2.1. Improved Gut Health and Enhanced Immune Function

Probiotics play an important role in supporting a harmonious balance of microorganisms in the gut, contributing to a healthy gut environment. When they establish themselves in the gastrointestinal tract, they hinder the proliferation of harmful bacteria, lower the likelihood of infections, and improve the digestion and absorption of nutrients [14]. They are commonly found in the guts of healthy animals, where they help maintain a balanced microbial community and contribute to overall health and well-being. Probiotics have been shown to improve gut health, improve immune function, and reduce the risk of gastrointestinal diseases in poultry [15]. One of the key benefits of probiotics is their ability to prevent or reduce the incidence of disease in poultry. Probiotics can inhibit the growth of pathogenic bacteria by competing for nutrients and adhesion sites in the intestinal tract, ultimately reducing colonization and proliferation of harmful bacteria. Various studies have shown that probiotics reduce the incidence of diseases such as necrotic enteritis [16], coccidiosis [17], and B. subtilis [18].

In addition to disease prevention, probiotics have been explored as alternatives to antibiotics in poultry production [19]. Antibiotics have been widely used in poultry production to promote growth and prevent diseases; however, their use has been associated with the emergence of antibiotic-resistant bacteria. Probiotics offer a natural and sustainable approach to disease management, without relying on antibiotics [20]. Several studies have shown that probiotics can improve growth performance and feed efficiency in poultry [21], while reducing the need for antibiotics [22]. Probiotics have also been associated with stress management in poultry. Stress can harm poultry health and productivity; however, probiotics have been shown to modulate the stress response by regulating the production of stress hormones and improving communication of the gut-brain axis [23]. This can lead to better overall health and performance of poultry [24].

Probiotics can activate the immune system of poultry. They regulate the generation of immune cells such as T and B lymphocytes, macrophages, and natural killer cells, resulting in enhanced immune reactions against harmful microorganisms. This increased immune activity contributes to protecting poultry from diseases, minimizing the need for pharmaceutical treatments. Probiotics can activate and strengthen the immune system of poultry, increasing their resistance to a wide range of diseases and infections. Improved immune function is a significant advantage of incorporating probiotics into poultry production [25]. Studies have shown that probiotics can effectively modulate and fortify the immune system of poultry, resulting in enhanced protection against various diseases and infections. One of the ways probiotics achieve this is by stimulating the production of cytokines, which are important signaling molecules involved in the regulation of the immune response. Cytokines are signaling molecules that play a critical role in the activation and regulation of immune cells. Research has shown that specific probiotic strains can stimulate the production of cytokines in poultry. In a study by Muhammad et al. [26], broiler chickens were administered Lactobacillus acidophilus as a supplement, resulting in an improved immune response characterized by increased cytokine production and improved immune cell function [17]. In addition to cytokines, probiotics can also influence the production of immunoglobulins, which are essential components of the immune system. Immunoglobulins, including IgA, IgG, and IgM, play a crucial role in the defense of the body against pathogens by neutralizing them and preventing their colonization [25]. Studies have indicated that supplementing poultry with probiotics can enhance immunoglobulin production. According to Babaei et al. [27], broiler chickens received probiotics containing Lactobacillus acidophilus and Lactobacillus casei, resulting in an increase in immunoglobulin production compared to the control group [27]. Additionally, probiotics can support the development and maturation of gut-associated lymphoid tissue (GALT), an essential component of the immune system in the gastrointestinal tract. Healthy GALT is crucial for the formation of effective immune responses against pathogens. As shown in Figure 1, the “Mechanism of action of probiotics and their uses,” probiotics play a crucial role in improving poultry health and performance through various mechanisms, ultimately contributing to sustainable poultry production practices. The figure outlines how probiotics can enhance gut health, improve immune response, and promote growth in poultry, highlighting their multifaceted benefits. By leveraging these positive effects of probiotics, the figure suggests that they can be leveraged to support more sustainable poultry production practices (Figure 1).

Probiotics can regulate intestinal microflora, creating an environment that supports the growth and function of intestinal-associated lymphoid tissue (GALT). Probiotics stimulate the production of immune-modulating compounds and improve the activity of immune cells, thus improving the ability of birds to fight infections [28]. Probiotic supplementation has been shown to increase antibody production and improve the overall immune response in poultry [29]. As shown in Figure 2 in the following, this in turn improves the overall gut health and immune function of poultry. However, it is important to consider that the effects of probiotics on immune function can vary depending on factors, such as the specific strain used, the dosage, and the duration of administration. Different probiotic strains may have different abilities to stimulate immune responses. Therefore, it is essential to carefully select probiotic strains that have been extensively studied and have shown positive effects on immune function in poultry.

2.2. Pathogen Displacement and Production of Antimicrobial Substances

Probiotics have been shown to have a significant potential to prevent or reduce various diseases in poultry. These beneficial microorganisms can help maintain healthy gut microbiota balance, enhance immune response, and reduce colonization of harmful bacteria in the gut. Some probiotic strains can effectively displace harmful pathogens that adhere to the intestinal wall by occupying receptor sites. This displacement mechanism inhibits pathogen colonization and the consequent infection [30], decreasing the likelihood of diseases such as salmonellosis [5], clostridiosis, and coccidiosis [31]. Poultry are common carriers of Salmonella, and transmission to humans can occur through contaminated poultry products. Probiotics, such as Lactobacillus acidophilus and Enterococcusfaecium, have been shown to reduce Salmonella colonization in the intestine of poultry, thus reducing the risk of transmission to humans [29].

Probiotics can generate antimicrobial substances, including organic acids and bacteriocins, that hinder the growth of harmful bacteria. These compounds create an unfavorable environment for pathogens, leading to a decrease in their population and effective prevention of disease outbreaks. Probiotics have been increasingly recognized as alternatives to antibiotics in various applications. The overuse of antibiotics in animal production has led to the emergence of antibiotic-resistant bacteria that pose a significant threat to public health. Probiotics, on the other hand, offer a natural and safe alternative to antibiotics in various applications. Antibiotics have been shown to improve feed efficiency and increase weight gain in poultry and livestock. The use of antibiotics as growth promoters has been associated with the emergence of antibiotic-resistant bacteria. Probiotics have been shown to have similar effects on growth promotion in poultry and livestock [32].

2.3. Improved Nutrient Utilization

Probiotics help break down and ferment complex carbohydrates, enhancing the utilization of nutrients such as fiber and starch. This improved availability of nutrients supports the optimal growth and performance of poultry, thus reducing the likelihood of nutrient-related disorders. Live microbes use nutrients and energy for growth and proliferation within the host [21]. According to a study by Jha et al. [33], probiotic supplementation improved intestinal barrier function and increased the ratio of villus height to crypt depth, leading to greater absorption in the intestine and a concurrent improvement in the ATTD of nutrients [33]. The enhanced digestibility and absorption of nutrients can be attributed to the production of extracellular enzymes by the vegetative form of B. subtilis [34]. Jin et al. [35] investigated the effects of L. acidophilus and a mixture of Lactobacillus strains on the utilization of nutrients in 180-day-old Arbor Acres chicks. Their results showed that both supplements increased amylase levels in the small intestine and reduced β-glucuronidase and β-glucosidase activities in the intestinal and fecal β-glucuronidase and β-glucosidase activities at 40 days of feeding [35]. Singh et al. [36] investigated the effects of combining enzymes with probiotics (three Bacillus spp.) on nutrient digestibility in Cobb 500 broilers. They observed that this combination increased the digestibility of most amino acids, except arginine and serine, compared with the control group [36]. These findings suggest that probiotics can selectively influence the utilization of key nutrients. As illustrated above in (Figure 2), probiotics have been shown to significantly improve nutrient utilization in poultry, leading to improved feed efficiency and overall health benefits for birds.

2.4. Stress Reduction

Poultry production is frequently accompanied by numerous stressors, such as transportation, handling, and dietary adjustments [37]. These stressors can compromise the immune system and make birds more vulnerable to diseases [38]. Probiotics have demonstrated the ability to alleviate stress by regulating stress hormone production and influencing the gut-brain axis. By reducing stress levels, probiotics help maintain a robust immune system, ultimately promoting overall poultry health [39]. There are many disputable hypotheses regarding why probiotics eliminate heat stress problems. Probiotics increase the activity of thyroid hormones, whose secretion is abnormally reduced during heat stress. Thyroid hormones have a significant effect on body metabolism and normal growth and development [40]. Returning the T3 (triiodothyronine) and T4 (thyroxine) hormones to the correct levels could reduce the number of abnormal changes in intestinal tissues and increase the growth of birds [41]. Heat stress creates problems in the poultry industry, negatively affecting the immunology, physiology, and microbiology of birds. Excess ambient temperature alters intestinal morphology, disrupts absorption and digestion, and facilitates the penetration of toxins and luminescent antigens into the blood [42]. Ischemia and hypoxia in intestinal tissues are other negative effects of heat stress. The use of probiotics reduces stress in laying hens, improving their well-being and productivity. Probiotics support intestinal health, boost immunity, and manage stress responses, benefiting hen welfare and egg production sustainability [43]. Modern breeds of chickens are particularly vulnerable to the negative impacts of heat stress. Broiler chickens, known for their rapid growth, generate more heat, leading to increased susceptibility to heat stress [44]. Figure 3 illustrates the detrimental impact of heat stress on the intestinal barrier and outlines the mechanisms through which probiotics provide protective effects, adapted from Abd El-Hack et al. [7].

2.5. Antibiotic Alternatives

As concerns about antibiotic resistance increase, finding effective alternatives has become imperative in poultry production. Probiotics present a promising solution, as they offer similar advantages to antibiotics without associated risks [45]. Unlike antibiotics, probiotics do not contribute to antimicrobial resistance and can be used safely for extended periods. Probiotic supplementation of broilers with B. licheniformis and B. subtilis may significantly decrease the load of Salmonella in the ileal and cecal microflora [46, 47]. When broilers challenged with Salmonella enteritidis were fed a Bacillus coagulans diet, this diet helped increase Lactobacilli and Bifidobacterium but reduced coliform and Salmonella concentration in the cecum [33]. Probiotics have been identified as alternatives to antibiotic growth promoters (AGP). Given the diversity between strains and species of microorganisms, a particular characteristic is that they do not leave residues in animal products, as opposed to antibiotics [17]. Figure 4 shows the effectiveness of probiotics as an alternative to antibiotics. This is an important consideration given the growing concerns around antibiotic resistance and the need to explore alternative treatment options. The data shown in the figure indicates that probiotics can be as effective, or in some cases, even more effective, than traditional antibiotic therapies. This review highlights the potential for probiotics to serve as a complementary or alternative approach to addressing various health conditions, potentially reducing the overreliance on antibiotics.

2.6. Cost-Effectiveness and Environmental Benefits

Inclusion of probiotics in poultry production can result in cost savings [48]. Probiotics can help reduce feed expenses by improving nutrient utilization and improving feed efficiency. Furthermore, the reduction in the occurrence of infections and the decrease in antibiotic dependency can lead to lower vet costs, ultimately reducing overall production expenses on poultry farms [49]. This cost-effectiveness establishes probiotics as viable and sustainable options for disease management. Incorporation of probiotics into poultry production can produce environmental benefits. By decreasing antibiotic dependency, the release of antibiotic residues into the environment can be minimized. Furthermore, probiotics can reduce the excretion of harmful pathogens from poultry waste, reducing the risk of contamination in the surrounding environment [50].

2.7. Consumer Preference

As the demand for natural and organic poultry products continues to increase, the use of probiotics is due to consumer preferences. Probiotics are considered a natural method of treating diseases and are appealing to consumers who desire products containing minimal synthetic additives or antibiotics. The use of probiotics can improve the quality of meat and eggs, which is beneficial as the demand for meat and eggs continues to grow as the global population increases [5]. The main goal of poultry farming is to provide healthy and safe poultry products. Probiotics have become alternative growth promoters that increase the quality and productivity of animal products. Probiotics have a positive effect on the overall weight of the carcass, with reduced abdominal fat leading to better carcass quality for poultry. Increased carcass yield has been observed in chickens, regardless of sex [51, 52]. The use of cholesterol by probiotic bacteria contributes to the reduction of cholesterol levels in meat [53]. Probiotics containing Bacillus licheniformis have resulted in significant improvements in the amount of protein and essential amino acids [54]. The combined supplementation of A. awamori and S. cerevisiae synergistically improves growth performance by promoting muscle protein metabolism [55]. Probiotics can improve chicken meat flavor, increase intestinal microbiota diversity, and improve meat oxidative stability. An important aspect for consumers is to improve the taste, smell, and color of poultry meat [56]. The quality of the eggs was improved by increasing their strength and shell thickness [57]. The fertility and hatching capacity of the eggs improved in the flocks where probiotics were administered [58]. In the groups of laying hens fed probiotics, the dry matter of the eggs and the percentage of protein increased. This can cause an increase in egg weight. Improvement in the quality of life of animals due to probiotics also has positive economic effects, as the number of broken eggs is reduced due to a stronger shell structure [59]. Table 1 provides a comprehensive summary of the beneficial probiotics that are used in the effective management and control of poultry diseases. These probiotics play a crucial role in promoting poultry health, improving disease resistance, and improving overall well-being in poultry production systems.