Genomic Diversity and Resistance Backgrounds of Enterobacteriaceae from Gulls and Coastal Environments

Background/Objectives: Enterobacterales are widely distributed in animals and environmental matrices and represent important reservoirs of antimicrobial resistance within a One Health framework. Gulls are particularly relevant because of their synanthropic behavior and frequent contact with anthropogenic waste, which may facilitate the acquisition and dissemination of resistant bacteria. Therefore, this study aimed to investigate the diversity, genomic backgrounds, and resistance determinants of Enterobacterales recovered from gulls and coastal environments in Portugal. Methods: A total of 265 samples, including seagull feces, saltwater, stagnant water, and sand, were collected from four coastal locations. Enterobacterales isolates were recovered and characterized by whole-genome sequencing, and comparative genomic analyses were performed for selected chromosomal beta-lactamase regions. Results:Enterobacterales were recovered from 41 samples and comprised 11 Klebsiella spp., 11 Leclercia spp., 9 Enterobacter spp., and 3 Citrobacter spp. isolates. Isolates were recovered mainly from seagull feces, followed by sand, and Berlenga Grande and Miramar showed the highest diversity. Klebsiella spp. and Enterobacter spp. concentrated most clinically relevant resistance determinants, including chromosomal bla_SHV-like, bla_OXY-like, and bla_ACT-like genes. By contrast, Leclercia pneumoniae was mainly associated with sand and showed a limited resistome, whereas Citrobacter braakii carried relevant genes such as bla_CMY and qnrB10. Comparative genomic analysis showed conserved chromosomal contexts surrounding bla_SHV-like, bla_OXY-like, and bla_ACT-like regions. Conclusions: These findings indicate that gull-associated fecal contamination may play an important role in the introduction and dissemination of resistant Enterobacterales in coastal ecosystems, with subsequent spread to sand and water. Coastal gulls and beach-associated matrices may therefore act as relevant reservoirs and indicators of antimicrobial resistance circulation at the human-environment interface.