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BackgroundAntimicrobial resistance is a growing concern in canineStaphylococcus pseudintermediusdermatitis. Treatment with rifampicin (RFP) is considered only in meticillin‐resistant and multidrug‐resistantS. pseudintermedius(MDR‐MRSP). Hypothesis/ObjectivesTo determine an optimal RFP dosing for MDR‐MRSP treatment without induction of RFP resistance and identify causal mutations for antimicrobial resistance. Methods and materialsTime–kill assays were performed in a control isolate and three MDR‐MRSP isolates at six clinically relevant concentrations [32 to 1,024 × MIC (the minimum inhibitory concentration)]. Whole‐genome resequencing and bioinformatic analysis were performed in the resistant strains developed in this assay. ResultsThe genomic analysis identified nine antimicrobial resistance genes (ARGs) in MDR‐MRSP isolates, which are responsible for resistance to seven classes of antibiotics. RFP activity against all four isolates was consistent with a time‐dependent and bacteriostatic response. RFP resistance was observed in six of the 28 time–kill assays, including concentrations 64 × MIC in MDR‐MRSP1 isolates at 24 h, 32 × MIC in MDR‐MRSP2 at 48 h, 32 × MIC in MDR‐MRSP3 at 48 h and 256 × MIC in MDR‐MRSP3 at 24 h. Genome‐wide mutation analyses in these RFP‐resistant strains discovered the causal mutations in the coding region of therpoBgene. Conclusions and clinical relevanceA study has shown that 6 mg/kg per os results in plasma concentrations of 600–1,000 × MIC ofS. pseudintermedius. Based on our data, this dose should achieve the minimum MIC (×512) to prevent RFP resistance development; therefore, we recommend a minimum daily dose of 6 mg/kg for MDR‐MRSP pyoderma treatment when limited antibiotic options are available. 

ABSTRACT Liberibacter pathogens are the causative agents of several severe crop diseases worldwide, including citrus Huanglongbing and potato zebra chip. These bacteria are endophytic and nonculturable, which makes experimental approaches challenging and highlights the need for bioinformatic analysis in advancing our understanding about Liberibacter pathogenesis. Here, we performed an in-depth comparative phylogenomic analysis of the Liberibacter pathogens and their free-living, nonpathogenic, ancestral species, aiming to identify major genomic changes and determinants associated with their evolutionary transitions in living habitats and pathogenicity. Using gene neighborhood analysis and phylogenetic classification, we systematically uncovered, annotated, and classified all prophage loci into four types, including one previously unrecognized group. We showed that these prophages originated through independent gene transfers at different evolutionary stages of Liberibacter and only the SC-type prophage was associated with the emergence of the pathogens. Using ortholog clustering, we vigorously identified two additional sets of genomic genes, which were either lost or gained in the ancestor of the pathogens. Consistent with the habitat change, the lost genes were enriched for biosynthesis of cellular building blocks. Importantly, among the gained genes, we uncovered several previously unrecognized toxins, including new toxins homologous to the EspG/VirA effectors, a YdjM phospholipase toxin, and a secreted endonuclease/exonuclease/phosphatase (EEP) protein. Our results substantially extend the knowledge of the evolutionary events and potential determinants leading to the emergence of endophytic, pathogenic Liberibacter species, which will facilitate the design of functional experiments and the development of new methods for detection and blockage of these pathogens. IMPORTANCE Liberibacter pathogens are associated with several severe crop diseases, including citrus Huanglongbing, the most destructive disease to the citrus industry. Currently, no effective cure or treatments are available, and no resistant citrus variety has been found. The fact that these obligate endophytic pathogens are not culturable has made it extremely challenging to experimentally uncover the genes/proteins important to Liberibacter pathogenesis. Further, earlier bioinformatics studies failed to identify key genomic determinants, such as toxins and effector proteins, that underlie the pathogenicity of the bacteria. In this study, an in-depth comparative genomic analysis of Liberibacter pathogens along with their ancestral nonpathogenic species identified the prophage loci and several novel toxins that are evolutionarily associated with the emergence of the pathogens. These results shed new light on the disease mechanism of Liberibacter pathogens and will facilitate the development of new detection and blockage methods targeting the toxins.