Effects of nest density and gender on prevalence of antibiotic resistant Gram- negative bacteria in the GI tract of Herring Gulls (Larus argentatus)

Over the past few decades, antibiotic resistant bacteria have become increasingly common in clinical settings and are now a major global public health concern. Wildlife may aid in the dissemination of antibiotic resistant (AR) bacteria and AR genes. Herring Gulls may be particularly important reservoirs of AR bacteria because they forage and roost at humanmodified settings where they may obtain AR bacteria. Although AR bacteria have been found in gulls, there is very little understanding of the ecological and demographic factors influencing carriage and transmission of AR by gulls. In this study, we tested for potential effects of nesting density and gender on the prevalence of Gram negative bacteria resistant to tetracycline and ceftazidime in Herring Gulls. We found that the prevalence of Gram-negative bacteria resistant to ceftazidime increased significantly in gulls nesting at high density sites at the end of the breeding season; there was no change at low density sites. There was no effect of nest density on prevalence of Gram-negative bacteria resistant to tetracycline. We found no significant effects of gender on the proportion of Gram-negative AR bacteria. We identified a subset of isolates from gull feces at the genus and/or species level including several E.coli strains and Proteus species. Our results suggest that males and females nesting at low and high densities play an equal role in dissemination of AR. However, the lack of observed differences may result from a small sample size relative to the high variability among samples. In conclusion, our study indicates that there is a relationship between high nesting densities and antibiotic resistance prevalence so that gulls at high density sites play a more important role in AR prevalence than gulls at low density sites.