaning pigs reduce nutrient absorption for the host and cause gastrointestinal disorders such as diarrhea [
25]. With a decreased population of pathogenic bacteria, it can improve gut health and reduce diarrhea frequency in weaning pigs [
2,
26].