Evaluation of dietary Bacillus-based commercial probiotics in broiler chickens

Commercial Bacillus-based probiotics are widely used in poultry production, but their biological mechanisms of action in broiler chickens are still not fully understood. This study evaluated the effects of dietary supplementation with Bacillus-based commercial probiotics on the performance, meat and tibia traits, gut health (ileal morphology and cecal volatile fatty acids), antioxidant status, nutrient digestibility, and intestinal gene expression in broiler chickens. Day-old Ross 308 broiler chicks (288 in total) were randomly assigned to one of four dietary treatments, with six replicate pens of 12 birds each in an environmentally controlled facility. The treatments included a control group fed a basal diet and three probiotic-supplemented diets: Bacillus subtilis KCCM13366P at 5.0 × 107 cfu/kg (BS), Bacillus licheniformis KCCM11751P at 1.0 × 108 cfu/kg (BL), or an equal blend at 2.0 × 108 cfu/kg (BSL). On days 21 and 35, one or three birds per pen were sampled for feathers, blood, small intestine, and cecal digesta to determine the indicators of gut health (i.e., secrtory immunoglobulin A, gut morphology, cecal volatile fatty acids, and intestinal gene expression) and physiology (i.e., serum parameters, corticosterone, and antioxidants parameters). In addition, meat quality and ileal nutrient digestibility were assayed on day 35. Data were analyzed using the general linear model procedure in SAS followed by pairwise differences using Tukey's post-hoc test. Statistical significance was preset at P < 0.05. The trial lasted for 35 days. Dietary BSL tended to increase feed intake during the grower phase by 6.33% (P = 0.087). Dietary BS and BSL improved the feed conversion ratio by 7.83% and 5.22%, respectively, during the starter phase (P < 0.05) and BSL reduced overall feed conversion ratio by 1.99% compared to the control group (P < 0.05). At 21 days, dietary BSL increased the ileal villus height:crypt depth ratio by 40.66% (P < 0.05). The activity of glutathione peroxidase in the ileal mucosa, but not in the serum samples, was decreased by 36.34% in the BS group and 38.06% in the BSL group at 21 days (P < 0.05). Serum glutamic oxaloacetic transaminase concentrations were lower by 22.3% on day 21, and ileal digestibility of crude protein and crude ash were lower by 6.6% and 21.9% on day 35, respectively, in birds fed the BSL diet than in the control group (P < 0.05). The interleukin-8 and interferon-alpha expression levels in the ileum were increased by 2.0-fold in BL-fed broilers compared to the control group (P < 0.05). The expression of genes encoding antioxidant-related parameters, tight junction proteins, mucin, and glucose transporters was upregulated by 2.0- to 4.0-fold in dietary BL and BSL groups on days 21 and 35 (P < 0.05). We concluded that dietary BS improved growth performance, whereas dietary BL enhanced cytokine, gut barrier, mucin, antioxidant, and nutrient transporter mRNA levels, resulting in additive effects of BSL on chickens. Based on the enhanced intestinal gene expression induced by dietary BS or BSL, further studies are needed to determine protective immunity against enteric pathogens in broiler chickens.