Pro- and postbiotics in broiler feed: the matrix reassessed

Probiotics (beneficial live micro-organisms, often bacteria), and more recently postbiotics (dead beneficial micro-organisms and/or compounds produced during their growth), have received increasing interest by feed producers to improve the growth and health of broiler chickens. This thesis describes research to determine the effects of the heat process to produce a postbiotic (Post) from a probiotic (Pro) on the composition and the way both affect the growth and physiology of broilers.

In the first laboratory experiment, the composition of small molecules (= metabolome) of Pro and Post additives was assessed and what impact the heat treatment has. It was shown that the heat treatment to produce Post had minor effects on the metabolome. Next, the effect of Post on an immune cell (macrophages) of chickens was investigated and showed that Post was increasing the inflammatory response of the cells.

To assess effects of Pro and Post in broiler chickens, three experiments were conducted under practical housing conditions (= in vivo). The first experiment evaluated if the composition of the diet (standard v.s. growth-depressing challenge diet) can influence the way Pro and Post work. Both feed additives contained their commercial carriers (= mixture used to carry an additive as a dry powder). Pro and Post increased growth and modified the caecal bacterial population (= the microbiota) only in broilers fed the challenge diet. In the second experiment, the reproducibility of this growth promoting effect of Pro and Post with their carriers was evaluated using two different challenge diets. Unlike the first experiment, Pro and Post did not improve broiler growth. The third in vivo experiment had two aims. Firstly, it investigated if the commercial carriers change the way Pro and Post affect the broiler chicken. Second, it assessed if expression of genes in the blood, small intestinal segments (jejunum and ileum) and in an immune organ of the caeca (the tonsils), as well as on the microbiota and metabolites in ileal and caecal digesta are affected by Pro and Post. The results showed that broiler growth was improved by Pro but not by Post, whilst the presence of their commercial carriers reduced and increased growth, respectively. Pro (without its carrier) lowered inflammation in the jejunal and caecal tonsil tissues, while Post had very limited and unclear effects. The data of the latter third experiment were further analysed to identify parameters associated with broiler growth. The gene expression data indicated that broilers reaching a greater body weight at 35 days had reduced inflammation in jejunal tissue.

For Pro without carrier, and for Pro and Post with carrier to be beneficial for broiler growth, challenging conditions are required. The carriers changed the effects of Pro and Post. Pro without carrier had a beneficial effect on growth and lowered intestinal inflammation. This effect was likely due to the viable cells in Pro and/or components that are degraded by the heat treatment to produce Post from Pro. Furthermore, lowering intestinal inflammation may allow for improvements in growth and health in broiler production.