Concern for global health security and the environment due to the emergence of antibiotic-resistant bacteria and antibiotic residues in meat and other livestock products has led many countries to restrict the use of antibiotics in animal feed. This experiment was performed to assess the impact of dietary supplementation with a probiotic (Bacillus subtilis) and a postbiotic (Saccharomyces cerevisiae fermentation product) on growth performance, carcass traits, blood haemato-biochemical profile, gut microbiome, gut morphology, and immune response in broilers as an alternative to antimicrobials in poultry production system to minimize the effect on global health security. A total of 324-day-old Ven Cobb 400 broiler chicks were randomly divided into three dietary groups, each containing 12 replicated pens (n = 12), .and each replicate contained nine broiler chickens. The dietary groups consisted of 1) a basal diet without any growth promoting (T1), 2) the basal diet augmented with Bacillus subtilis at 200 g/MT feed (T2), and 3) the basal diet supplemented with Saccharomyces cerevisiae fermentation product at 1.25 kg/MT feed (T3). To calculate body weight gain, all birds and residual feed were weighed on a replicated basis over days 0, 7, 14, 21, 28, 35, and 42; mortality was recorded daily. Each replicate's mortality percentage was calculated and used to adjust BW, ADFI, and FCR calculations at the trial's end. At the end of the trial (42 d), two chickens from each replicate (24 chickens per group) were slaughtered for carcass traits, gut microbiome, and morphology measurements. Blood samples were collected for the haemato-biochemical profile on 35d and antibody titer on 28d and 35d.Feeding with SCFP (T3 group) significantly improved average daily feed intake (ADFI) and average daily gain (ADG) of chickens compared to the T1 (control) and T2 (probiotic) groups from 1 to 14 days of age. FCR was significantly improved (P<0.05) in SCFP-fed birds (T3) relative to the control (T1)over the entire experimental period (1-42 days). Carcass traits and blood haemato-biochemical parameters remained unaffected (P>0.05) by any diets. However, cholesterol levels and concentrations of corticosterone were significantly lower (P<0.05) in SCFP-fed birds (T3) compared to those of the probiotic (T2) and control (T1) groups. Total E. coli, Enterohaemorrhagic E. coli, ESBL-producing Enterobacteriaceae, and Salmonella counts were significantly higher in the T1group compared to T3 and T2 groups (P<0.05) and Salmonella counts were lower in T3 when compared to T2 (P<0.05). However, there was no significant difference in Lactobacillus count among treatment groups. PCR based characterization of ESBL- producing Enterobacteriaceae revealed maximum presence of blaCTX-M-Type (6/15, 40%), followed by blaSHV-Type (5/15, 33.3%) and blaTEM-Type (4/15, 26.6%). No variation in the occurrence pattern of ESBL/beta-lactamase associated genes in the studied ESBL- producing Enterobacteriaceae isolates was observed between the treatment groups. A significant increase in villi height and villi height to crypt depth ratio (VH: CD) (P<0.05) was observed in both T3 and T2 groups. Contrarily, villi widths did not vary across the dietary groups (P>0.05). The control group (T1) displayed a notable deeper in crypts when compared to the T3 and T2 groups (P<0.05). On day 28, the SCFP-fed birds (T3) and those fed the probiotics (T2) exhibited a significant increase (P<0.05) in antibody titers against Newcastle disease virus and Infectious bursal disease virus. Increase in antibody titre against IBDV in birds with the addition of SCFP in the diet was detected for the first time. However, there were no significant differences observed on day 35. The phagocytic activity of neutrophils and lymphocytes in broiler chickens did not show any noticeable variations among the different groups at day 35(P>0.05).It can be concluded that Saccharomyces cerevisiae fermentation product and Bacillus subtilis probiotic could be viable alternatives to antimicrobials in poultry production considering beneficial impacts in broilers fed antibiotic-free diet.
