One Health emphasizes the interconnectedness of animal, human and environmental health. The emerging risk of zoonotic and antibiotic-resistant bacteria in fattening poultry is a significant concern for public health and consumer protection. This underlines the necessity for an integrated approach and comprehensive strategies that encompass veterinary, public health, and environmental perspectives. This thesis comprehensively examines the spread of zoonotic and antibiotic-resistant bacteria among animals, within their immediate environment, and throughout the surrounding areas of barns, as well as other stages in the production chain. Initially, we focused on methicillin-resistant Staphylococcus aureus (MRSA) and later expanded our research to extended-spectrum beta-lactamase/ plasmid-mediated AmpC beta–lactamase Escherichia coli (ESBL-/pAmpC- E. coli). The rising number of antimicrobial resistance (AMR) was and is a highly relevant issue in human and veterinary medicine worldwide. This thesis describes transmission routes for the selected resistant bacteria in broiler chickens and turkeys. The focus is on their spread into the environment as well as their (re-)introduction into herds and their transmission across the broiler production chain. This involved analyzing many different environmental and animal samples. Subsequently, we explored Campylobacter transmission pathways within broiler chicken farms, considering the farm environment. Campylobacteriosis is the most frequently reported foodborne illness in the European Union (EU) in 2021. Across all studies, we conducted additional investigations on isolates from different habitats such as pulsed-field gel electrophoresis (PFGE) or whole genome sequencing (WGS) to reveal epidemiological links. The findings of all 10 publications within this thesis are thematically summarized into two main categories: 1. the dissemination in the environment via air or dust and through fecal associated matrices on broiler chicken and partly turkey farms, and 2. the transmission across the production chain in broiler chickens. The results of the studies show that these bacterial species enter the environment of the farms and can thus be a source of spread or (re)-entry into the production chain. They also illustrate the different behavior of the various bacterial species and show that spread scenarios can occur simultaneously at different stages of the production chain. ESBL-/pAmpC- E. coli pose a significant risk of contamination from feces. Further investigations of the isolates proved a pseudo-vertical transmission of ESBL-/pAmpC- E. coli via contaminated egg surfaces from the parent flock through the hatchery to the fattening flock, and a horizontal transmission between consecutively fattened flocks. Furthermore, the resistant bacteria persist in the barn, surviving even intensive cleaning and disinfection (C&D) measures inside the barns and thus represent a source of colonization for subsequent animals. This shows the crucial role of C&D measures. Critical points for controlling C&D in broiler houses were identified, which should receive more attention in the future. Fragile building structures and mobile equipment are particularly important. Contrarily, MRSA transmission in the environment is less linked to fecal emissions and probably more so to the release of bioaerosols. Investigations revealed that MRSA with the same staphylococcal protein A (spa) types were present on ground surfaces surrounding the barns, sporadically in ambient air samples, in animal samples, and in dust and air samples from inside the broiler chicken and turkey houses. This indicates that MRSA can spread to the outside via the barn air, sediment and persist on the ground. At the time of the study, these were the first investigations on this topic in poultry farming. Thermophilic Campylobacter, similar to E. coli, colonizes the gut. Campylobacter spp. were sporadically found in environmental samples from chicken farms at low concentrations within this thesis. Despite the limited sample size, whole-genome analyses of various isolates revealed a high similarity between those obtained from puddles and water retention basins in the outdoor area and those from the indoor area of the barn, including air samples. Identifying dominant transmission pathways proved challenging, nevertheless, potential scenarios were demonstrated. For the first time, investigations of the the presence of Campylobacter in the viable but non-culturable (VBNC) state, both within the barn and in the surrounding environment, revealed that VBNC Campylobacter (C.) jejuni was predominantly detected in boot swab samples collected from ground surfaces around Campylobacter-positive farms. This was achieved using viability quantitative polymerase chain reaction (v-qPCR) after propidium monoazide (PMA) treatment. Intensive sampling, detailed analysis of isolates, and careful evaluation of metadata are crucial for accurately describing transmission routes. Applying modern methods to analyze and compare isolates from various habitats, which were not available during the initial studies of this thesis, promises to yield further insights. While this work focuses on specific environmental aspects of conventional fattening poultry houses, the One Health concept opens up additional interesting areas for future research, in particular concerning different animal housing systems.
