The spread of antibiotic resistance has become a global issue in recent decades. Clinically healthy animals can transfer antibiotic-resistant bacteria or resistance genes to other animals and humans. If resistance genes are in turn transferred to pathogenic bacteria, the lack of response to antibiotic therapy can lead to diseases that are almost impossible to cure. In livestock farming, antibiotic-resistant bacteria are particularly common in fowl. Therefore, alternatives are demanded in poultry production to keep birds healthy and thus reduce the use of antibiotics and the spread of antibiotic resistance. Various measures have already been investigated in recent years, with varying outcomes. One promising approach could be to combine different strategies in order to achieve a more comprehensive protection. The aim of this work was to investigate whether a preventive strategy with a preselected pre-and probiotic combination and a strain-specific parent stock vaccine can affect the growth of antibiotic-resistant Enterobacteriaceae and furthermore performance, gut health, and excretion of antibiotic resistance genes in broilers exposed to a multidrug-resistant Escherichia coli (E. coli) strain. The methodical approach included both an ex vivo assay and an animal trial. First, the impact of different pre- and probiotics (inulin, fructooligosaccharide, Bacillus coagulans DSM 32016, Enterococcus faecium DSM 7134) and their combinations on the growth inhibition of antibiotic-resistant Salmonella Enteritidis (S. Enteritidis) and E. coli strains in caecal contents of broiler breeders were investigated. The samples were obtained from three different flocks (five birds per flock) at two different sampling timepoints (25 and 50 weeks of life). Subsequently, the most promising pre-/probiotic combination was added to a basal diet and further examined in an in vivo study with broiler chickens exposed to the E. coli strain from the ex vivo assay (challenge on day eight of life). Beside the pre- and probiotic group, the animal trial also included a group descended from vaccinated hens; a group from vaccinated hens that additionally received the pre-/probiotic combination; and a negative and positive control group. Each treatment group consisted of five replicates, each comprising a pen with nine chicks. Performance parameters were measured weekly until the end of the trial on day 28. In addition, data on microbial metabolites, pH values and microbial compositions in crop and caecum digesta samples were collected and antibiotic resistance genes were determined in the excreta. Of the pre- and probiotics tested in the ex vivo study Enterococcus faecium (E. faecium) together with fructooligosaccharide (FOS) or inulin proved to be most efficient in extending the lag time of E. coli O1/O18 and S. Enteritidis compared to the control without additives. Moreover, a higher age of the broiler breeders (50 compared to 25 weeks) was also a significant factor impairing the fitness of the antibiotic-resistant strains. Interestingly, both the single applications and other product combinations did not inhibit the growth of the selected bacteria strains and, in some cases, even improved it. The poultry trial revealed synergistic impacts of the preselected combination of E. faecium and FOS together with maternal vaccination on body weight and body weight gain of broilers exposed to the multidrug-resistant E. coli. In addition, this multifactorial strategy elevated L-lactate concentrations in crop contents, decreased pH levels in crop and caecal contents, and reduced the relative abundance of Proteobacteria therein. The ex vivo preselection of a pre- and probiotics for the bacterial strain and the poultry species thus appeared to be reasonable in order to achieve the above-mentioned effects. However, the excretion of specific antibiotic resistance genes could not be diminished by the chosen approach. In conclusion, the investigated strategy comprising a preselected pre- and probiotic combination and a specific parent stock vaccination has the potential to positively influence the zootechnical performance and gut health of broilers exposed to multidrug-resistant E. coli. Future studies should examine whether these effects are sufficient under field conditions and can serve to protect against infections with highly pathogenic Enterobacteriaceae.
