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The development of new feeding strategies to maintain gut health in newly-weaned pigs, in order to minimize the use of antimicrobial compounds such as antibiotic growth promoters has become essential to avoid the emergence of antibiotic resistant bacteria. Since the late 1980s, the use of pharmacological concentrations of zinc oxide (ZnO) was studied to prevent diarrhea and increased growth rates in weaning piglets (Chapter 1).
In Chapter 2 a summary of the literature on gut microbiota – host interactions and zinc as feed additive is given. The main objective of this thesis was to investigate the pig gut microbiota under the influence of high dietary ZnO to improve our knowledge for structural and functional aspects of gut microbial communities and to estimate new feeding strategies to substitute antibiotic growth promoters. Detailed information on the study aims can be found in Chapter 3.
The two different experimental approaches (Chapter 4 & 5) used in this thesis are based on the same animal trial where different diets containing high and low amounts of ZnO on newly-weaned piglets are tested. Briefly, piglets were fed diets containing 57 (low) or 2425 (high) mg kg-1 zinc from analytical grade ZnO. For the first experiment (Chapter 4) intestinal contents from stomach, jejunum, ileum and colon were sampled from 32, 39, 46 and 53d old piglets and analyzed for bacterial cell numbers and main bacterial metabolites. The most drastic effects of high dietary zinc could be found one week after weaning in the stomach and small intestine. Cell numbers of Enterobacteriaceae, the Escherichia group as well as from Lactobacillus spp. and three abundant Lactobacillus spp. were reduced. The influence of high dietary zinc was transient for enterobacteria but permanent for Lactobacillus spp. No impact could be observed at cell numbers of bifidobacteria, enterococci, streptococci, Weissella spp. and Leuconostoc spp. as well as the Bacteroides-Prevotella-Porphyromonas group.
Among the microbial metabolites molar acetate ratios increased and propionate decreased in the proximal intestine and lower lactate concentrations were observed in the high dietary zinc group throughout the feeding trial. However, the differences between the dietary groups in cell numbers and in microbial metabolites diminished in older animals.
For the second experiment (Chapter 5) digesta samples from the stomach and jejunum of 32, 39, 46 and 53d old animals (n = 6 per group) were sampled and incubated for 16h under anaerobic conditions in a complex media containing 80, 40, 20 and 0 μg mL-1 soluble Zink. Specific growth rate, maximum growth and lag time were calculated according to the obtained growth curves. The highest rate of growth and lowest lag times could be observed in the samples from animals fed the low dietary zinc concentration in media without added zinc. The samples from animals fed the high dietary zinc concentration showed highest growth rates and lowest lag time in the media supplemented with zinc. Bacterial growth of digesta samples from the high dietary zinc group was less influenced by zinc and recovered growth more rapidly than in the low dietary zinc group and samples from older animals of the low dietary zinc group also showed reduced growth depression. Bacterial cell number analysis from PCR results showed that lactobacilli were reduced by the zinc treatment, while bifidobacteria and enterococci showed increased growth in samples from the high dietary zinc treatment. Enterobacteria from samples of the high dietary zinc group showed an increased growth rate at high zinc concentrations in the medium. No differences were observed for the analyzed clostridial cluster and the Bacteroides-Prevotella-Porphyromonas group.
The studies have shown that the supplementation of feed with high doses of zinc leads to a reduced ex vivo - bacterial growth rate of bacteria from the stomach and jejunum and to transient and lasting effects during the development of the intestinal microbiota along the whole GIT in vivo, affecting composition as well as metabolic activity of weaned piglets. In view of the rapid bacterial adaptation to dietary zinc ex vivo as well as in vivo , the administration of ZnO as feed additive for weaned piglets might only be beneficial in a short period after weaning.
In Chapter 6 the results of both experimental approaches are discussed and summarized with contributions to evaluate new feeding strategies in modern animal nutrition in respect of microbial communities in the gut.