Browsing by Subject "antimicrobial susceptibility"

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    Characterization of emerging Brachyspira pathogens in swine: Applications to disease control
    (2016-12) Mirajkar, Nandita
    Swine dysentery (SD) is a mucohemorrhagic diarrheal disease of pigs and is a welfare and economic issue globally. Its re-emergence in the U.S. after two decades of quiescence was associated not only with B. hyodysenteriae but also with a novel spirochete, Brachyspira hampsonii. The overall objective of this dissertation was to determine the genotypic, antimicrobial susceptibility and virulence factors of these pathogens in order to understand the re-emergence of SD in the U.S. As the re-emergent isolates were genetically related to historic isolates, it is likely that B. hyodysenteriae persisted in the U.S. over the last two decades without causing overt clinical disease. B. hampsonii isolates from migratory birds and pigs were found to be closely related, thus highlighting the potential role of migratory birds in its emergence in swine herds. When the broth microdilution method was compared with the agar dilution/strip test methods for antimicrobial susceptibility testing (AST) of various Brachyspira species, with the exception of carbadox, all other evaluated antimicrobials (tiamulin, valnemulin, lincomycin, tylosin and doxycycline) showed high agreement between the methods. Most Brachyspira isolates demonstrated high susceptibility to tiamulin, valnemulin and carbadox, heterogeneous susceptibility to doxycycline and low susceptibility to lincomycin and tylosin. Decreased tiamulin susceptibility, when rarely identified, was associated with the genotype, stage of production and production system from which the isolate originated, possibly reflecting local antimicrobial usage. Overall, antimicrobial resistance did not seem likely to be the cause of the re-emergence of SD in the U.S. The genomic comparison of U.S. B. hyodysenteriae field isolates revealed several mobile elements that showed variation between the isolates and are likely associated with the evolution of this species. Genes involved in carbohydrate and cell wall biosynthesis, cell survival and host-pathogen interaction were predicted to be potentially associated with B. hyodysenteriae virulence. Based on a comprehensive polyphasic approach (multiple whole-genome comparison, genotypic and phenotypic data), Brachyspira hampsonii sp. nov. was classified as a unique species with genetically diverse yet phenotypically similar ‘genomovars’. Finally, this work also provided information that can directly (genotyping, ASTs) and indirectly (gene candidates for diagnostic tests and vaccines) be applied to disease control.
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    Evaluation Of Pcr And Sequencing-Based Assays For The Detection Of Mycoplasma Hyopneumoniae Antimicrobial Susceptibility-Associated Mutations.
    (2023-08) De Abreu, Cipriano
    Mycoplasma hyopneumoniae (M. hyopneumoniae) is a difficult bacterium to grow that is responsible for enzootic pneumonia (EP), a respiratory disease affecting pigs of all ages. High economic losses have been associated with M. hyopneumoniae infections mainly due to decreased performance, and an increased need for antibiotic treatments. Antibiotic therapy remains a significant approach to control and treat M. hyopneumoniae infections. Selection of antimicrobials in the field against M. hyopneumoniae is often based on veterinary expertise rather than on antimicrobial susceptibility testing (AST). Reports of decreased susceptibility in M. hyopneumoniae isolates have become available over time, raising concerns about its potential impact on the persistence of infections and the efficacy of treatment strategies applied in affected herds. Antimicrobial susceptibility testing of M. hyopneumoniae is not commonly performed, primarily due to the low success rate of the culture and isolation process for this bacterium. In this research, we assessed the presence of genetic mutations associated with decreased antimicrobial susceptibility in M. hyopneumoniae isolates as well as in clinical specimens as molecular tools such as PCR and sequencing. Amplification and sequencing of five different gene segments (gyrA, gyrB, parC, parE and 23S rRNA) was performed, and mutations were recorded and contrasted to previous reports. Findings revealed synonymous and non-synonymous mutations in the assessed gene segments. In the parC gene, a mutation previously associated with decreased susceptibility to fluoroquinolones (C248A) was detected in isolates and clinical specimens, whereas, in the 23S rRNA gene, a macrolide-susceptibility associated mutation (A2071G) was detected only in clinical specimens. Results underscore the potential utility of molecular-based methods for routine monitoring of the emergence of susceptibility-associated traits in M. hyopneumoniae, and its potential to guide treatment selection and consequently improve the treatment and control of EP.