Synbiotics for Broiler Chickens—<i>In Vitro</i> Design and Evaluation of the Influence on Host and Selected Microbiota Populations following <i>In Ovo</i> Delivery

<div><p>Synbiotics are synergistic combinations of prebiotics and probiotics. In chickens, synbiotics can be delivered <i>in ovo</i> to expedite colonization of the gut by beneficial bacteria. We therefore aimed to design synbiotics <i>in vitro</i> and validate them in broiler chickens upon <i>in ovo</i> delivery. The probiotic components of the synbiotics were <i>Lactobacillus salivarius</i> and <i>Lactobacillus plantarum</i>. Their growth was assessed in MRS medium supplemented with different prebiotics. Based on <i>in vitro</i> results (hatchability and growth curve), two synbiotics were designed: S1 –<i>Lactobacillus salivarius</i> with galactooligosaccarides (GOS) and S2 –<i>Lactobacillus plantarum</i> with raffinose family oligosaccharides (RFO). These synbiotics were delivered to Cobb broiler chicken embryos on day 12 of incubation at optimized doses (10⁵ cfu egg^-1 of probiotic, 2 mg egg^-1 of prebiotic). Post hatching, 2,400 roosters were reared (600 individuals group^-1 divided into eight replicate pens). Microbial communities were analyzed in ileal and cecal digesta on day 21 using FISH. Gene expression analysis (<i>IL1β</i>, <i>IL4</i>, <i>IL6</i>, <i>IL8</i>, <i>IL12</i>, <i>IL18</i>, <i>IFNβ</i>, and <i>IFNγ</i>) was performed on days 7, 14, 21, and 42 for the spleen and cecal tonsils with RT-qPCR. Body weight and feed intake of the roosters did not differ by the treatments. Microbial populations of <i>Lactobacillus</i> spp. and <i>Enterococcus</i> spp. in the ileum were higher in S1 and S2 than in the control. In the cecum, the control had the highest bacterial counts. S1 caused significant up-regulation of <i>IL6</i>, <i>IL18</i>, <i>IL1β</i>, <i>IFNγ</i>, and <i>IFNβ</i> in the spleen on day 21 and <i>IL1β</i> on day 7 (<i>P</i> < 0.05). In cecal tonsils, S1 caused significant down-regulation of <i>IL12</i>, <i>IL8</i>, and <i>IL1β</i> on day 42 and <i>IFNβ</i> on day 14 (<i>P</i> < 0.05). S2 did not elicit such patterns in any tissues investigated. Thus, we demonstrate that divergent effects of synbiotics in broiler chickens were reflected in <i>in vitro</i> tests.</p>