Antibiotic resistance in Aeromonas hydrophila associated with exposure to subtherapeutic levels of oxytetracycline

Introduction The aquatic environment is an ideal ecosystem for the development of antimicrobial resistance (AMR) as it can be impacted by many sources of antibiotic residues. Methods To better understand the mechanism for AMR associated with exposure to subtherapeutic concentrations of antibiotics, we monitored the genetic and transcriptional changes in Aeromonas hydrophila exposed to incremental doses of oxytetracycline (OTC), a commonly used antibiotic in aquaculture. Results Our results showed that all three independent lineages rapidly developed OTC resistance, starting after 3 days of exposure, with consistent adaptation patterns across replicates. Whole-genome sequencing identified recurrent point mutations, particularly in genes AHA_ 2, 785, AHA₂910 (aheB), and AHA₀308 (rpsJ). Transcriptomic analysis identified over 1, 000 differentially expressed genes across bacterial replicates exposed to 100 ppm OTC, and further revealed coordinated changes in pathways associated with efflux pumps, outer membrane proteins, and ribosomal proteins. Discussion These combined genetic and transcriptional changes likely drove the progressive development of resistance during incremental antibiotic exposure. Our findings support the notion that AMR can arise in aquatic bacteria through coordinated regulation of several intrinsic mechanisms and highlight potential risks during in-feed antibiotic usage in aquaculture.