1. Introduction
Poultry meats have been considered as an essential source of animal protein in human food. Due to the massive production system, broiler chickens are virtually exposed to different types of stress factors such as cages, density, temperature, pests, parasites, and disease agents [
1]. Such stressors may cause colibacillosis diseases induced by the activation of
Escherichia coli (EC) in the digestive system of broiler chickens [
2]. EC infection is one of the most critical challenges that counteracts the broiler growth and leads to a considerable economical drop for the business sector of meat production [
3,
4]. EC can invade the host via digestive or pulmonary tracts, then translocate across epithelium membranes to the bloodstream, and then colonize in various tissues [
5].
Poultry infection with EC is commonly associated with several diagnostic symptoms such as airsacculitis, salpingitis, perihepatitis, peritonitis, pericarditis, and septicemia, which subsequently results in sudden death [
4]. When EC is colonized in the intestinal tract, it disrupts the epithelial cells and adversely affects the absorption of nutrients [
6]. It was reported that EC infection in broiler chickens increased the expression of some proinflammatory cytokines including interleukins (ILs), such as IL-1, IL-6, and IL8, to alert the immune system against the source of inflammation or infection [
7]. In addition, EC caused oxidative stress and antioxidant-defense system depletion in the infected broiler chicks [
8]. Reactive oxygen species (ROS) and free radicals were generated as a response to inflammation and immunological reaction in the EC-infected birds, and consequently, harm hematological and tissue cells [
9]. Moreover, EC has a pathogenic component, lipopolysaccharide (LPS), which can induce immune stress in broilers [
10]. It was reported that infection with avian pathogenic EC in broiler chickens had adverse effects on antibody production and cellular immunity [
11] and caused an increase in the intestinal pathogenic bacteria against the beneficial bacteria [
12].
Spirulina platensisis (SP) is a microscopic spiral-shaped blue-green alga existing in sea and river water [
13]. Dried spirulina contains 50–70% protein with all essential amino acids, and 5–7% lipids including about 45% polyunsaturated fatty acids [
14,
15]. SP also comprises a wide range of fundamental nutrients, including vitamins (thiamine, riboflavin, nicotinamide, pyridoxine, folic acid, ascorbic acid, retinol, and α-tocopherol), minerals (potassium, calcium, chromium, copper, iron, magnesium, manganese, phosphorus, selenium, sodium, and zinc), and phytopigments (β-carotene, Xanthophylls, zeaxanthin, Chlorophyll, and phycocyanin) [
13,
16]. Additionally, SP has immunostimulant [
17,
18], anti-inflammation [
19], anti-carcinogenic [
20], and antioxidant activity [
21] properties. It was reported that growth efficiency, nutrient digestibility, antioxidant enzyme activity, anti-inflammatory response, and the cecal microflora population of broiler chickens were remarkably enhanced by the dietary supplementation of the diets with SP at 0.25–1.0% [
15,
22,
23]. Interestingly, SP supplementation into broiler diets was found to be beneficial under stress conditions. It was found that dietary supplementation with SP powder improved the growth, oxidation/reduction, and immunity of broiler chickens exposed to elevated temperature stress [
24,
25,
26,
27].
It is well-known that using antibiotic therapy in poultry breeding is increasingly not allowed worldwide due to the antibiotic resistance and residues, which compromise human and animal health [
28]. This has prompted the scientists and poultry-business holders to try natural substitutes to overcome the problems related to antibiotics in feed. With the wide biological properties of SP, it could be used as a feed supplement to the broiler diets during the intensive production or stressful condition. According to our knowledge, there is a lack of sufficient studies discussing the possible effect of SP administration to broilers challenged by EC infection. Therefore, the current study aimed to highlight the prospective impact of feeding SP to broilers challenged by EC infection on their growth outputs, redox activity, immune response, and microbial count and acidosis of lower intestine.
4. Discussion
The present work was designed to assess the potential effect of SP inclusion into broiler diets on their growth, antioxidant activity, immune response, and intestinal microbial count and acidosis of broiler chickens challenged by EC infection. We used a selected level of SP (10 g/kg as fed) which was recommended based on a previous study [
25]. As per the results of this study, dietary SP supplementation at levels of 5, 10, and 15 g/kg to broilers exposed to heat stress relieved the deterioration that occurred in the productive performance, carcass quality, redox status, and blood metabolites, with the best outputs recorded in the chickens fed 10 g/kg SP.
EC infection occurs in broiler chickens when they are exposed to substances or tools contaminated with an avian pathogenic strain of EC per se [
2] or exposed to various stress factors that activate EC in their digestive system [
1]. The current study manifested that EC infection deteriorates the growth performance of challenged broilers. The depression in feed intake reached 8% in EC-challenged broilers compared to non-EC-challenged broilers. The repression of feeding behavior may be due to the harmful action of nitric oxide and interleukin cytokines on the brain tissue of EC-infected birds [
34,
43]. The decrease in feed intake can directly cause a significant decrease in BWG and FCR [
44]. We recorded a reduction of 20% and 28% in the BW
42 and BWG, respectively, and an increase of 29% in the EC-infected broilers. Similarly, Boratto et al. [
37] found a reduction in BWG by 16% and a reduction in FI by 7%, as well as an increase in FCR by 9% in the EC-infected broilers compared to non-infected birds [
45].
The impairment in the broiler growth performance induced by EC infection in the present study was accompanied by an approximately 1.5-fold reduction in the GSH and SOD antioxidants, and a 2-fold increase in the CP and MDA levels. It was reported that
E. coli produces excess free radicals and ROS that can induce irreversible damage in the antioxidant defense system and, in turn, dramatically reduce the productive outputs in poultry [
46]. In addition, da Rosa et al. [
8] reported that chickens infected with
E. coli represent poor growth aspects and attributed this to the oxidative stress remarked by high levels of serum and hepatic ROS. In contrast, SP treatment improved the growth performance of both non-challenged and challenged birds with EC infection (
Table 3). The SP treatment also increased the antioxidant activity and decreased the oxidative stress in the broilers (
Table 4). Similar effects for SP were reported on the productive performance, redox activity, and immune response in normal broiler chickens [
15,
22,
23] and in broilers exposed to heat stress [
24,
25,
26,
27]. The positive effects of the SP treatment may be accounted for by its antioxidant and anti-inflammatory effects [
47]. The antioxidant properties of SP were evidenced via the chemical analysis of SP in our study (
Table 1) and other studies [
48]. The polyphenols and flavonoids that existed in the SP are sufficient to promote the scavenging action against the free radicals produced by EC in the infected broilers.
SP supplementation in poultry nutrition improved the performance not only by their biological functions but also by their rich nutrients [
13,
16]. According to the chemical analysis of SP in our study, it contained high amounts of protein (56.4%) and amino acids (
Table 1). It calculatedly means that supplemental 1% (10 g/kg) SP can increase the crude protein of the diet by 0.5% (5.6 g/kg). Thus, the improved performance of broilers treated with SP could be attributed to the contribution of SP to increase the high-quality protein content of the diets.
The EC challenge in the broilers dramatically suppressed all immunological parameters examined in this study. One of the major reasons for disturbing the immune system in EC-challenged birds is the presence of lipopolysaccharides (LPS) on the outer membrane of EC [
49]. LPS can produce large amounts of ROS and proinflammatory cytokines, which consequently disrupt the normal function of target cells, including immune cells [
50]. After the challenge, the lymphocyte proliferation was poor and a reduction of 73% and 50% was observed in the TLP and BLP, respectively. These data are consistent with earlier studies that found a suppressive effect of EC on the proliferative responses of avian splenocytes [
51,
52]. On the contrary, SP is well recognized for its anti-inflammation and immunomodulation properties [
23,
48,
53]. When the broilers were supplemented with SP, most of the immunological parameters were remarkably improved in both challenged and non-challenged broilers. The positive effect of SP on broiler immunity could be referred to its high nutritional value, especially with protein [
54]. In addition, the presence of essential amino acids in SP, as indicated in
Table 1, may contribute to several immunological functions such as the activation of T and B lymphocytes, the regulation of immune-cellular redox status, the lymphocyte proliferation, and the production of antibodies [
55,
56].
The intestinal microbial population and acidosis were also investigated in this study to illustrate the effectiveness of SP on the broilers’ health and their ability to resist pathogens before and after the EC challenge. Our results indicated that EC infection not only deteriorated the growth performance and suppressed the immune response, but also exhibited an expansion of harmful bacteria such as SLM and inhibited the beneficial bacteria such as LAB (
Table 6). However, it is fortunate that SP supplementation contrasted with these negative effects in the EC-challenged broilers. Interestingly, a recent in vitro study pointed out that spirulina extracts have antimicrobial activity against various pathogenic microorganisms [
57]. It was reported that SP bioactive compounds, such as phenols, act on the cell integrity and permeability of pathogens, thus restraining their invasion, attachment, motility, and biofilm formation into animal guts [
58,
59]. In addition, the development of LAB in the SP-supplemented broiler groups created an acidic-pH micro-ecology coming from the release of volatile fatty acids (VFAs) in the caeca and colon [
41]. These lipophilic VFAs penetrate the out-membrane of the pathogenic bacteria and produce hydrogen ions, which in turn lead to eliminating or destroying the bacterial cell [
60].