Acquired antibiotic resistance traits in commensal bacteria of wild land iguanas from a remote and protected island of the Galápagos Archipelago

In recent years, the presence of acquired resistance traits has been investigated in the wildlife commensal microbiota, to better understand the ecology and dynamics of antibiotic resistance determinants dissemination. Most of the available data concern mammals and birds from remote or more promiscuous settings . This study was aimed at collecting similar data on commensal bacteria from land iguanas living in a remote and protected environment, free from exposure to the selective pressure related by the use of antimicrobial agents and where close interactions with humans and other animal species are overall minimal. Methods: Cloacal swabs were collected from 96 iguanas in Santa Fè island (Conolophus pallidus) where tourists are restricted to well defined trails for animal viewing and human settlements are not allowed. Coliform bacteria were selected on MacConkey agar to determine the dominant species, and susceptibility to 10 antimicrobials (AMK, AMP, CIP, CHL, GEN, KAN, NAL, TET, STR, SXT ) was determined. Non-dominant resistant coliforms were detected by plating swabs onto MacConkey agar and looking for colonies grown inside the zone of inhibition of the antibiotic disks applied directly to the seeded plate. Results: In each animal, the dominant gut microbiota growing on McConkey medium consisted of one or two coliform species which included Escherichia coli in 77% of animals. In no case the antimicrobial susceptibility profile was suggestive for acquired resistance traits. Non-dominant resistant coliforms were obtained from 82 (85.4%) animals (overall, 126 non-duplicated isolates of various species). Of these, only 5 E. coli carried acquired resistance traits to AMP (n=3), TET + NAL (n=1), or to all the tested antibiotics but CIP, NAL and AMK (n=1). blaTEM, Tet A and Tet B were found as acquired resistance genes. Conclusions: Coliforms are normally represented in the gut microbiota of terrestrial iguanas. In this remote and protected natural environment acquired resistance traits were exceedingly rare. This study provides a “zero level” antibiotic resistance basis, which could prove useful also in bioconservation approaches.