1. Introduction
Gut health determines the whole health of the animal because the gut is not only a vital organ for digestion and nutrient absorption but also an immune organ [
1]. Kittens have underdeveloped gut; therefore, they are more prone to intestinal health problems and exhibit symptoms of vomiting, diarrhea, reflux, and weight loss [
2]. Notably, intestinal health in early life is critical to health and disease development in later life [
3]. The colonization of gut microbes in infancy may define the lifelong composition of the microbiome [
4]. Moreover, gut microbes influence nutritional intake, regulate intestinal homeostasis and physiological function, and drive immune responses in pets, thereby affecting their health [
5]. Nutritional interventions in the diet can rapidly and repeatedly modify the composition and function of the gut microbiota [
6]. Numerous studies have indicated that altering the dietary composition of cats can significantly influence the gut microbiome within 1 to 2 weeks [
7,
8]. There were significant differences in the composition of the intestinal microbiota between young and adult cats, and a marked increase in the structural and functional diversity of microbes was reported in adult cats [
9]. Therefore, optimizing the intestinal microbiota structure through dietary interventions may be an essential means of improving the gut health of kittens. However, very few studies have used kittens as test subjects, with little data available for reference. Current studies have generally been conducted on adult cats.
Prebiotic supplementation is currently among the significant methods for improving the intestinal health of pets. Seaweed is rich in various bioactive compounds with anticancer, antioxidant, anti-inflammatory, and antibacterial properties [
10,
11,
12]. More importantly, most of these active substances can resist gastric acid and host digestion, making seaweed a promising substrate for prebiotics [
13]. Seaweed and its extracts can enhance nutrient digestibility and immune system function, thereby improving the growth and production performance of economic animals [
14,
15,
16]. Furthermore, studies have shown that polysaccharides in seaweed can scavenge free radicals and enhance the endogenous antioxidant system of animals, thereby treating various diseases [
17]. Meanwhile, seaweed polysaccharides are responsible for anticancer effects through inducing apoptosis of cancer cells, inhibiting lysosome-dependent autophagy, and suppressing cancer cell invasion and metastasis [
18]. Conversely, research on the impacts of seaweed on pets is limited. A study showed that dogs fed a seaweed-supplemented diet exhibited no change in nutrient digestibility, fecal microbiota, and the metabolome [
19]. Moreover, studies on the effects of seaweed on cat intestinal health have not yet been conducted.
Probiotics alter the host gut microbiota through different mechanisms, such as by changing the abundance of pathogenic bacteria, promoting microbiota interactions to stimulate the growth of resident bacteria, or indirectly influencing the microbiota structure through metabolite secretion [
20]. Notably, probiotics are effective only if they tolerate stomach acid and bile salts, and adhere to and colonize the intestinal lumen. Otherwise, they will be excreted as passers-by bacteria. Factors such as resident microbiota, host characteristics, and diet determine the colonization success of probiotics [
21]. In addition, some studies have questioned whether probiotics are safe for use in pets [
22]. Therefore, additional studies are required to track the benefits of probiotics as well as the frequency and severity of their adverse outcomes in pets over time. Accumulating evidence suggests that
Saccharomyces boulardii (
S. boulardii) can inhibit the colonization of pathogenic microorganisms, improve intestinal barrier function, and regulate immunity [
23]. This microbe is widely used as a commercial probiotic in pets.
S. boulardii supplementation can improve the intestinal status, reduce stress, and act as an effective adjunct to the treatment of chronic enteropathies in dogs [
24,
25]. However, studies on the application of
S. boulardii in cats are few, and its effects on the intestinal health of cats and the underlying action mechanisms remain unclear.
Ragdoll kittens were selected in this study because they have delicate intestines and a higher incidence of intestinal diseases. The study compared the effects of the enzymolysis seaweed powder and S. boulardii on intestinal health in kittens, particularly focusing on the gut barrier function and microbiota composition.
4. Discussion
The intestinal development of young animals is not sound, and they lack a stable intestinal microbiota, which makes them prone to intestinal microecology disorders. It is well known that dietary supplementation with probiotics and prebiotics is beneficial to animal intestines and, therefore, related products have been used in pet food. In vivo studies have focused on revealing the action mechanisms of specific dietary components. However, because of the large individual differences, the number of in vivo trials conducted on pets is very low, and adult animals are commonly used for experiments. In this study, it will be of special significance to take the lead in using kittens with larger scale and better sample uniformity. Numerous studies have shown that enzymolysis seaweed powder has a crucial application value in the breeding of economic animals and positively influences the production performance and body health of livestock and poultry [
14,
28,
29]. Unfortunately, no application research has been conducted on enzymolysis seaweed powder in cats. Therefore, to provide data supporting the application of enzymolysis seaweed powder in pet food, this study explored the effects of this seaweed powder on immunity, antioxidant properties, intestinal permeability, and microbiota composition of kittens compared with the common commercial
S. boulardii.
Immunoglobulin is a globulin with antibody activity and is widely found in mammalian serum, interstitial fluid, and exocrine fluid. The plasma immunoglobulin level can indirectly reflect the animal’s ability to resist exogenous stimuli and pathogens. It is a critical parameter to evaluate the immunity of animals. As the major component of serum immunoglobulin, IgG accounts for 75–80% of total serum immunoglobulin. It is mainly synthesized and secreted by plasma cells in the spleen and lymph nodes [
30]. IgG plays a pivotal role in combating infections in young animals [
31]. IgA is crucial for local anti-infection action of the body’s mucosa. The blocking of IgA synthesis increases susceptibility to local infections with microorganisms [
32]. Studies have shown that dietary supplementation with seaweed by-products increased IgA concentration in the serum of chicks [
33]. Serum IgG concentration in rats supplemented with heat-treated dried brown seaweed increased significantly after 16 weeks [
34]. These results are in line with our findings, which showed that direct dietary supplementation with enzymolysis seaweed powder increased IgA and IgG levels. In addition, supplementation with seaweed extract increased IgA and IgG levels in maternal blood and colostrum, further enhancing the circulating IgG concentration and the percentage of leukocytes and lymphocytes phagocytosing Escherichia coli in lactating juvenile animals. This suggested that enzymolysis seaweed powder could improve the immune function by increasing immunoglobulin levels in young animals [
35,
36].
A stable reactive oxygen species (ROS) is essential for maintaining a normal physiological function. Under normal circumstances, the free radical content is at an equilibrium level. These free radicals are involved in cell signal transduction, synthesis and metabolism of substances, and energy production in the body. However, oxidative stress can trigger disease development when an imbalance between ROS production and antioxidant networks occurs [
37]. Owing to their high sensitivity and activity, cellular stress levels may be higher in young animals [
38]. SOD scavenges superoxide free radicals, and MDA is a lipid peroxidation product generated in the metabolism of oxygen free radicals in the organism. In this study, compared with the basal diet and the diet supplemented with
S. boulardii, the addition of enzymolysis seaweed powder to the diet significantly increased the SOD level and reduced the MDA level in kittens. Consistent with our findings, a study showed that seaweed polysaccharides may increase SOD, CAT, and GSH levels and decrease the MDA level by regulating the NRF2 signaling pathway [
39]. In addition, seaweed polysaccharides exert a beneficial effect on acetaminophen-induced acute liver injury in rats by elevating GSH, GSH-Px, and SOD expression and decreasing the MDA concentration [
40]. Notably, supplementation of seaweed-derived polysaccharides increased the SOD level of weaned piglets at 24 days of age, along with changes in intestinal microbiota and increase in acetic acid and butyric acid concentrations [
41].
Studies have confirmed the role of gut microbes in contributing to growth and development, maintaining health, and regulating the occurrence and treatment of various diseases in pets [
42,
43]. Moreover, the intestinal microbes and their functional products affect host health [
44,
45]. Cats are obligate carnivores and need a protein-rich diet. Therefore, the gut microbiota composition of cats is different from those of humans and many other mammals. However, the amount of microbiome research conducted in cats is relatively small compared with that in humans and other mammals.
Firmicutes,
Bacteroidetes,
Actinobacteria,
Proteobacteria, and
Fusobacteria were the predominant phyla in previous feline microbiota reports that used different sequencing technologies [
7,
46,
47], and these results are consistent with those of our study. Aging in the gut is related to changes in microbial composition, developing from colonization in early life to relative stability in adulthood and then decreasing diversity in old age [
7,
9,
48]. Bacteroides were less represented in the kitten intestines but significantly increased in the adult cats, probably balanced by a reduced abundance of
Firmicutes and
Actinobacteria [
7,
49]. In our study, enzymolysis seaweed powder increased the Shannon index, decreased the Simpson index, and increased the abundance of
Bacteroidetes, indicating that enzymolysis seaweed powder is beneficial for the stability and maturity of kitten intestinal microbiota. The abundance of Prevotellaceae and Bacteroidaceae (members of the phylum Bacteroidetes) as well as Lachnospiraceae and
Faecalibacterium (members of the phylum Firmicutes) was significantly reduced in dogs with chronic enteropathies [
50,
51,
52,
53]. These gut bacteria are known to produce SCFAs. In our study, enzymolysis seaweed powder added to the basal diet increased the abundance of these bacteria. In addition, Muribaculaceae, much like Bacteroides, ferment plant polysaccharides to produce propionate [
54]. Consistent with the study findings, dietary addition with brown seaweed elevated the relative abundance of Muribaculaceae in the gut of high-fat- and high-sugar-diet-induced obese rats [
55]. Although
Eubacterium hallii cannot degrade complex polysaccharides and oligosaccharides, it can produce butyrate by using fermentation metabolites such as acetate and lactic acid [
56]. Supplementing enzymolysis seaweed powder significantly increased the abundance of
Eubacterium_hallii_group, suggesting that SCFA generation changes to some extent. Thus, according to these results, enzymolysis seaweed powder may promote intestinal health by increasing the abundance of beneficial bacteria. Moreover, the abundance of
Sutterella significantly increased in dogs with acute diarrhea [
51]. Meanwhile, the abundance of
Desulfovibrio increased markedly in the intestines of cats with inflammatory bowel disease [
57].
Erysipelatoclostridium and its related metabolite, ptilosteroid A, have been considered markers of radiation-induced intestinal damage [
58]. In this study, dietary addition with
S. boulardii markedly reduced the relative abundance of
Desulfovibrionaceae,
Erysipelatoclostridium, and
Sutterella, suggesting that
S. boulardii improves the gut microbiota structure in kittens by lowering harmful bacteria.
The SCFAs are the main metabolites of intestinal microorganisms and serve as a source of energy for gut epithelial cells. They have various biological functions, such as enhancing the gut mucosal barrier function and participating in intestinal immune regulation [
59]. Acetic acid, propionic acid, and butyric acid are the most abundant SCFAs in the animal’s digestive tract. Similar to the case in other species, probiotics and prebiotics can affect intestinal SCFA production in felines by regulating the intestinal microbiota structure, especially by increasing the amount and activity of SCFA-producing bacteria [
1]. The acetate and propionate contents of pre-digested red seaweed increased significantly after fermentation in a colonic model in vitro [
60]. Furthermore, dietary supplementation with 400 mg/kg seaweed-derived polysaccharides significantly increased the cecal concentrations of acetate and butyrate in weaned piglets [
41]. However, dietary addition with seaweed had no effect on the SCFA composition and concentration in dog feces [
19], which is consistent with our findings in kittens. In addition, we did not find an effect of S. boulardii on SCFAs in kitten feces. Similarly, a study reported no significant effect of
S. boulardii on SCFAs in the feces of breeding dogs [
25]. Conversely, a recent study found that the addition of
S. boulardii and
Pediococcus acidilactici to feline diets increased the concentration of total SCFAs and butyric acid in cat feces, which may be beneficial for intestinal health [
61]. Additionally, humanized mice supplemented with
S. boulardii can increase the colonic contents of acetate, propionate, and butyrate, thereby alleviating colitis induced by dextran sulfate sodium [
62]. As obligate carnivores, cats possess a distinctive intestinal structure and digestive system that enable them to efficiently digest and absorb high-protein and high-fat diets. Nevertheless, their capacity for fiber digestion is relatively poor [
63,
64]. Although some in vitro studies have shown that cats may increase the production of SCFAs by digesting fiber [
65,
66], current commercial cat foods are rich in protein and fat while limiting the use of fiber-rich ingredients. Similarly, only sweet potatoes, potatoes, and rice in this experimental diet could provide cats with a small amount of fiber. Most SCFAs are bacterial metabolites produced by specific anaerobic bacteria of the colon after the fermentation of dietary fiber. It is possible that the low fiber intake of experimental cats resulted in a lack of fermentable substrates for the gut microbiota to produce SCFAs. Therefore, the effects of enzymolysis seaweed powder and
S. boulardii on SCFAs were nonsignificant under the test conditions.
The gut mucosal barrier is pivotal in preventing the invasion of pathogenic antigens and maintaining a normal intestinal function. Increased intestinal permeability indicates damage to the gut epithelial barrier, causing the penetration of toxins and pathogens [
67]. D-LA and LPS are intestinal bacterial metabolites that disrupt the intestinal mucosal barrier, invade the bloodstream, and promote the release of inflammatory factors [
68,
69]. DAO is a highly active enzyme in the epithelial villi of the mammalian intestinal mucosa, and iFABP mainly exists in the mature intestinal epithelial cells of the small intestine. Under normal conditions, they are present at very low levels in the blood. Damage to the gut mucosal barrier increases permeability, resulting in the release of large quantities of the above substances into the bloodstream [
70,
71]. This study found that the dietary addition of enzymolysis seaweed powder significantly reduced plasma levels of D-LA, LPS, DAO, and iFABP in kittens compared with the basal diet and the
S. boulardii diet. Consistent with our results, sodium alginate improved the gut mucosal barrier function in cyclophosphamide-induced immunosuppressed mice, which reduced serum concentrations of D-LA and LPS [
72]. Furthermore, dietary supplementation with seaweed-derived polysaccharides could promote the levels of claudin-1, occludin, and ZO-1 in the jejunal mucosa and significantly reduced DAO activity and serum D-LA concentrations in weaned piglets [
41]. Seaweed extract may improve intestinal health by strengthening the gut barrier function.
The gut is the largest immune organ of the body. Intestinal physical, chemical, and microbial barriers influence and alter host inflammation by regulating homeostasis as well as tolerating and preventing pathological immune responses. Cytokines, a class of endogenous polypeptides generated mainly by immune system cells, have many biological effects and can mediate various immune responses. The dynamic balance between pro-inflammatory and anti-inflammatory cytokines is the key for maintaining the body’s normal immune status and physiological activities [
73]. IL-1β, IL-6, and TNF-α are typical pro-inflammatory cytokines. Among them, IL-1β and TNF-α are pleiotropic, and their local activation can lead to elevated levels of secondary inflammatory mediators including IL-6 [
74]. Laminarin can significantly downregulate the expressions of colon mucosal inflammatory factors IL-6, IL-17, and IL-1β, thereby improving the intestinal health of piglets [
75]. Additionally, the brown seaweed extract reduced IL-6, IL-8, and TNF-α expressions in piglets with LPS-induced colitis, thereby inhibiting the pro-inflammatory factor response [
76]. Consistent with the findings of other studies, we found that a dietary addition of enzymolysis seaweed powder in kittens reduced TNF-α, IL-1β, and IL-6 levels, probably by inhibiting NF-κB activation to reduce the inflammatory response [
77]. Seaweed polysaccharides were reported to inhibit NF-κB activation and downregulate the mRNA expressions of TLR-4, MyD88, and IκBα, thereby inhibiting IL-6 and TNF-α expressions [
77]. IL-10 is a pleiotropic cytokine mainly secreted by antigen-presenting cells. It has powerful anti-inflammatory properties that inhibit the levels of pro-inflammatory cytokines by activating macrophages [
78]. In our study, compared with CON diet- and
S. boulardii diet-fed kittens, the serum IL-10 level in kittens fed with enzymatic seaweed powder significantly increased, indicating that enzymolysis seaweed powder improved the anti-inflammatory ability of kittens to some extent.