Necrotic enteritis (NE) is a type of enterotoxemia caused by Clostridium perfringens [1], it is a common disease affecting the poultry industry and the high contamination rates of poultry by C. perfringens can cause a threat to public health through the food chain [2]. An NE outbreak in broiler chickens often results in high mortality rates and reduced growth performance [3, 4]. Various in-feed antibiotics have been used to prevent and control this disease [4–6]. However, the use of a large amount of antibiotics as growth promoters can cause antibiotic-resistant genes to spread extensively by promoting the selection of antibiotic-resistant bacteria in animals. In view of this concern, many countries have limited the use of non-therapeutic antibiotics in poultry feed; as a result, NE incidences have increased significantly over the past decade [7, 8]. This disease costs the poultry industry $2 billion annually in the purchase of drugs to treat NE and in the lost body weight gain (BWG) [9, 10]. With the banning of in-feed antibiotics use, alternative methods of preventing NE outbreaks must be developed.

A potential approach to NE control is to supplement probiotics in the diets of broiler chickens to manipulate their gut ecosystems. Over the past few decades, Bacillus spp., such as B. licheniformis and B. subtilis, have been used in competitive exclusion experiments. B. subtilis competitively exclude C. perfringens from broiler chicks; in addition, these bacteria significantly improved body weight and feed efficiency [11–15]. B. licheniformis, which are “generally recognized as safe” bacteria, have long been extensively used in the poultry industry. This bacteria can serve as an alternative to antibiotics to enhance growth performance in poultry [16] and is a useful prebiotic for overcoming NE in a commercial-like condition [17, 18].

In broilers, the intermediary metabolism of lipids and energy usually occurs in the liver [19], as does the majority of the de novo fatty acid synthesis process [20, 21]. In the modern poultry industry, chickens are subject to various stress factors that can thus influence lipid metabolism [22–24]. Studies show that the livers of broilers suffering from NE undergo pathological changes [4, 25, 26]; nonetheless, changes in the lipids of broilers infected with C. perfringens are rarely investigated. At present, few studies have demonstrated the efficacy of B. licheniformis as prophylactic agents against NE in broilers. Therefore, the objectives of our present study are to investigate the effect of B. licheniformis on growth performance, on lipid metabolism, and on the hepatic expression of lipogenic genes in broilers infected with NE.

In the present study, the growth performance of broilers infected with C. perfringens was significantly poorer in the PC group than in the other groups. The reduced BWG and impaired FCR can be indicated by the damaged intestinal mucosa that accompanies C. perfringens infection [27, 28], and this damage can affect feed absorption and thus utilization [18].

Bacillus sp. microbes are among the most extensively used, direct-fed growth promoters [15]. These bacteria serve as an alternative to antibiotics. In the current experiment, a diet supplemented with B. licheniformis can significantly improve BWG and FCR despite C. perfringens infection. These effects are mainly attributed to the fact that B. licheniformis can enhance nutrient digestion and utilization in broilers by producing several enzymes, such as lipase, protease, and amylase [29, 30]. The diets supplemented with Bacillus sp. as a growth promoter can improve BWG and feed efficiency [15, 24, 31, 32]. Nonetheless, the effects of B. licheniformis on positive responses to growth performance, such as changes in gut flora, immunity responses, and direct substrate digestion, remain unclear.

The level of fatty acid circulation between the liver and adipose tissue is related to the containment of serum lipids and lipoproteins [33]. Several studies observed that probiotics can significantly reduce these levels in broilers [34, 35]. These results may be attributed to the ability of probiotics to bind cholesterol in guts; probiotics can remove cholesterol through binding onto cellular surfaces [36] and also can convert cholesterol to coprostanol for direct excretion via feces [37, 38]. Increased fecal-lipid and bile-acid output can reduce accumulate serum levels and liver lipids in the body [39].

The present data demonstrate that the birds infected with NE exhibited the highest level of GLU, TC, and LDL-C among those in all of the groups. NE-infected birds that were co-treated with B. licheniformis in this study had low concentrations of TC, GLU, and LDL-C but high levels of HDL-C. This result agrees with the finding presented by Yeon et al., who suggested that B. licheniformis can improve lipid metabolism in mice who were fed a high-fat diet [40].

Researchers hypothesize that oxidative stress influences growth performance and lipid metabolism in animals [15, 24, 41, 42]. In the present study, SOD, CAT, and GSH-Px serum activities were assayed as indices for serum antioxidant capacity, whereas MDA content served as an indicator to determine serum lipid peroxidation levels. MDA is among the most studied products of polyunsaturated fatty acid peroxidation, and its lipid peroxidation is facilitated by reactive oxygen species (ROS). The BL group in the current study displayed a low MDA level, thus indicating that B. licheniformis treatment can reduce oxidative stress. The inhibition of SOD, GSH-Px, and CAT activities contributes to the onset of many diseases [43]. In the present study, NE infection lowered CAT and GSH-Px serum activities. This result is similar to that of the research conducted by Lee et al., which postulated that SOD, GSH-Px, and CAT activities were inhibited in birds infected with NE [44]. In a previous study, dietary B. licheniformis supplementation significantly enhanced the antioxidant capability of triangular breams [45]. In this research, BL group improved the antioxidant capacities in the serum of broilers by increasing GSH-Px and CAT activities and by decreasing MDA compared with the PC group. This result showed that dietary B. licheniformis supplementation is an effective strategy to reduce the oxidative stress of experimental avian NE.

The liver is an important organ in which the majority of lipid metabolism occurs. Thus, the gene expression in livers, which can change the capacity of enzymes in relevant metabolic pathways, plays an important role in altering digestive capability [19]. ACC, which is the first key enzyme in fatty acid synthesis and converts acetyl-CoA into malonyl-CoA, plays an important role in the regulation of fatty acid synthesis in animal tissues. This enzyme often combines with FAS as rate-limiting lipogenesis enzyme [46]. In the current study, both ACC and FAS levels increased in three of the groups, especially in the BL group, unlike in the NC group. This phenomenon may be ascribed to the diet change made to build the NE model. Huang et al. determined that diets supplemented with 0.5–2 % soy lecithin can significantly increase the expression of FAS and ACC and change the biochemical status of serum lipids [47]. Zhao et al. reported that C. butyricum supplementation is related to increased FAS and ME activities, enhanced FAS, ME, and ACC mRNA levels in the liver, and increased intramuscular fat content in broilers [48]. In the current research, B. licheniformis supplementation can generate the same effect, possibly because B. licheniformis can enhance the digestion and utilization of nutrients from a diet containing a high percentage of fishmeal. This diet was used in the NE experimental model and was difficult for broilers to digest. SREBP-1c is a basic/helix-loop-helix/leucine zipper transcription factor that contributes to lipogenic enzyme expression [49]. This transcription factor can bind to the promoters of several lipogenic enzyme genes and induce their expression [50]. Researchers also report that SREBP-1c is a lipogenic nuclear transcriptional regulator that can directly influence the expressions of ACC, FAS, ME, and SCD [51]. In our present study, however, SREBP-1c gene expression decreased significantly in contrast to the expression of lipogenic enzyme genes. This result may be attributed to the fact that fat composition is increasingly exuberant in the subsequent growth stage of broilers [52], especially as the diet changes and probiotics supplementation increases nutrient digestion and utilization in these chickens.

Different diets can alter the expression of PPARs in broiler livers [53]. PPAR-α plays an important role in lipid metabolism; an increase in the expression of this isoform can upregulate the expression of fatty acid catabolism-related genes (CPT-1, ACOX1) and enhance fatty acid β-oxidation [54, 55]. In the present study, B. licheniformis can increase both PPAR-α and CPT-1 expression. The expression of these genes was lowest in the NE-infected birds.

In summary, the present study shows that NE infection can change the expression of genes related to fatty acid synthesis and oxidation. B. licheniformis supplementation can adjust the levels of these genes by enhancing the expression of fatty acid β-oxidation-related genes to alleviate the negative effects of such infection.

The results of our present study showed that dietary B. licheniformis supplementation effectively alleviates the negative effects of NE infection. This supplement can also reduce antioxidant stress, enhance growth performance, and adjust the expression levels of certain key genes related to lipid metabolism. Furthermore, the study data may provide a new insight into the prevention and treatment of NE in broilers.