Recent empirical challenges to the “everything is everywhere” concept of microbial distribution have encouraged more fine-grained studies of both biogeographical and ecological drivers of bacterial population and community dynamics. In this study we explore the role of both spatial proximity and ecological similarity of hosts on the transmission of the gastrointestinal bacterial group, Salmonella, among Galapagos iguanas on different islands of the Galapagos. We isolated Salmonella from fecal samples obtained from marine iguanas (Amblyrynchus cristatus) on two islands (South Plaza and Santa Fe), from land iguanas (Conolophus subcristatus) on South Plaza and from land iguanas (Conolophus pallidus) on Santa Fe. Genomic similarity among isolates was evaluated by cluster analysis of repetitive extragenic palindromic PCR (Rep-PCR) banding patterns. We hypothesized that spatial proximity would more strongly drive clustering of isolates creating reduced similarity among Salmonella isolates from marine iguanas residing on different islands as compared to the similarity of isolates from land and marine iguanas residing on the same island.

Results/Conclusions

Genomic fingerprinting revealed high strain diversity within and among the four iguana populations. Contrary to the expectation that isolates would cluster relatively exclusively by island of origin, we found two deep isolate clusters composed predominantly of Salmonella obtained from both marine and land iguanas from South Plaza and one deep cluster (approximately 1/3rd of the sample set) composed of marine iguana-derived isolates from both South Plaza and Santa Fe. Our data suggest that the more intimate geographical association of marine and land iguanas cohabiting a single island certainly encourages bacterial transmission among iguana populations. However, the unexpected clustering of marine iguana isolates from both islands also suggests that Salmonella travel between islands and that inter-island exposures are specific for the ecology of marine iguanas.  Future work will explore the possible modes of transmission of gastrointestinal bacteria among islands (e.g. movements among islands by shore birds or migration of marine iguanas among islands).