This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Conjugative transfer of antibiotic resistance genes by plasmids has greatly contributed to the rapid spread of drug resistance among bacterial pathogens, thereby decreasing the effectiveness of various treatment options for infectious diseases in humans. While some plasmids only transfer and stably replicate in a narrow range of hosts (NHR), so-called broad-host-range (BHR) plasmids transfer and replicate in distantly related bacteria, thereby shuffling resistance genes across taxonomic barriers. Our long-term goal is to limit the rapid spread of multi-drug resistance to important human pathogens, but first a fundamental understanding of the evolution of plasmid host range is essential. Our overall hypothesis is that the host range of plasmids can evolve, much like that of parasites, to become wider or narrower over evolutionary time. To test this hypothesis, we propose the following specific aims: 1) To elucidate evolutionary changes that permit a narrow-host-range plasmid to expand or shift its host range, and 2) To elucidate evolutionary changes that cause host range contraction of a broad-host-range plasmid as a result of long-term association with a single host. To address the first aim, we will evolve the NHR mini-replicon mini-F in four bacterial hosts that show different initial levels of plasmid stability. The DNA sequences of the evolved plasmids will be determined to identify genotypic changes that cause a host range shift or expansion. We postulate that plasmid adaptation resulting in improved stability in one poor host will also improve the stability in other hosts. Under Specific Aim 2, we will examine evolved mutants of the BHR mini-replicon mini-pBP136 that were recently shown to have undergone a shift in host-range. We postulate that the observed shift is due to changes in the interaction of the replication initiation protein TrfA1 with the host helicase. For both Aims mathematical models and statistical analyses will be developed and used to identify the parameters that affect these evolutionary processes. Relevance to public health: This work will unravel the mechanisms of plasmid-host interactions that determine the host range of multi-drug resistance plasmids. This may reveal attractive targets for specific drug therapy in the fight against the alarming spread of drug resistance in human pathogens.