The emergence of multidrug-resistant bacterial pathogens and the imposition of ban on the usage of antimicrobials in animal production have led to a resurgence of interest in phage therapy . Research on reducing zoonotic pathogens with the application of BP as a viable option in food animals has also focused on reducing the impact of infections in the animals themselves  thereby improving the production and performance of animals.
In the present study, commercially available BP consisting of Salmonella gallinarum, S. typhimurium, S. Enteritidis, E. coli and Clostridium perfringes was assessed for its suitability as feed additive for the enhancement of performance and production of broiler chickens under normal physiological state (without bacterial challenge).
In agreement with the findings of Kim et al.  who demonstrated that FI and FCR were unaffected by supplementing the broilers diet with anti-SE bacteriophage, the inclusion of BP as feed additive at 0.05% and 0.1% levels in the present study showed no effects on FI and FCR throughout the trail, except for a trend in linear reduction in FCR during day 22–35. However, the present study showed significant linear effect in increasing the BWG with the increase in BP levels during the initial starter and finisher phases and overall experiment period indicating that BP supplementation had no detrimental effect on feed consumption but promoted the BWG. In contrast, Huff et al.  suggested that the BWG were not affected by the inclusion of bacteriophage in broiler chickens without bacterial challenge; and Wang et al.  noted that the supplementation of BP consisting of mixture of Salmonella gallinarum, S. typhimurium, and S. Enteritidis at the ratio of 3:3:4 at the dose level of 0.05% improved FCR during day 1–14 in broiler chickens. In broiler production, increase in body weight is an important parameter since lower body weight equates with increased cost for broiler meat production . The increase in BWG when BP was used as a feed additive instead of antibiotics in animal feed might be due to the inhibitive or lytic effect on harmful bacteria replication in the gastrointestinal tract of broiler chickens . Moreover as expected, the inclusion of sub-therapeutic dose of antibiotic as positive control in the diet of broiler chickens led to higher BWG and FI than the birds fed basal diet without antibiotics which agrees with several other studies [33–35] suggesting that improvement in BWG might be due to increase in FI.
The supplementation of antibiotics or bacteriophage to the basal diet did not have significant effect on nutrient digestibility. In line with the findings of Wang et al. , the ATTD of nutrients was not affected by the supplementation of increasing levels of BP. Further experiments are needed to confirm the lack of response of antibiotics or BP on nutrient digestibility.
Salmonella is the major cause of food borne diseases worldwide with chickens as the main reservoir. Other zoonotic pathogens include Clostridium, Campylobacter, E. coli. For the control of these pathogens in poultry, bactericidal bacteriophages may provide a natural, nontoxic, feasible and non-expensive component. Previous works indicated that Salmonellae can be controlled by the use of bacteriophages [18, 36, 37, 38]. Early studies with E. coli also demonstrated that phage therapy can be as efficient as antibiotics [39, 40]. The reduction in E. coli and Salmonella counts in the excreta of broiler chickens by the application of bacteriophage has been reported . Conversely, in the present study, dietary supplementation of BP did not have significant effect on the pathogenic bacteria such as E. coli, Salmonella as well as Clostridium counts isolated from the caecal digesta. However, a trend in linear increase in Lactobacillus count was observed in birds fed BP diets. The possible reason for non-significant effect of BP on nutrient digestibility and pathogenic food borne bacterial counts among the treatments might be the birds were raised in hygienic environment and were not experimentally challenged with bacteria due to which gastro intestinal tract might not have been colonized by harmful micro-organisms thereby maintaining gut in healthy state.
The gastrointestinal microbiota plays a crucial role in gut associated host immune system. Moreover, the physiological development, health, and productivity is also influenced by gut microbiota. Poultry diets have tremendous impact on the gut microbiome in regard to diversity and composition . The manipulation of the microbial community through the inclusion of feed additives such as phage is feasible in order to enhance chicken growth and control either human or animal pathogens. Several studies have reported the use of bacteriophages as a feed additive in animals in order to control bacteria transmitted by foodstuffs. These models include the use of phages to control Salmonella and Campylobacter in broiler chickens [8, 42]. Microbiome analysis showed that Firmicutes, Bacteroidetes, and Proteobacteria are the predominant phyla in the avian gut , which is also supported by the results from our study. In TR1 group, Firmicutes and Bacteroidetes were composed as predominant, whereas in TR2 group besides, Firmicutes, and Bacteroidetes the members of phylum Proteobacteria was also predominant. The presence of members of phylum Proteobacteria in TR2 may indicate that BP dose of 0.1% may not be favorable as increase in Proteobacteria may be associated to increase in E. coli. In PC group, in addition to Firmicutes, Bacteroidetes, Proteobacteria, members of phylum Deferribacteres (1.03%) was also present. Members of phylum Firmicutes was reduced whereas members of Proteobacteria and Bacteroidetes were increased in pigs receiving antibiotic treatment. The composition of microbiota at the genus and species level was modified, decreasing the abundance of Lactobacillus at the genus level in PC as compared with CON, TR1 and TR2 as well as an increase in the relative abundance of Prevotella and Bifidobacteria in phage treated groups compared with CON and PC groups. The genus Lactobacillus plays a crucial role in the homeostasis of the gastrointestinal tract of metazoans .
At the species level, the Lactobacillus salivarus population in ileum mucosa in phage treated groups was twofold as compared with CON and PC suggesting the efficacy of the phage in promoting the beneficial bacteria which eventually contributes in improved performance and gut health.
With regards to meat quality, a significant quadratic response in the redness and lightness values of meat color was observed with the increase in the level of bacteriophage. Although, the meat color is closely associated with the meat pH , we found that the pH of breast muscle was not different among treatments indicating that change in color was not due to pH. In partial agreement to our finding, Wang et al.  demonstrated that meat pH as well as meat color were not affected by the addition of bacteriophage in the basal diet of broiler chickens. Besides pH, other reported factors affecting color inside the muscle include myoglobin content, muscle fiber orientation and the space between the muscle fibers . Further studies on these factors with bacteriophage application could help explain the changes in color observed. With regards to organ weight, a tendency to increase in the relative weight of gizzard in PC than NC and BP groups was observed. The possible reason for increase in the relative weight of gizzard in PC compared with NC may be due to increase in FI in PC groups. The weight of spleen and Bursa of Fabricius relative to the percentage of body weight in NC was higher than PC group during day 35. However, the inclusion of the increasing level of bacteriophage to NC diet linearly reduced the weight of spleen and Bursa of Fabricius relative to the percentage of the body weight indicating BP at 0.05% is better among the levels tested. As the spleen and bursa are associated with immune function (as lymphoid organs) this may explain that BP level higher than 0.05% may not be effective in improving the immune functions.