High abundance of Stenotrophomonas maltophilia in slaty-backed gull breeding in Northern Japan.

Stenotrophomonas maltophilia is a gram-negative bacillus that is widely distributed in the natural environment and is known to cause opportunistic infections. While it has been isolated from diverse wildlife, the relationship between migratory birds and this bacterium is poorly understood. In this study, we investigated fecal samples from migratory slaty-backed gulls (Larus schistisagus) breeding in Northern Japan by 16S rRNA sequencing and examined the characteristics of multidrug-resistant bacteria. We found that S. maltophilia represented 41% of all bacterial 16S rRNA gene sequences. S. maltophilia capable of growing on agar medium supplemented with multiple antibiotics was present at levels up to 10⁴ CFU/g, and most of the 60 isolates exhibited strong biofilm-forming ability and swimming motility. Although trimethoprim-sulfamethoxazole and minocycline were relatively effective, 18% of the isolates showed resistance to both trimethoprim-sulfamethoxazole and levofloxacin, which are widely used as therapeutic drugs. All strains carried genes involved in multidrug efflux pumps and aminoglycoside resistance. The frequencies of virulence-associated determinants were as follows: stmPr1 (major extracellular protease), 56.7%, and smf-1 (type 1 fimbriae, biofilm-related gene), 100%. These findings suggest that migratory gulls play a role in the dissemination of antibiotic-resistant S. maltophilia in natural environments.IMPORTANCEStenotrophomonas maltophilia has emerged as an opportunistic pathogen that causes pneumonia and bloodstream infections. Despite its clinical importance, there are few reports describing its association with migratory birds. This study revealed that the intestines of slaty-backed gulls harbor a high proportion of this bacterium. Among the multidrug-resistant bacteria examined, all isolates were identified as S. maltophilia. While most isolates were susceptible to commonly used antimicrobial agents such as trimethoprim-sulfamethoxazole, minocycline, and levofloxacin, some exhibited resistance to these antibiotics. Furthermore, all isolates showed strong biofilm-forming capacity and swimming motility, similar to clinical isolates, thereby possessing the potential to adhere to and develop biofilms on artificial surfaces such as medical devices, as well as on mucosal surfaces. Collectively, these findings indicate that migratory gulls may carry and transport antibiotic-resistant S. maltophilia.