Genetic Diversity and Antimicrobial Resistance of Extraintestinal Populations Pre- and Post-Antimicrobial Therapy on Broilers Affected by Colisepticemia.

The aim of the present study was to investigate the genetic diversity and antimicrobial resistance (AMR) of during enrofloxacin therapy in broilers affected by colisepticemia. Three unrelated farms with ongoing colibacillosis outbreaks were sampled at day 1 before treatment and at days 5, 10 and 24 post-treatment. A total of 179 isolates were collected from extraintestinal organs and submitted to serotyping, PFGE and the minimum inhibitory concentration (MIC) against enrofloxacin. PFGE clusters shifted from 3-6 at D1 to 10-16 at D5, D10 and D24, suggesting an increased population diversity after the treatment. The majority of strains belonged to NT or O78 and to ST117 or ST23. PFGE results were confirmed with SNP calling: no persistent isolates were identified. An increase in resistance to fluoroquinolones in isolates was observed along the treatment. Resistome analyses revealed and genes along with mutations in the , and genes. Interestingly, despite a fluoroquinolone selective pressure, -carrying plasmids did not persist. On the contrary, two conjugative AMR plasmid clusters (AB233 and AA474) harboring AMR genes other than were persistent since they were identified in both D1 and D10 genomes in two farms. Further studies should be performed in order to confirm plasmid persistence not associated (in vivo) to antimicrobial selective pressure.