Antimicrobial resistance patterns differ amongst Salmonella enterica recovered from synanthropic flies captured...

Synanthropic flies are recognized as mobile reservoirs of enteric foodborne pathogens, yet their utility as antimicrobial resistance (AMR) sentinels within livestock production systems remains under-explored. This study aimed recovered the enteric foodborne pathogen Salmonella enterica from whole-body fly homogenates and characterized phenotypic AMR patterns across sampled Texas counties differing by livestock production intensity groups (low, medium, and high). Each livestock intensity group consisted of two counties. Flies were collected from five sampling locations throughout each county at multiple distances (0-15 km) from a designated epicenter. Collected flies were identified to at least family level, homogenized and then screened for Salmonella. Pathogens were biochemically presumptively confirmed molecularly. Confirmed pathogens were subjected to serotyping and phenotypic AMR screening. Besides descriptive statistical analyses, logistic regression evaluated associations between individual counties, Salmonella serovar, and AMR patterns. Overall, 89/230 samples (38.7%) yielded confirmed Salmonella. Elevated AMR frequencies for isolated Salmonella were observed for tetracycline (77.5%), streptomycin (61.8%), amoxicillin/clavulanic acid (56.2%), azithromycin (39.3%), and cefoxitin (41.6%) across sampled counties. Heatmaps revealed clear spatial differences in AMR patterns. Generalized estimating equations (GEE) analysis showed counties Go and S, both belonging to the high livestock intensity group, and the antimicrobial tetracycline were strongly associated (FDR p < 0.05) with the occurrence of AMR in Salmonella isolated from flies. Synanthropic flies demonstrated the presence of variability in Salmonella AMR patterns, and thus may be a useful sentinel for surveiling localized differences in AMR patterns of human pathogens in food animal production environments.